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A COMPARATIVE STUDY OP THE STRUCTURE AND FUNCTION
OP THE ADHESIVE APPARATUS OF THE CYCLOPTERTDAE AND GOBIESOCIDAE
by
' GEORGE SHIRO ARITA B.A., The University of Hawaii, 1962 M.Sc, The University of Hawaii, 196^
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF
THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE
in the Department
of
Zoology
We accept this thesis as conforming to the required standard
THE UNIVERSITY OF BRITISH COLUMBIA August, 196? In presenting this thesis in partial fulfilment of the requirements
for an advanced degree at the University of British Columbia, I agree
that the Library shall make it freely available for reference and
Study. | further agree that permission for extensive copying of this
thesis for scholarly purposes may be granted by the Head of my
Department or by h i.-s representatives. It is understood that copying
or publication of this thesis for financial gain shall not be allowed
without my written permission.
Department of
The University of British Columbia Vancouver 8, Canada
Date a 2- m i
ABSTRACT
The ventral adhesive discs of the cyolopterid Eumicrotremua orbis and the gobiesocid Gobiesox Eaeandricus were compared in regards to structure and efficiency. The disc of E. orbis was found to be the more efficient, being able to support, on the average, 843.6 grams per square centimeter of disc area as compared to ^53«8 g/crn^ for G. maeandricus. The firmer, more compact nature and the greater use of the muscles in the function of the cyclopterid disc accounted for this greater efficiency. The creation of a negative pressure during the adhering process in both species was empirically demonstrated for the first time. G. maeandricus was shown to create a greater suction than E. orbis while supporting an equivalent weight. The pectoral fins were found to be vital in the function of the disc of G. maeandricus. It was concluded that the efficiency of the discs is related to body form; thus
E. orbia. with a globular, unstreamlined body, has the more efficient disc, while in G. maeandricus. in which the body is flattened and streamlined, the disc is less efficient. A new mechanism, described as a sliding and bending property of the lepidotrichs of the fin-rays, is presented; the role of this mechanism in the function of the adhesive disc of G. maeandricus and in the locomotive function of fins in general is discussed* ii
TABLE OP CONTENTS
Page
ABSTRACT . i
TABLE OP CONTENTS ii
LXST OF TABLES ...... «•.....••• v
LIST OP FIGURES vi
ACKNOWLEDGEMENTS ...... ix
INTRODUCTION ...... 1
MATERIALS AND METHODS 3
Descriptions of the Species and Collection of the Specimens ... 3
Eumicrotremu8 orbis (Gunther)...... 3
Gobiesox maeandricus (Girard)...... 7
Preparation of Specimens for Anatomical Study ...... 10
Definition and Methods of Measurements...... 10
Standard Length...... ».«••••. 10
Dimensions
AX*@£L Of "til 6 Dd.SC oooeooo9ooooooooo««««* 11
Determination of the Maximum Adhesive Strength of the Disc. ... 11
Determination of the Hydrostatic Pressure Within the Disc in Relation to Weight Supported...... 13
Determination of the Hydrostatic Pressure Within the Disc in Relation to Current ...... 15
Behavior of E. orbis in Relation to Current ...... 18
Determination of Body Drag in a Current ...... 19
Removal of Various Structures Believed to be Associated With the Adhesive Function of the Disc ...... 21 iii
I§ge
Pectoral Fins ...... 21
Axial Flaps in G. maeandricus ...... 21
Transection of the Spinal Nerves Innervating the Disc. 22
Determination of the Maximum Adhesive Strength of the Disc of Freshly Killed Specimens ...... 23
Morphology of the Adhesive Disc...... 23
External Morphology of the Disc of E. orbis 23
External Morphology of the Disc of G. maeandricus 26
Osteology of the Disc of E. orbis 32
Osteology of the Disc of G. maeandricus 3k
Myology of the Disc of E» orbis ...... 39
Description of the Muscles of the Adhesive Disc. .... 39
Action of the Muscles During the Adhering and
Releasing Processes...... k6
Myology of the Disc of G. maeandricus ...... k?
Description of the Muscles of the Adhesive Disc. .... ^7
Action of the Muscles During the Adhering and Releasing Processes...... 56 Maximum Adhesive Strength of the Disc...... 58 Hydrostatic Pressure Within the Disc in Relation to Weight
Function of the Adhesive Disc in Relation to Current ...... 60
Behavior of the Animal in Still and Flowing Water ...... 60
Body Drag in Relation to Current...... 65
Hydrostatic Pressure Within the Disc in Relation to Current . 65 iv
Page
Structures Associated With the Function of the Adhesive Disc of E. orbis 69
Maximum Adhesive Strength of the Disc of Freshly- Killed Specimens...... 69
Inhibition Of Nervous Control of the Disc ...... 69
P@ C "tl O I*£L X FiriS 9««oeo««o«o««*iao««*0««eo 70
Structures Associated With the Function of the Adhesive Disc
Of Cr a IHSlG£lXld.I*l CU.S ooeooooooeoooooooootoo** 70
M\iS C1 ©S » o • 9 o o o e o o 9 c o o 0 o o 0 o o o • « 0 • • 0 70 Maximum Adhesive Strength of the Disc of Freshly Killed Specimens...... 70
Inhibition of Nervous Control of the Disc ...... 72
P©CtOI*£tX FLriS «0O60oooooeao9«0«oo0O«00 73
SL X FX £L]D «0oo0oao«oe««>ooooO0o«90*0 73
DISCUSSION 0»«0«000e0eoooooooooeeo9oea000 73
SUMM^LRY S00QOOO eoooo O00««oa«o oooooeo9e»o 36
LITERATURE C ITED 0900*00000000000000000000*0 89 V
LIST OF TABLES
Page
Table I. Orientation of Eumicrotremus orbis while attached and while swimming in still water and in a current. The values represent the amount of time (in minutes) the four specimens were observed facing in each direction. The values in parentheses represent the fo of total minutes in each row. "Forward" refers to facing "into" the current ...... 64
Table II. Summary of the effects of the removal of varying numbers of the ventralmost rays of the pectoral fins on the adhesive strength of the disc of Eumi c rot remus Ol*bXS • ««»eo«oaO0»»«aoooo« 7^-
Table III. Summary of the effects of the removal of various structures on the adhesive strength of the disc of Gobiesox maeandricus...... 7k vi
LIST OF FIGURES
Page
Fig. 1. Eumicrotremus orbis. mature female, 3.6k cm SL, lateral view...... 5
Fig. 2. The collecting area of E. orbis adjacent to the swimming pool at Lumberman's Arch, Stanley Park, Vancouver, British Columbia: at low tide ...... 6
Fig. 3. Gobiesox maeandricus. mature male, 7.78 cm SL,
Fig. 4. The collecting area of G. maeandricus at Second Narrows Burrard Inlet, Vancouver, British Columbia; at low tide ... 9
Fig. 5« The counterweight balance apparatus used for determining the maximum adhesive strength of the disc 14
Fig. 6. The adhering platform used for determining the negative pressure developed within the chamber of the disc in relation to weight supported. ... 14
Fig. 7. The current chamber used for determining the negative pressure developed within the chamber of the disc in 1*61 si/ion "to cux*x*6n>t oo«»«ooo«ooa«»o«*»««« \tS
Fig. 8. The modified current chamber used for measuring the body drag in different currents...... 20
Fig. 9« The adhesive disc of Eumicrotremus orbis, mature female, 3*32 cm SL 9 ^5 • 7 • ao©o©©«oo© « o » • » • ©•«©«2^ Fig. 10. Sexual difference in the relationship between standard length and disc diameter in Eumicrotremus orbis 27 Fig, 11. The adhesive disc of Gobiesox maeandricus. mature male, 6 o ^4*3 cm SL« ^3 *3**,<,**oo*°ooo*oo****#**28 Fig. 12. Relationship between standard length and disc diameter in Gobiesox maeandricus ...... 31 Fig. 13. Osteology of the adhesive disc of Eumicrotremus orbis. A. Dorsal view. B. Ventral view. Fig. 14. Osteology of the adhesive disc of Gobiesox maeandricus. dorsal view. X8...... 35 vii Page Fig. 15. Osteology of the adhesive disc of Gobiesox maeandricus. ventral view. X8 36 Fig. 16. Dorsal musculature of the adhesive disc of Eumicrotremus orbis. dorsal view. X10 40 Fig. 17. Deeper view of the dorsal musculature of the adhesive disc of Eumicrotremus orbis. dorsal view. XI0 41 Fig. 18. Ventral musculature of the adhesive disc of Eumicrotremus orbis. ventral view. X10 kk Fig. 19. Ventral musculature of the adhesive disc of Eumicrotremus orbis with pelvic rays removed, ventral view. X10 45 Fig. 20. Dorsal musculature of the adhesive disc of Gobiesox maeandricus. dorsal view. X8 ...... 48 Fig. 21. Deeper view of the dorsal musculature of the adhesive disc of Gobiesox maeandricus, dorsal view. X8 49 Fig. 22. Deepest dorsal musculature of the adhesive disc of Gobiesox maeandricus. dorsal view. X8 50 Fig. 23. Ventral musculature of the adhesive disc of Gobiesox maeandricus. ventral view. X8. . . 54 Fig. 24. Deeper view of the ventral musculature of the adhesive disc of Gobiesox maeandricus. ventral, view. X8. . 55 Fig. 25« Relationships between disc size and maximum adhesive strength of the discs of Eumicrotremus orbis and Gobiesox maeandricus 59 Fig. 26. Relationship between the negative pressure within the disc and weight supported per unit area of disc in Eumicrotremus Fig. 27. Relationship between the negative pressure within the disc and weight supported per unit area of disc in Gobiesox Fig. 28. Relationships between the body drag per unit area of disc and current in Eumicrotremus orbis and Gobiesox maeandricus 66 viii Page Pig. 29. Relationship between the negative pressure within the disc and body drag per unit area of disc in Eumicrotremus orbis 67 Fig. 30. Relationship between the negative pressure within the disc and body drag per unit area of disc in Gobiesox maeandricus 68 ix ACKNOWLEDGEMENTS I wish to thank Dr. N. J. Wilimovsky, Institute of Fisheries, The University of British Columbia, for his encouragement and advice throughout the course of the study, and Drs. I. E. Efford and P. A. Larkin for reading and criticizing the manuscript. Dr. J. D. McPhail, Mr. F. K. Sandercock and Mr. G. I. MoT. Cowan offered valuable criticisms and helpful suggestions. To the several fellow students who assisted with the collection of specimens, I am greatly indebted. Most notable of these were Mr. J. P. Wiebe and Mr. M. Dickman. Finally I wish to thank the National Research Council of Canada for financial assistance through a grant supplied to Dr. Wilimovsky and The Fisheries Association of British Columbia for their financial support during the initial phases of this study. -1- INTRODUCTION Many animals of the aquatic environment live under conditions which constantly expose them to the effects of flowing water. Some of these ani• mals remain at the mercy of their surroundings and merely drift along with the currents; others develop into moderate to strong swimmers to combat the currents and travel on their own accord. Some animals living near or at the bottom learn to seek refuge in crevices or under rocks. Finally others simply attach themselves to the substrate to resist the forces of the flow• ing water. For this purpose, organs of attachment of various types have been developed in many different groups of animals. Many invertebrates rely on sticky secretions or hooking devices as a means of attachment (Hora, 1930). For example, platyhelminths and many molluscs use the mucous secretions not only for locomotion but also to keep themselves affixed to the substrate, and many crustaceans and pupae of in• sects develop gripping claws or posteriorly directed hooks and spines for the purpose of attachment. Most fishes can contend with the currents by their swimming ability, but some fishes have developed sucking organs for the purpose of attachment. Most notable are the remoras (family Echeneidae) with the large dorsal sucker on the head, the cyprinid Garra and some sisorids with an oral sucker formed by parts of the lower lip (Hora, 1922), and the fishes of the families Sisoridae, Homalopteridae, Gobiidae, Cyclopteridae and Gobiesocidae, in which the pelvic fins or both pelvic and pectoral fins have become modified to form -2- a ventral sucking disc. The present study was undertaken to examine the structure and to deter• mine and compare the functional efficiencies of the adhesive organs of the Cyclopteridae and Gobiesocidae, and also to relate any detected differences in efficiency to the differences in structure. The families Cyclopteridae and Gobiesocidae are not closely related (Gill, 1864; Putman, 1874), and marked structural differences occur between the ventral adhesive organs of the two families (Gunther, 1861:495). There has been no previous comparative study between the two families; however, the external and internal structures of the adhesive disc of the largest cyclopterid, Cyclopterus lumpus. have been described by Borckert (1889), and Guitel (I896) reported on the embryology of the disc of the same species. Morphological studies on other species of this family have not been recorded in the literature. The adhesive organ of the Gobiesocidae has been given somewhat greater consideration by previous workers. Niemiec (1885) described the structure of the disc of the eastern Atlantic species, Lepadogaster gouanii. and Guitel (1889) furthered the knowledge on the same species with a description of the actions of the various parts of the disc in the adhering and releasing processes; he also gave brief descriptions of the discs of other genera including Gobiesox. The osteology of the disc of Gobiesox maeandricus was briefly considered by Starks (1905), while Briggs (1955)» in a systematic monograph of the family, presented descriptions of the external features and osteology of the adhesive discs of the various -3- species. Mcintosh (189?) showed that the adhesive apparatus in Lepadogaster bimaculatus did not develop until during the later stages of larval life. The same situation was reported for Gobiesox strumosus by Runyan (1961) and Dovel (1963). Experimental studies on the function and efficiency of the adhesive disc have been limited entirely to Cyclopterus lumpus. Borckert (1889) showed that the disc of this species could support a weight j/h of a theo• retical maximum amount calculated on the basis of the creation of a perfect vacuum between the disc and the substrate; thus a fish with a disc 1.9 cm long and 1.3 cm wide, which should theoretically be able to support a weight of 2.6 kg, was found to be able to support a weight of 1.9 kg. Gill (1907) reported an incident where a large specimen of £. lumpus was able to lift a bucket of water weighing 43 pounds. Studies on the larval development of C_. lumpus indicate that the adhesive disc is already formed and functional at hatching (Mcintosh, 1886, 1897; Merriman and Sclar, 1952; Kobayashi, 1962). Bishai (i960) showed experimentally that the newly hatched larvae attach almost immediately when confronted by a current and are able thereupon to withstand currents of up to 44.7 cm/sec. MATERIALS AND METHODS Descriptions of the Species and Collection of the Specimens Eumicrotremus orbis (Gunther) The spiny lumpsucker was first described by Gunther (l86l) as Cyclopterus orbis from a specimen taken from the Esquimalt area on Vancouver Island, British Columbia, and later placed in the genus Eumicrotremus by Gill (I863). In addition to the presence of a characteristic ventral adhesive disc, E, orbis (Pig. l) can be distinguished by the presence of dermal tubercles arranged in definite rows on the head and body (Lindberg and Legeza, 1955)• It is a small species; the largest specimen taken in this study, a female, measured less than 5»0 cm in standard length. Further details on the description of the species are provided by Lindberg and Legeza (1955) and Clemens and Wilby (1961:337). The live specimens of the spiny lumpsucker used in this study were collected by seining along the beach area (Fig. 2) adjacent to the swimming pool at Lumberman's Arch, Stanley Park, Vancouver, British Columbia. The bottom of the offshore area at which the specimens were collected is made up mostly of sand and small rocks with only a few large rocks which occasionally hampered the seining operation. The vegetation in this area consists mainly of eelgrass (Zostera marina) and kelp (Nereocystis luetkeana). The collections were made at low tide during the months of October to April (1965-67) when the lumpsuckers migrate inshore to spawn. The maximum tidal current in this area during these months is approximately 6 knots (Canadian Hydrographic Service, 1965:262). The low tides during this period occur usually at night. The peak abundance of -5- Fig. 1. Eumicrotremus orbis. mature female, 3.64 cm SL, lateral view. Fig. 2. The collecting area of E. orbis adjacent to the swimming pool at Lumberman's Arch, Stanley Park, Vancouver, British Columbia; at low tide. -7- E. orbis was between late November and late February, and most of the specimens caught at this time were sexually mature, the females being swollen with numerous large orange eggs. Juvenile specimens (0.5-1.0 cm in standard length) were abundant in the early April collections. No specimens were caught in this area at times other than during the breed• ing season. All the specimens collected were kept in an 80-gallon holding tank until needed; the temperature of the water in the tank was kept at approximately 10° C. Gobiesox maeandricus (Girard) The clingfish occurs along the shores, concealed under rocks, in areas relatively free of mud. The conspicuous adhesive disc and flat, broad head make its identification obvious when seen (Fig. 3), but its cryptic coloration with dark reticulated patterns and clandestine be• havior make it difficult to be seen. Briggs (1955slOl) gives the systematic account and a more detailed description of this species (also see Clemens and Wilby, 1961096). The collections were made at Second Narrows, Burrard Inlet, Vancouver, British Columbia, along the northern shore adjacent to the Canadian National Railways bridge (Fig. k) during the lowest tides occurring between May and September (1966-67). The offshore tidal currents in this area may be as high as 5-5 knots (Canadian Hydrographic Service, 1965:273)i and light waves caused by the changing tide or -8- FiS« 3. Gobiesox maeandricus. mature male, 7.78 cm SL, lateral view. -9- -10- passing boats periodically strike the shore. The fish were found hiding under rocks at the low water mark and frequently even under rocks a meter above this level. Most of the larger specimens collected were already spent; on three occasions in mid-May and early June (l°66), adult males were captured guarding egg masses. The eggs, which were laid on the undersides of large rocks, were brought into the laboratory for rearing and additional study. The clingfish specimens were kept in the same holding tank in which the lumpsuckers had been kept during the winter and spring; the temperature was maintained at approximately 12° C. Preparation of Specimens for Anatomical Study Specimens were stained using Alizarin Red in 2% KOH. For the study of the musculature, to provide contrast between the muscles and the red-stained skeletal parts, the specimens were also lightly stained with methylene blue. Definition and Methods of Measurements Standard Length The standard length was measured from the tip of the snout to the base of the caudal fin. Dial calipers were used for this measurement which was determined to the nearest 0.01 cm. Dimensions of the Disc In both species the discs are elliptical; therefore, measurements -11- of the diameter of the disc had to be made along both longitudinal and transverse axes. Measurements were made to the nearest 0.01 cm. Dial calipers were used for measuring preserved specimens, but in determin• ing the disc dimensions of live specimens, the fish were placed in a 10 cm X 8 cm X 8 cm Plexiglas box, the bottom of which was marked with a grid of 0.1 cm intervals. The box was partially filled with sea water containing a solution of MS222-Sandoz (approximately 1:10,000), which permitted easy measurement of the dimensions after the specimen became quiescent and adhered to the bottom. In E. orbis the disc diameters were measured from the outer edges of the large papillae along the longitudinal and transverse axes. In G. maeandricus the longitudinal diameter was measured from the outer edges of the outermost row of papillae at the anterior and posterior parts of the disc, and the transverse diameter was measured from the outer edges of the second pelvic fin rays of the two sides of the disc. Area of the Disc The area of the adhesive disc was calculated using the equation for determining the area (A) of an ellipse, A = if ab , where a and b represent the longitudinal and transverse axes, respectively. Determination of the Maximum Adhesive Strength of the Disc A counterweight balance system (Fig. 5), consisting of a 66 cm long brass beam on a Dexion supporting frame, was used to determine the maximum adhesive strength of the disc. A sliding weight attached to the beam -12- pennitted zero-balancing. The arm of the balance to which the specimen is attached is longer than the arm on which the pan for the weights is attached by a ratio of 2.08:1, so that a weight of 100 g on the long arm is counter• balanced by a weight of 208 g on the other. Therefore, the total weight accumulated in the weighing pan had to be corrected by dividing by 2.08. This ratio permits a more precise measurement of the force exerted on the fish. A series of brass metric weights ranging from 10-5000 grams was used. Strong nylon crinolin fabric was used to harness the fish to the bal• ance. The width of the harness was measured according to the length of the specimen and the length of the harness was made sufficient to encircle the body of the specimen and still allow about 1.5 cm for the attachment of the line from the balance; a pin was used to tighten the harness around the ani• mal. An oval hole was cut in the middle of the harness to allow the disc and pectoral fins to protrude unhindered. The specimen was lightly anaesthetized in the disc measuring box. After recording the dimensions of the disc and the standard length, the specimen was placed in the harness and attached to the balance. With E. orbis the body is firm enough so that the specimens could be placed directly into the harness; however, with the clingfish, a length of 1.5 mm diameter wire passing from the mouth to the anus had to be used to support the soft body. The wire was passed through the stomach and intestine, avoiding damage to any vital organ or muscles of the disc. That the wire did not markedly harm the animals was shown by the fact that all of the specimens not preserved after the test recovered in the holding tank. -13- After the specimen was attached to the balance, it was placed in a 10- gallon aquarium filled with sea water to a depth of 2k cm. Upon recovery from the effects of the anaesthetic the fish was made to adhere to the clean, glass bottom of the aquarium and weights were successively placed on the pan on the opposite end of the balance. The adhesion broke immediately when the maximum weight was exceeded, so to prevent fatigue, the fish was subjected to the tension of each successive increase in weight for only about one second; if the force was successfully withstood, the tension was released and another weight was added. This procedure was continued until the fish was finally pulled off the substrate. Then the process was repeated two or three times to verify the maximum value. After the test, the lumpsucker specimens, with their first dorsal fins clipped, were returned to the holding tank to be used again in other tests. Since G-obiesox specimens were more abundant, these were usually labelled and preserved in 10^ formalin after the test. Determination of the Hydrostatic Pressure Within the Disc in Relation to Weight Supported The same counterweight balance described in the previous section was used in this experiment. However, the 10-gallon aquarium used in this case was fitted with a Plexiglas bottom on which a 8.0 cm X 8.0 cm Plexiglas platform was mounted. Through the center of this platform a hole, 0.8 mm in diameter, was bored and a length of polyethylene tubing (Clay-Adams Intramedic Polyethylene Tubing, PE 60) with an inside diameter of 0.76 mm and an outside diameter of 1.22 mm was fitted to the hole (Fig. 6); the -14- Fig. 5« The counterweight balance apparatus used for determining the maximum adhesive strength of the disc. 0.8 m» - • • * ^ 13 PE tubing to transducer Fig. 6. The adhering platform used for determining the negative pressure developed within the chamber of the diso in relation to weight supported. -15- tubing was led to the outside through another hole, 1.25 mm in diameter, through the bottom of the aquarium. The polyethylene tubing was connected to a pressure transducer (Statham Pressure Transducer Model P23AA) and the pressure changes occurring within the disc were recorded on a Beckraan Type R Dynograph k Channel Pen Recorder. The specimen was prepared in the same manner as described in the pre• vious experiment. After the harnessed specimen had recovered from the effects of the anaesthetic, it was made to attach itself over the hole in the center of the platform. Weights were then placed on the pan at 20 to 50 gram increments (depending on the size of the specimen) until the point at which the specimen could no longer support the weights was reached. Two or three trials were conducted for each specimen. Determination of the Hydrostatic Pressure Within the Disc in Relation to Current The current chamber consisted essentially of a cylinder enclosed in a larger outer cylinder with the current created by a propeller driven by an electric motor (Fig. ?). The outer chamber was made from a 38.5 cm length of Plexiglas cylinder with an inside diameter of 14.6 cm and 0.7 cm thick walls. The inner chamber consisted of a Plexiglas cylinder 28.5 cm in length, with an inside diameter of 7«6 cm and 0.7 cm thick walls. The actual swimming chamber, in which the fish was placed, was 18.0 cm long. A baffle, 4.0 cm long, was fitted between the propeller and the swimming chamber to reduce the spiral flow of water. A shorter removable baffle (l.5 cm in length) was fitted at the end of the Pig. 7. The current chamber used for determining the negative pressure developed within the chamber of the disc in relation to current. -17- chamber to prevent the specimen from swimming out. A 0.7 cm thick Plexiglas plate, 18.0 cm long and 5«5 cm wide, was fitted at the bottom of the swimming chamber. Two holes, 0.8 cm in diameter, placed along the midline 5«0 cm and 8.5 cm from the anterior end of the plate, were fitted with polyethylene tubings as described in the previous section. The surface of the plate outside a radius of approximately 1.5 cm from each hole was covered with sand stuck to the Plexiglas. Three strips of 0.7 cm thick Plexiglas were attached to the outside of the inner chamber to serve as braces, and the inner chamber was then fitted into the larger, outer chamber. The ends of the outer chamber were sealed by two Plexiglas end-plates. A circular Teflon section was fitted into a 4.2 cm diameter hole in the center of the anterior end-plate to serve as a bearing and seal for the shaft of the propeller. The entire chamber was fastened to the bottom of a 59*0 cm X 30.5 cm X 24.0 cm Plexiglas container which served as a water bath to prevent air bubbles from entering the system. The propeller shaft was passed through another Teflon bearing at one end of the container. The current was generated by a three-blade, nylon model airplane pro• peller mounted on a 20.0 cm long, 0.5 cm diameter stainless steel shaft. Each blade of the propeller was cut to a length of 3°5 cm to fit into the anterior end of the inner chamber. The shaft was connected by pulleys to a Black & Decker Type E, -J- inch Utility Drill, which was mounted directly above the shaft on a Black & Decker Type 1, \ inch Horizontal Stand; and a rheostat (Variac Powerstat Type 116, 7»5 amperes, The Superior Electric Co., Bristol, Connecticut) was used to vary the speed of the motor. The velocity •18- of the current was determined using a Pitot tube following the method de• scribed by Welch (l°48). The polyethylene tubings leading from the holes in the Plexiglas plate in the inner chamber were connected to a Statham Pressure Transducer Model P23BB, and the pressure changes within the disc were recorded on a Gilson Polygraph Recorder, Model M5P. The specimen was placed in the inner chamber and left undisturbed for about 30 minutes, after which time it usually settled in the vicinity of one of the holes. Often the specimen had to be prodded with a flexible wire to induce it to adhere directly over the hole. The motor was then started, running first at the lowest speed and then at progressively higher speeds after 30 to 60 second intervals. Each point of increase in speed was in• dicated on the recording so that any corresponding change in pressure with• in the disc could be correlated. The readings on the Pitot tube were also simultaneously recorded. r Four or more trials were conducted on each specimen with the fish left undisturbed for at least 30 minutes between each trial. The disc dimensions and standard length of each specimen were measured after the tests - Behavior of E„ orbis in Relation to Current The current chamber, slightly modified, was also used in this experiment. A Plexiglas plate, without sand or holes, was fitted to the bottom of the inner chamber in place of the one previously described. The apparatus was placed behind a cardboard screen to avoid visual disturbance, and observa• tions were made through a one-way mirror. The activities of the specimens -19- were recorded on a 20-channel event recorder (Esterline Angus Operation Recorder Model A). Four lumpsuckers of varying sizes were observed individually in the current chamber under two conditions, still water and current of 17.0 cm/sec. Each fish was placed in the chamber 30 minutes prior to the start of the ob• servation periods and six 2-minute periods were conducted for each fish under each condition. A two-minute rest period separated each observation period. The fish were not fed for 24 hours prior to the study. The following actions of the fish were recorded: (l) instances and duration of attachment and free swimming, and (2) orientation in relation to the direction of the current. Tests were not conducted at currents of higher velocities because it was found that a fish was not able to overcome the current and attach at these speeds. Determination of Body Drag in a Current The inner cylinder of the current chamber was modified and used for the determination of the body drag. The cylinder was anchored vertically (Fig. 8) to the bottom of a large plastic tub filled with sea water and fitted with plastic piping to a water pump (Model #3, Little Giant Pump Company, Oklahoma City, Oklahoma), the speed of which was controlled by the rheostat. The freshly killed specimen was suspended by a thin thread from a triple-beam balance into the center of the cylinder. Four specimens of varying sizes of each species were used. The disc dimensions and standard length of each specimen were recorded and the speci• men was suspended from the balance into the cylinder. The weights of the -20- Fig. 8. The modified current chamber used for measuring the body drag in different currents. -21- specimen in still water and in the progressively increasing current were recorded, and the difference in the weights in still water and in the various current velocities was taken as a measure of body drag at the various current velocities. The velocities of the current were calculated by determining the volume of water flowing through the system per unit time at the various pump speeds and dividing these values by the cross-sectional area of the chamber. Removal of Various Structures Believed to be Associated With the Adhesive Function of the Disc Pectoral Fins The extent of the role of the pectoral fins in the adhesive function of the disc was examined by clipping varying numbers of the lower rays of the pectoral fins. After allowing at least four days for recovery from the treatment, the maximum adhesive strength was determined. Axial Flaps in G. maeandricus Each axial flap behind the pectoral fins in G. maeandricus was removed by cutting along the base of the flap and across the membrane connecting the flap to the posterior part of the disc. The thin mem• brane passing from the posterior part of the disc to the base of the fifth pectoral fin ray was not disturbed. After allowing at least four days for recovery from the treatment, the maximum adhesive strength was determined. -22- Transection of the Spinal Nerves Innervating the Disc In the lumpsucker, spinal nerves 1-4 innervating the pectoral muscles and most of the muscles of the disc pass together from the spinal column ventrally along the posterior margin of the skull. These nerves were cut at this point behind the skull by passing a scalpel blade through an inci• sion at the posterodorsal edge of the large tubercle directly over the gill aperture. Extreme care had to be taken to avoid damaging the anterior part of the kidney lying adjacent to the nerves. Success of the treatment was indicated by the immobility of the pectoral fins. Three control specimens were prepared by making incisions into the musculature at the same locality. Both treated and control specimens were tested for maximum adhesive strength after at least seven days of recovery. Spinal nerves 5> 6 and possibly 7 innervate the posterior part of the adhesive disc. However, no attempt in transecting these were made because of the difficulty. In G. maeandricus spinal nerves 1 and 2 pass posterolaterally from the spinal cord to innervate the muscles of the pectoral fins, and spinal nerves 3 and k innervate the anterior half of the adhesive disc. These two nerves converge laterally from the spinal cord and turn ventrally together to the disc. Spinal nerves 5 and 6 pass directly laterally from the spinal cord and turn ventrally to innervate the posterior half of the disc. Spinal nerves 1-4 were transected by making a longitudinal incision midway between the dorsal midline and the gill opening. The use of the disc following the operation was observed and the maximum adhesive strength was determined after seven days following the operation. The success of the -23- operation was later verified by dissection. Determination of the Maximum Adhesive Strength of the Disc of Freshly Killed Specimens Specimens anaesthetized by a high overdose of MS222 were killed by severing one of the branchial arteries. After twenty to thirty minutes each specimen was tested for maximum adhesive strength. RESULTS Morphology of the Adhesive Disc External Morphology of the Disc of E. orbis The morphology of the adhesive disc of E. orbis is very similar to that of Cyclopterus lumpus, as described by Borckert (188°). As in the latter, the bilaterally symmetrical disc of E„ orbis (Fig. 9) also consists of a cupped, inner portion supported by the proximal ends of the pelvic fin-rays and a flat surrounding rim supported by the central and distal portions of the rays. The inner portion of the disc is lined with tough but flexible skin firmly attached to the fin- rays. The rim consists of three parts: a single innermost row of large pavement-like dermal papillae, an intermediate section of numer• ous, similarly-shaped but smaller dermal papillae and a margin repre• sented by a thin flap-like membrane. A ring of numerous rows of minute, finger-like projections, or fimbriae, separates the membrane from the section of small papillae. The single, innermost row is made up of sixteen large dermal Fig. 9. The adhesive disc of Eumicrotremus orbis, mature female, 3.52 cm SL. X5.7. -25- papillae, one each in the midline at the anterior and posterior ends of the disc and seven others between these two in each lateral half of the disc. All of the papillae, except the two medial ones and first anterior pair, are supported internally by the pelvic fin-rays. The surfaces of the papillae are coarse and appear to be free of slime. During the adhering process these papillae are pressed on the substrate by the action of the fin-rays and serve to seal the chamber of the cupped, inner portion of the disc as well as to prevent the disc from sliding over the substrate. The smaller and closely-packed papillae making up the intermediate section decrease in size laterally but otherwise appear to be randomly arranged in two to four indefinite rows around the row of large papil• lae. The total width of this section is about equal to the width of the single row of large papillae. As in the larger papillae, the sur• faces of these papillae are also coarse and slime free. This section of small papillae reinforces the row of large papillae in keeping the disc from sliding and in forming the seal of the disc chamber. Numerous rows of minute fimbriae border the outermost extent of the section of small papillae. These rows overlap one another and completely encircle the disc. At the anterior end of the disc at the symphysis of the pectoral fins, the fimbriae are slightly enlarged and thickened and serve as the anterior margin of the disc. The thin, flap-like membrane forming the disc margin does not completely encircle the disc. It is widest along the posterior half of the disc where it is supported by the extremities of the pelvic rays. -26- It then tapers towards the sides and ends along the anterolateral edges of the disc adjacent to the ventral rays of the pectoral fins. The disc margin at this point is then formed by the rows of fimbriae. The membrane and the rows of fimbriae act as flexible flaps to prevent water from entering the disc chamber during the adhering process. The pectoral fins reinforce the rim of the disc to a certain extent. Eight to ten of the lowermost pectoral fin-rays are arranged horizontally enclosing the anterolateral and lateral margins of the disc. The membrane of this portion of the pectoral fin is greatly thickened and the underside is lined with numerous fimbriae similar in size and shape as those on the rim of the disc. That this part of the pectoral fin supports the disc is suggested by the frailty of the adja• cent part of the rim of the disc, where it is narrowest and lacks the internal support of the pelvic rays; indeed, in the tests to determine the maximum adhesive strength of the lumpsucker disc, it was noticed that this part of the rim was the first to collapse inward as the fish was being pulled off the substrate. Males tend to have a larger disc than females; this difference is more pronounced in larger fish but not apparent in smaller individuals (Fig. 10). Test of covariance shows a significant difference between the slopes of the two regression lines (F = 6.537; f = 1,76; P<0.5). External Morphology of the Disc of G0 maeandricus The adhesive disc of the clingfish (Fig. ll), although somewhat more complex, parallels that of E. orbis in the basic structure. -27- Fig. 10. Sexual difference in the relationship between standard length and disc diameter in Eumicrotremus orbis.' Figo 11c The adhesive disc of Gobiesox maeandricus. mature male, 6.45 cm SL. X3.5. -29- Here, too, the disc can be divided into a cupped, inner portion and a surrounding rim. However, the rim in this case is not continuous around the disc but is interrupted by two lateral clefts which separate the disc into anterior and posterior halves. The anterior half is formed by the pelvic fins and girdle, and the posterior half is formed by the distal postcleithra of the pectoral girdle. The inner portion of the disc is lined with tough but flexible skin which is supported internally largely by the pelvic girdle and partly by the proximal ends of the pelvic rays and the muscles of the disc. Ignoring the lateral clefts, three parts can be distinguished in the rim. The innermost part is covered with numerous rows of small dermal papillae which have the same texture as the papillae of the disc of E. orbis and apparently serve the same function. The papillae de• crease in size laterally but an innermost row of greatly enlarged papillae like those found in E. orbis is not present in this disc. There are two patches of papillae at the posterolateral sections of the inner portion of the disc? the function of these patches was not determined. The fimbriae encircling the rows of papillae are larger and flatter than those in E. orbis. Two rows of overlapping fimbriae form the margin at the anterior end of the disc, but these rows end at the anterolateral corners of the disc. A single row of fimbriae then pro• ceeds along the lateral margin of the disc where the pelvic rays support the rim. The fimbriae at this region are arranged along the outer edges ~30~ of each ray. A single row also continues around the posterior half of the disc. A thin, flap-like membrane occurs only along the margin of the posterior half of the disc and between the pelvic rays. The nature of this membrane is similar to that of E„ orbis. The ventral rays of the pectoral fins are not modified as is the case in the lumpsuckers. However, there are two flaps which join the pectoral fins to the disc. The first passes from the fourth pelvic ray, along the anterior base of the pectoral fin, to the dorsal surface of the fourth pectoral ray. The other passes from the anterolateral end of the posterior part of the disc to the posterior base of the ventral side of the fifth pectoral rayo These two flaps, in essence, join the anterior and posterior halves of the disc via the pectoral fin. The membrane lining the edge of the posterior half of the disc turns dorsally along the lateral edge of the disc and unites with a large fan-like cartilaginous flap which is an external extension of the proximal postcleithrum. This flap, emerging from the posterior angle of the pectoral fin, is called the "axial dermal flap" by Briggs (1955=8). No sexual difference in disc size was detected in the specimens examine d (Fig. 12). -31- 0.8 A 1 1 1 1 ' 1 1 1 1— 3.0 4.0 5.0 6.0 7.0 8.0 ' 9.0 10.0 11.0 STANDARD LENGTH (cm) Pig. 12. Relationship between standard length and disc diameter in Gobiesox maeandricus. -32- Osteology of the Disc of E. orbis The emphasis in this present description will be placed on the structure of the pelvic bones and rays, which provide the actual skeletal foundation of the disc. Sewertzoff (193*0 and Shelden (193?) were referred to in determining the names of the various parts of the pelvic girdle. The skeletal foundation of the disc of E. orbis is similar to that of Cyclopterus lumpus as described by Borckert (1889). Each pelvic bone has three processes (Fig. 13). Most prominent is the broad ex• ternal process which extends anteriorly from the pelvis and connects the pelvic girdle to the cleithrum of the pectoral girdle. A strong cartilage joins the anterior ends of the two external processes. The internal process is a slim strut projecting anteriorly and slightly medially so that the process of one side of the girdle approaches the process of the other side. This process is a continuation of the rim along the lateral edge of the pelvis on which the fin-rays articulate. The dorsal process projects anterolaterally from the dorsal surface of the pelvis. It tapers gradually and ends in a strong tendon, to which various muscles are attached. The six rays of each lateral half of the disc are attached by ligaments to the ventrolateral edge of the pelvic bone. These rays, instead of being segmented and bifid as in the typical fin-ray, are firmly ossified into a single piece, except at the distal ends of the last four rays, which bear traces of segmentation as in the typical ray. Each ray is curved posteriorly, the curvature being the smallest cartilage external process internal process pelvic spine dorsal process 5th pelvic ray B Fig. 13. Osteology of the adhesive disc of Euiaicro*..re:nu: Dorsal view. B. Ventral view. ELO. -34- in the anteriormost ray and the largest in the posteriormost. On the anterodorsal edge of each ray adjacent to the point of attachment to the pelvic hone, there is a small process for the attachment of the muscle from the dorsal side of the pelvic girdle. This knob-like pro• cess appears to be the rudiment of the proximal end of the dorsal half of the fin-ray. The proximal end of the ventral half of the fin-ray is greatly enlarged and extends almost to the midline on the ventral side of the pelvis. The distal ends of the ossified parts of the rays are flattened ventrally to provide for the attachment of six of the large dermal papillae. Osteology of the Disc of G. maeandricus The osteology of the adhesive disc of G. maeandricus has been dealt with by Starks (1905) and Briggs (1955 s 9)o Two symmetrical bones form the triangular pelvic girdle (Fig. 14 & 15), which lies under the anterior part of the disc with the apex directed posteriorly. Two pro• cesses can be distinguished anteriorly. The external process at the anterolateral corner of each bone serves for the attachment of the pelvic girdle to the cleithrum of the pectoral girdle. An internal process projects medially from the anterolateral corner on the dorsal surface of each pelvic bone and meets the process of the other side anteromedially. Two large fenestrae occur on each pelvic bone. The first, the anterolateral fenestra, is visible ventrally at the anterolateral corner of each bone and is formed by an arch at the base of the Fig. H Osteology of the adhesive disc of Gobiesox maeandricus. dorsal view. Fig. 15.- Osteology of the adhesive disc of Gobiesox maeandricus, ventral view. X8. -37- external process. The second, the central fenestra, is a prominent opening through the center of each pelvic bone. It is formed jointly by the medial extension of the external process and by the arch of the internal process. Both of these openings serve for the passage of muscles from the internal process to the fin-rays, A single opening, formed by the meeting of the internal processes at the anteromedial end of the girdle, is covered by ligament and does not serve as a passageway for any muscle. A strong ligament is attached to the center of the pelvic girdle along the middorsal line. It passes dorsally from the girdle and attaches to the ventral surface of the esophagus. The modified pelvic spine and the four pelvic rays attach along the lateral edge of each half of the pelvis. The spine is a short and flattened Y-shaped bone which articulates by a ball-and-socket joint at the anterolateral corner of the pelvis lateral to the external process. It supports the anterolateral part of the rim of the disc. The four soft rays are loosely attached to the pelvis by ligaments. As in the pelvic rays of E. orbis, these four rays are also curved posteriorly; however, the rays in this case are not completely ossified into solid pieces but retain the paired and segmented condition of the typical rays. The segments are closely spaced and greatly flattened, permitting a high degree of flexibility in the ray; also the two halves, or lepidotrichs, of the paired rays are not equal in size but the ventral lepidotrich is wider than, and slightly encloses, the dor• sal. The two lepidotrichs are loosely joined throughout the length of -38- the ray except at the tips where they are fused by the actinotrichs (Goodrich, 1903). As a result of this arrangement, if a muscle attached to one lepidotrich contracts, it pulls the lepidotrich medially causing it to slide over the other lepidotrich; but because the two lepidotrichs are joined at the tips, this sliding forces the entire ray to bend to• wards the sliding lepidotrich. The significance of this bending effect in the adhering process will be discussed below. The lepidotrichs of the first three rays are arranged dorso- ventrally, permitting the bending movements in that plane. Thus the rays can be pressed onto or lifted off the substrate depending on which lepidotrich is pulled. The lepidotrichs of the fourth ray, which is placed slightly dorsal to the third, are arranged laterally. Conse• quently, the bending motions of this ray are directed laterally on or off the side of the body. When this ray is bent medially, it presses the pectoral fin onto the axial flap, thereby closing the gap between the anterior and posterior halves of the disc and completely enclosing the inner chamber of the disc. The third ray is the largest of the four rays. The distal postcleithra are the only members of the pectoral girdle directly involved in forming the disc. These are broad, ante• riorly curved bones lying flat under each half of the posterior part of the disc. The bones are joined to each other medially, and to the apex of the pelvic bones anteriorly, by ligaments. Laterally the distal postcleithra are attached by ligaments to the proximal postcleithra, which pass ventrally from the coracoid at the dorsal part of the cleithrum. -39- Myology of the Disc of E. orbis Description of the Muscles of the Adhesive Disc The pelvic musculature has become greatly modified in both species in conjunction with the new function of the pelvic appa• ratus. Homology and nomenclature were determined by comparing the origins, insertions, and functions of these muscles with those of other teleosts as described by Grenholm (1923) and Shelden (1937)• In cases where the homologies or names could not be ascertained, the muscles are referred to by function; no attempt has been made to present new names. The dorsal musculature of the disc is shown in Figures 16 and 17» Most prominent is the adductor superficialis, which in this case inserts only on the spiny ray. It originates partly on the fascia at the midline of the girdle and partly on the mid- dorsal surface of the pelvic bone and inserts along the dorsal surface of the shaft of the spine. This muscle draws the distal end of the spine dorsally; along with this movement, the proximal end of the spine on the ventral side of the disc moves slightly ventrally. The adductor profundus consists of five separate bundles in• serting on the five soft rays. The bundle to the first ray origi• nates partly on the dorsal surface of the pelvis deep to the adductor superficialis and partly on the dorsal process of the pelvis; the remaining bundles originate only on the dorsal process. -40- Fig. 16. Dorsal musculature of the adhesive disc of Eumicrotremus orbis, doraal view. XLO -41- Fig. 17. Deeper view of the dorsal musculature of the adhesive disc of Eumlcrotremus orbis, dorsal view. X10. -1*2- Each bundle inserts on the dorsal surface of the shaft of the corresponding ray. The distal ends of the rays are drawn dorsally and the proximal ends ventrally by the contraction of the adductor profundus. The bundle of the adductor profundus to the fifth ray may, in fact, be the extensor proprius; but since its points of attachment and .function are so similar to those of the other bundles, it will be considered here as part of the adductor profundus complex. Two other muscles are attached to the dorsal process of the pelvis. The first, tentatively called protractor ischii II, origi• nates on the underside of the pterotic of the skull and inserts on the dorsal process by a tough tendon. It appears to function in keeping the dorsal process erect during the contraction of the adductor profundus muscles„ The other muscle originates partly on the tendon of the protractor ischii II and partly on the tip of the dorsal process, and inserts on the posteroventral surface of the lower pharyngeal bone. It is a retractor of the lower pharyn• geal bone and does not appear to function in the adhesive process. The protractor ischii I is only partly visible from the dor• sal view. It originates partly on the cartilage between the two external processes and partly on the cleithrum of the pectoral girdle; its insertion is partly on the base of the external pro• cess and partly on the anterodorsal surface of the internal pro• cess. It holds the pelvis to the cleithrum and also serves to lift the anterior margin of the disc which is attached to the belly of the muscle by connective tissue. -43- The retractor ischii originates on a strong tendon attached to the posterior end of the pelvis. It is continuous with the abdominal and trunk muscles. The muscles of the ventral surface are shown in Figures 18 and 19. The abductor superficialis is a wide but short muscle originating along the midventral line of the pelvic girdle. Five short branches insert on the tips of the proximal ends of the five rays. This muscle draws the proximal ends of the five rays anterodorsally, causing the distal ends to move posteriorly and ventrally, in effect spreading the distal ends apart and pressing them onto the substrate. The abductor profundus occurs in two distinct bundles. The first bundle inserts on the apices of the angles of the first and second soft rays and originates on the cartilage between the ex• ternal processes dorsal to the point of origin of the protractor ischii I. The other bundle originates on the posteroventral sur• face of the internal process and inserts on the apices of the angles of the third, fourth and fifth rays. The two bundles draw the apices of the rays dorsally, causing the distal ends of the rays to move ventrally. The arrector ventralis originates near the tip of the lateral surface of the internal process and inserts on the anteroventral surface of the distal end of the pelvic spine. The arrector dorsalis also originates on the internal process but at a point proximal to the origin of the arrector ventralis. Its insertion -44- protractor ischii I abductor profundus Fig. 18 Ventral musculature of the adhesive disc of Eumicrotremus orbis, ventral view. XLO. -45- protractor ischii I abductor profundus abductor superficialis Fig. 19. Ventral musculature of the adhesive disc of Eunicrotremus orbis with pelvic rays removed, ventral view. X10. -46- is also proximal to that of the arrector ventralis on the antero- ventral surface of the distal end of the spine. These two muscles together draw the spine anteriorly and slightly ventrally to spread the rays apart. Action of the Muscles During the Adhering and Releasing Processes While the lumpsucker is swimming the ventral rays of the pectoral fins are kept abducted under the belly, streamlining this part of the body to some extent by covering the anterior half of the disc. As the animal settles to the bottom the pectoral rays are abducted and placed flat on the substrate around the anterolateral parts of the disc. The muscles of the ventral side of the disc are mainly in• volved in the adhering process, while the muscles of the dorsal side control the releasing process. Immediately before the lump- sucker settles, however, the adductor muscles of the dorsal side of the disc are contracted to draw the distal ends of the pelvic and rays (and thus the disc margin) up, and the proximal ends (thus the inner part of the disc) down, making the disc nearly flat, while at the same time the arrector muscles pull the distal end of the spine forward to spread the rays apart. After the disc contacts the substrate, these muscles relax, and as the disc tends to return to its original, slightly cupped form, the suction results. To increase the suction, the muscles of the ventral side are contracted. The abductor muscles pull the inner part of the -47- disc up and the distal ends of the rays down, in effect increasing the volume within the chamber of the disc while pressing the papil• lae on the margin of the disc firmer against the substrate. In the releasing process the abductor muscles of the dorsal side contract, raising the disc margin and papillae off the sub• strate and lowering the inner part of the disc. The latter action decreases the volume (and thus the partial vacuum) within the chamber of the disc while the former breaks the seal around the chamber. The suction ceases and the fish is now free to swim off. Myology of the Disc of G. maeandricus Description of the Muscles of the Adhesive Disc The muscles of the dorsal surface of the disc are shown in Figures 20, 21, and 22. The adductor superficialis complex is represented by five separate muscle bundles. Two large bundles, the adductor superficialis I and II, originate on the dorsal sur• face of the pelvis and insert by tendon on the posterodorsal sur- of the pelvic spine. These muscles pull the spine posteriorly and dorsally. The next two bundles, the adductor superficialis III and IV, originate along a diagonal ridge on the dorsal sur• face of the pelvis and insert by tendon on the posterodorsally directed processes at the proximal ends of the first and second rays. Adductor superficialis V originates along the same ridge but deep to adductor superficialis III and inserts by tendon on the posterodorsal corner of the base of the third ray. These s I Fig. 20. Dorsal musculature of the adhesive disc of Gobiesox maeandricus, dorsal view. X8. Fig. 21. Deeper view of the dorsal musculature of the adhesive disc of Gobiesox maeandricus, dorsal view. X8. Fig. 22. Deepest dorsal musculature of the adhesive disc of Gobiesox maeandricus, dorsal view. X8. -51- three muscles draw the corresponding rays posteriorly and dorsally. The adductor profundus is a single muscle originating on the posterodorsal surface of the internal process of the pelvis; it passes deep to the muscles of the adductor superficialis complex to insert by tendon on the dorsal surface of the base of the third ray. It draws the ray dorsally and slightly anteriorly. The extensor proprius originates along the ridge on the dor• sal surface of the pelvis anterior to the origin of the adductor superficialis III. It passes posterolaterally deep to all the other muscles to insert on the shaft of the outer lepidotrich of the fourth ray. Contraction of this muscle pulls the fourth ray anteriorly and laterally off the side of the body. A long, anterior branch of the trunk musculature originates on the posterolateral corner of the pelvis and passes posteriorly dorsal to the posterior part of the disc. Another muscle, the retractor ischii, is continuous with the lateral abdominal muscu• lature and originates by a strong tendon on the posterior tip of the pelvic bones. These two muscles have no apparent function in the adhesive process. The posterior part of the disc contains several large muscles. The muscle referred to here as the protractor of the postcleithrum originates on the dorsal surface of the posterior part of the pelvis and inserts on the dorsal surface of the medial end of the distal postcleithrum. It draws the medial end of this bone anteriorly. -52- A narrow band of muscle, which appears to be a branch of the lateral body musculature, inserts alongside the above muscle on the dorsal surface of the distal postcleithrum. This muscle serves as a retractor of the postcleithrum and draws the medial end of the bone posterolaterally and dorsally. Two large muscles, referred to as the adductors (I and Ii) of the postcleithra, fill the space between the anterior and posterior halves of the disc. These muscles are best seen from the ventral aspect (Fig. 23 & 24). The adductor I originates on the ventral surface of the posterior part of the pelvis and in• serts largely on the skin of the inner chamber of the disc and partly on the dorsal surface of the lateral end of the distal postcleithrum. The skin of the inner chamber attaches laterally to the ventral edge of the proximal postcleithrum, and a strong ligament from the anterior tip of the ventral edge of this bone passes to the shaft of the fourth pelvic ray. As a result of these various ligamentous connections, the contraction of the adductor I not only lifts the skin of the disc chamber and draws the proximal and distal postcleithra medially, but it also draws the fourth pelvic ray medially, antagonizing the action of the extensor proprius. By this action the fourth ray presses the ventral part of the pectoral fin against the axial flap, thereby closing the gap between the anterior and posterior halves of the disc. The adductor II originates partly on the ventral surface of the posterior tip of the pelvis and partly on the tendon of -53- the retractor ischii and inserts on the dorsal surface of the lateral half of the distal postcleithrum. It draws the distal postcleithrum medially. A narrow band of muscle referred to as the levator of the postcleithrum originates on the ventromedial surface of the cora- coid and passes over the proximal postcleithrum to insert on the dorsal surface of the lateral end of the distal postcleithrum. It draws the distal postcleithrum dorsally. The musculature of the ventral side of the disc is shown in Figures 23 and 2k. The arrector dorsalis lies under the anterior margin of the disc. It originates on the fascia at the midline and is firmly attached to the overlying anterior margin. It inserts by three branches on the spine and on the first and second rays. The action of this muscle pulls the spine and the two rays anteriorly and ventrally. The arrector ventralis originates on the arch of the external process over the anterolateral fenestra and inserts by tendon on the posteroventrally directed process at the base of the pelvic spine. It draws the spine ventrally. The abductor superficialis originates along the mid-ventral line and inserts by tendon on the posteroventrally directed pro• cess of the third ray. It draws the third ray posteriorly and ventrally. The abductor profundus complex occurs as three separate Fig, 23. Ventral musculature of the adhesive disc of Gobiesox rcaeandricus, ventral view. X8. Fig. 24. Deeper view of the ventral musculature of the adhesive disc of Gobiesox maeandricus, ventral view. X8. -56- muscles. The abductor profundus I is partly covered by the arrector ventralis and originates on the arches over the antero• lateral and central fenestrae. It inserts on the base of the third ray and draws the ray ventrally. The abductor profundus II lies deep to the previous muscle and originates on the ventral surface of the internal process of the pelvis. It then passes through the central fenestra and inserts on the base of the third ray deep to the insertion of the abductor profundus I; a small branch from this muscle also inserts by tendon on the anterior corner of the base of the second ray. The two rays are drawn ventrally by this muscle. The abductor profundus III originates on the ventral surface of the internal process adjacent to the origin of the abductor profundus II and passes through the anterolateral fenestra to insert by tendon partly on the base of the pelvic spine and partly on a nodule of cartilage between the bifid bases of the first and second rays. The contraction of this muscle not only draws the spine ventrally but also moves the articulation points of the two rays ventrally. Action of the Muscles During the Adhering and Releasing Processes For the inner chamber to become completely enclosed by the margin, the gaps in the margin between the anterior and posterior halves of the disc must be closed. This closure is brought about -57- by the adduction of the ventral rays of the pectoral fin as well as by adduction of the fourth pelvic ray, which, as previously described, is carried out by the contraction of the adductor I of the postcleithrum. These actions hold the ventral rays of the pectoral fin tightly against the axial flap, thereby closing the gap and completely enclosing the chamber; the adhesive process can then proceed. The arrector dorsalis pulls the spine and first two rays anteroventrally to spread the disc margin while pressing it against the substrate at the same time. The arrector ventralis also draws the spine ventrally while the muscles of the abductor groups draw the second and third rays ventrally. As previously mentioned, the action of the abductor muscles, and to some extent the arrector dorsalis, pulls the ventral lepidotrichs of the fin rays medially, sliding them along the shafts of the dorsal lepido• trichs, except at the very distal tips where the halves are fused by the actinotrichs. By this process the rays become bent ventrally thereby pressing the disc margin even firmer against the substrate. The termination of the suction is mainly the function of the adductor muscles and the extensor proprius muscle of the dorsal side of the disc. The adductor muscles draw the spine and first three rays dorsally off the substrate; in the same manner that the sliding of the ventral lepidotrichs causes the rays to bend ventrally during the adhering process, the action of the adductor muscles during the releasing process causes the dorsal lepidotrichs -58- to slide over the ventral members, thereby bending the rays dorsally and off the substrate. The extensor proprius pulls the lateral lepidotrich of the fourth ray medially, bending and draw• ing the ray laterally off the ventral part of the pectoral fin. Following this action, the abduction of the pectoral fin pulls the fin off the axial dermal flap, thereby opening the gap between the two halves of the disc; with this opening, the adhesion terminates. Maximum Adhesive Strength of the Disc The maximum adhesive strength of the disc was determined for 46 speci• mens of lumpsuckers and 41 clingfish (Fig. 25). Although the mean sizes of the discs of the two species greatly differ, where the sizes do overlap, there is a distinct difference in the weights supported by the discs. An analysis of covariance test shows a highly significant difference between the slopes of the two curves (F = 11.53; f = 1,83; P<0.0l). The mean values of the maximum adhesive strength expressed in terms of maximum weight supported per unit area of disc are 843.6 g/cm^ for E. orbis and 453• 8 g/cm*- for G. maeandricus. Within each species there appears to be a slight sexual difference in the adhesive strength. TJI both species the females are able to support more weight per unit area of disc than the males. In E. orbis the mean weight supported by eight females was 694.5 g/cm^, while the mean of eight males was 626.2 g/cm^. In the clingfish the mean values were 490.5 g/cm^ and 421.2 g/cm^ for 20 females and 20 males, respectively. -59- Fig. 25o Relationships between disc size and maximum adhesive strength of the discs of Eumicrotremus orbis and Gobiesox maeandricus. -60- Hydrostatic Pressure Within the Disc in Relation to Weight Supported Forty-two determinations, using ten specimens of E. orbis (Fig, 26), and 101 determinations, using eleven individuals of G. maeandricus (Fig. 27), were made to determine the relationship of hydrostatic pressure within the chamber of the disc to weight supported by the disc. The amount of weight applied was expressed in terms of grams per unit area of disc in order to eliminate the effects of size differences. At absolute vacuum, or — 760 mm Hg, at which point further suction is no longer possible, the disc should be supporting the maximum weight possible under the given conditions. The value of the weight supported per unit area of disc at absolute vacuum is about 660 g/cm2 for E. orbis (see Fig. 26), somewhat less than the mean value of 843 g/cm2 for the maximum adhesive strength determined in the previous section. For G„ maeandricus this value is approximately 450 g/cm2 (see Fig. 27), nearly equivalent to the mean value of 453«8 g/cm2 previously reported for the maximum adhesive strength. Function of the Adhesive Disc in Relation to Current Behavior of the Animal in Still and Flowing Water In the current chamber with no current, of a total of 48 minutes of observation time, the four specimens of E. orbis spent 9.0 minutes, or approximately 18.7% of the time swimming; the remainder of the time (39.0 minutes, or 81.3%) the specimens were attached to the bottom plate. With a current of 17.0 cm/sec passing through the chamber, during another 48 minutes of observation, the amount of time the speci• mens were observed swimming decreased to only 5«1 minutes, or 10.6% (42.9 minutes, or 89.4% of the time attached). -61- W EIGHT SUPPORTED / AREA OF DISC (g/cm2) Pig. 26. Relationship between the negative pressure within the disc and weight supported per unit area of disc in Eumicrotremus orbis. -62- 800 H WEIGHT SUPPORTED / AREA OF DISC (g/cm2) Fig. 27 • Relationship between the negative pressure within the disc and weight supported per unit area of disc in Gobiesox maeandricus. -63- In regards to orientation, when attached in still water the ani• mals showed a slight tendency to face towards the rear of the chamber (Table i), but this tendency was by no means clear-cut. With the cur• rent passing through the chamber, the tendency was now for the speci• mens to face either directly into or directly away from the direction of the current, but still the tendency was not overwhelming. In still water the swimming fish spent most of the time traversing the length of the chamber with its snout in contact with the wall of the cylinder, facing either perpendicularly or obliquely into the wall. However, when swimming in the current the specimens spent the greatest amount of the time facing directly into the current, as one would normally expect. In the holding tank the lumpsuckers were observed most of the time attached to the walls or to the rocks at the bottom of the tank. Swim• ming was usually confined to brief periods or when food material was thrown into the tank. The specimens of G. maeandricus kept in the holding tank were almost never seen swimming off the substrate. Only when specimens attached to the sides near the surface were disturbed or when the clingfish was in pursuit of live food-fish, was actual swimming observed. Most of the time the clingfish would remain concealed under the rocks at the bottom of the tank. In a strong current of over 20 cm/sec in the current chamber, the attached clingfish would usually remain facing in the same direction it had been facing prior to the start of the cur• rent; but in weaker currents the clingfish would eventually slowly turn to face directly into the current, Table I. Orientation of Eumicrotremus orbis while attached and while swinging in still water and in a current. The values repre• sent the amount of tine (in minutes) the four specimens were observed facing in each direction. The values in parentheses represent the i» of total minutes in each row. "Forward" refers to facing "into" the current. Orientation in Relation to Long Axis of Chamber Total Oblique Oblique Time Forward Forvard Sideward Backward Backward Fish 6.0 5.2 7.1 10.4 10.3 39.0 Attached (15.4) (13.3) (18.2) (26.7) (26.4) No Current Fish 1.4 2.0 3.2 2.0 0.4 9.0 Swimming (15.*) (23.0) (35.2) (22.0) (4.4) Fish 12.5 6.2 6.2 9.6 42.9 Attached (29.1) (1*.5) (1M) (19.6) (22.4) Current of 17.0 cm/sec Fish 3.7 0.7 0.6 0.1 0 5.1 Svirvsing (72.5) (13.7) (11.8) (2.0) (0) -65 Body Drag in Relation to Current The body drag of a fish in various current velocities was measured in grams. So that the values from one fish could be compared with those of other fishes of different sizes, the body drag was divided by the area of the disc of the given specimen. A total of 24 determinations using four lumpsuckers and 21 determinations from four Gobiesox speci• mens were made. The current was varied between 2.4 to 10.4 cm/sec. Although having a smaller body size, the specimens of E. orbis yielded higher values of body drag per unit disc area than G. maeandricus (Fig. 28). This difference is undoubtedly due to the globular, unstream- lined body shape of E. orbis as compared with the flattened, streamlined body form of the clingfish. Variation in the values relating to size difference within the species was not apparent. Hydrostatic Pressure Within the Disc in Relation to Current The suction created within the disc chamber with the attached fish facing currents ranging from 5 to 58 cm/sec was determined for two lumpsucker and six clingfish specimens. To show the actual force of the water acting on the animal, the current velocity values were con• verted to body drag per unit area of disc using the regression relation• ship between these two measurements (see Fig. 28). Figures 29 and 30 show the relationships of body drag per unit area of disc to pressure within the disc in E. orbis and G. maeandricus, respectively. In each figure an extension of the regression lines from Figures 26 and 27 was drawn to permit a comparison between the effects of weight supported and body drag on the pressure within the disc. -66- CURRENT (cm/sec) Fig. 28. Relationships between the body drag per unit area of disc and current in Eumicrotremus orbis and Gobiesox maeandricus. IE E E u rr I/) to Ixl or a. u > < o 111 T 1 1—r 0.6 0.8 1.0 BODY DRAG / AREA OF DISC (g/cm2) Fig, 29, Relationship between the negative pressure within the disc and body drag per unit area of disc in Eumicrotremus orbis. -68- BODY DRAG / AREA OF DISC (g/cm2) Fig. 30. Relationship between the negative pressure within the disc and body drag per unit area of disc in Gobiesox maeandricus. -69- In E. orbis the negative pressure values start below the extended regression line when the force of body drag is low and reach the line when the force increases to 1.0 g/cm2 (approximately 30 cm/sec current). In the clingfish, except for a few points at very slow current, the negative pressure values are generally below the extended regression line. Structures Associated With the Function of the Adhesive Disc of E. orbis Muscles Maximum Adhesive Strength of the Disc of Freshly Killed Specimens The adhesive discs of the dead specimens were still functional but the maximum adhesive strength of these was only about 77?° of that of live fish (646.3 g/om2 for dead fish and 84-3.6 g/cm2 for live fish). Inhibition of Nervous Control of the Disc The specimens in which spinal nerves 1-4 had been successfully severed were still able to adhere to a flat surface. The pectoral fins in these specimens were immobilized but swimming was main• tained by the dorsal, anal and caudal fins. One striking effect the treatment had on the function of the adhesive disc was in the difficulty exhibited by the specimens in detaching themselves from the substrate. In apparent attempts to detach, the specimens vigorously undulated the dorsal, anal and -70- caudal fins and often rotated around an axis passing through the center of the disc before successfully detaching. The mean value of the maximum weight supported per unit area of disc for the treated fish was 633 *k g/cm2. The control specimens appeared normal in their ability to adhere and detach. Pectoral Pins The pectoral fins of E. orbis contain 22 to 2k rays, of which eight to ten of the ventralmost lie adjacent to the adhesive disc. Removal of varying numbers of these ventralmost rays did not disrupt the adhering ability of the disc, although the maximum adhesive strength was affected somewhat (Table II), Structures Associated With the Function of the Adhesive Disc of G. maeandricus Muscles Maximum Adhesive Strength of the Disc of Freshly Killed Specimens The mean value of the maximum adhesive strength of eleven dead specimens tested was k36.k g/cm2. The value is only slightly less than the mean of the live specimens tested (453•8 g/cm2). -71- Table II. Summary of the effects of the removal of varying numbers of the ventralmost rays of the pectoral fins on the adhesive strength of the disc of Eumicrotremus orbis. No. of Ventralmost Mean of Pectoral Rays Clipped No. Tested Weight Supported/Area of Disc Normal Fish ' 46 843.6 5 2 682.3 6 2 537.6 7 3 467.4 8 2 587.6 9 2 757.7 10 1 780.2 12 1 631.1 ' -72- Inhibition of Nervous Control of the Disc Two specimens in which spinal nerves 3 and' k had been severed were still able to use their pectoral fins. The use of the adhe• sive disc in these specimens also appeared unaltered with the fish still being able to attach and release themselves off the sub• strate as in the normal and control specimens. In the two other specimens in which both pectoral and pelvic fins had been immobilized by severing spinal nerves 1-4, the func• tioning of the adhesive disc was greatly affected. The clingfish, in this case, were able to adhere to and slide over the smooth glass substrate, but they were unable to detach and lift them• selves off the substrate. This was demonstrated by placing a rod across the bottom of the aquarium. The specimens with only the pelvic fins immobilized were able to detach themselves and swim over the low barrier, but the specimens with both pectoral and pelvic fins paralyzed managed only to get their heads over the rod while struggling vigorously and unsuccessfully to detach them• selves from the substrate. When a flat rock was used as the substrate, the specimens with both paired fins nonfunctional were able to adhere but now they were unable to slide over the substrate as well as being unable to detach themselves from the substrate. The specimens with only the pelvic fins immobilized were able to detach and swim away as did the controls. -73- Pectoral Fins The five ventralmost rays of the pectoral fins are involved in closing the gap between the anterior and posterior halves of the disc during the adhering process. A membrane from the fourth pelvic ray attaches to the base of the dorsal side of the fourth pectoral ray, and another membrane from the posterior half of the disc (from the anterolateral end of the distal postcleithrum) attaches to the base of the ventral side of the fifth pectoral ray. Specimens, in which four or five of the ventralmost rays had been cut off at the base, were unable to adhere. However, adhesion was possible by specimens in which only two or three of the ventralmost rays had been removed. The mean weight supported by three specimens was less than that of the normal fish (Table III). Axial Flap No marked effects in adhering ability were apparent in specimens in which the axial flaps were cut off at' the base. The mean maximum weight supported by the four specimens tested was only slightly less than that of the normal fish (Table III). -74- Table III. Summary of the effects of the removal of various structures on the adhesive strength of the disc of Gobiesox maeandricus. Mean of Treatment No. Tested Weight Supported/Area of Disc Normal Fish 41 453.8 2 ventralmost pectoral rays clipped 1 3^1.0 3 ventralmost pectoral 1 305.6 rays clipped 4 ventralmost pectoral 5 unable to adhere rays clipped 5 ventralmost pectoral unable to adhere rays clipped Axial flap removed ^37.9 -75- DISCUSSION Information on the functional efficiencies of the adhesive discs of the Cyclopteridae and Gobiesocidae is almost completely lacking. The only study reported in the literature was presented by Borckert (I889) on Cyclopterus lumpus, in which he determined the maximum adhesive strengths of four specimens. According to the data Borckert provided, the four speci• mens were able to support, on the average, 857-5 grams per square centimeter of disc area. This value is slightly higher than that (843.6 g/cm2) deter• mined in the present study in Eumicrotremus orbis. Concerning the Gociesoc- idae, Guitel (1889) described the actions of the parts of the disc of Lepadogaster during the adhesive process but otherwise gave no information on the efficiency of the disc. Similarly, a comparative study of the morphology and function of the discs of these two families has never been previously undertaken. Bertin (1958), in a general treatise on the modifications of the fins, presented brief descriptions of the ventral adhesive disc of various families, in• cluding the two considered here; however, he did not attempt to compare the differences between the discs of the various groups. The results obtained in this present study indicate that the lumpsucker possesses a more efficient adhesive disc than the clingfish. This was shown by the greater weight per unit area of disc supported by the lumpsucker, which, on the average, was able to support a weight of 1.8 times greater than could _G. maeandricus. That this difference in adhesive strength is not directly due to the difference in the range of the disc size between -76- th e samples of the two species can be demonstrated by a comparison of the adhesive strengths of specimens having similar disc sizes, as well as by a comparison of the regression line for each species (see Fig, 25 )» A con• sideration of the morphological and anatomical differences between the two types of discs, therefore, should account for the difference in efficiency. The disc of E. orbis is a single, compact structure made of firm con• nective tissue and supported by ossified fin rays. These rays serve as ribs, which reinforce not only the rim but also the roof of the inner chamber of the disc, enabling these parts to withstand great stresses with• out collapsing. The rays are curved and arranged in such a manner that when the abductor muscles pull on the proximal ends of the rays to raise the roof and increase the volume of the chamber of the disc during the adhesive process, the distal ends become simultaneously drawn down to press the rim of the disc against the substrate. This association between the movements of the roof and rim of the disc ensures that any increase in suction be accompanied by a tightening of the seal around the disc. The disc of G. maeandricus. on the other hand, is complex in structure and not as compact and firm as but somewhat more flexible than the disc of E, orbis. The pelvic girdle, and thus the disc, appears to have been ex• panded laterally along with the head and abdomen in accommodation of the habits of the clingfish. The pelvic rays which support the lateral walls of the disc are highly flexible and loosely placed on the lateral edges of the pelvic bones, and the ball-and-socket joint of the modified spine and the loose articulation of the first two rays on the pelvis permit these elements to swing ventrally across a wide arc. Such a motion, in effect, -77- increases the height of the disc when the body of the fish is pulled upwards (e.g., by a very strong current or by an experimenter), making the walls more liable to succumb to the forces of the hydrostatic pressure acting on them. The presence of the lateral clefts between the anterior and posterior halves of the disc may also weaken the Gobiesox disc. The membrane which closes these gaps is not exceedingly heavy, and it is possible that this membrane could yield if the pressure differential on either side of it be• came too great; however, in view of the fact that the surface area of the parts of the membrane which come under stress while keeping the gaps closed is so small, it is more likely that the lateral walls of the anterior half of the disc would collapse before any damage is done to the membrane. Similarly the disc of E. orbis is not without imperfections. Although the pelvic rays efficiently reinforce most parts of the disc, there is no such internal support for the anterior and anterolateral sections of the rim. The anterolateral section is also narrower than the other parts of the rim, and as a result, appears to be the weakest part of the disc. To overcome this apparent weakness, this part of the rim is reinforced ex• ternally to a large extent by the ventral rays of the pectoral fin. The value of the pectoral fin in reinforcing the anterolateral section of the rim was demonstrated by the removal of varying numbers of the ventral rays of the pectoral fin. Such treated specimens were still able to adhere strongly, indicating that the disc can function independent of the pectoral fins (Table Ii). However, the maximum adhesive strength of these specimens showed a decrease of about 28%, which showed that the ventralmost pectoral -78- rays play a role in increasing the adhesive ability of the disc. Indeed, it was noted that as each of these treated specimens was being pulled off the substrate, it was the anterolateral section of the rim that was the first to collapse inward. In the clingfish the pectoral fins play an entirely different and more intimate role in the adhesive function of the disc. The membraneous flaps from the anterior and posterior halves of the disc attach onto the fourth and fifth ventralmost rays of the pectoral fin, respectively. The signifi• cance of the pectoral fins and the flaps in the closure of the gaps between the anterior and posterior halves of the disc during the adhesive process was conclusively demonstrated by the inability of the specimens to adhere after the four ventralmost pectoral rays were clipped. Removal of the first three rays affected the adhesive strength somewhat, but not as dras• tically. The removal of the axial dermal flap also lessened the adhesive strength, but again the effect was not a total disruption of the adhesive ability. In this case, apparently the pectoral fin was pressed directly against the side of the body to bring about the closure of the gap between the anterior and posterior halves of the disc. The role of the pectoral fins during the detaching process in the clingfish was also clearly demonstrated. Specimens in which the pectoral fins were immobilized by severing spinal nerves 1 and 2, which innervate these fins, were unable to detach themselves after adhering to the sub• strate. Other specimens in which only the spinal nerves innervating the pelvic fins had been cut (spinal nerves 3 and k) were still able to adhere and detach as in a normal fish. This clearly showed that the detaching -79- process is actively controlled by the movements of the pectoral fins. Under normal conditions in still water a resting fish need not expend much energy to maintain its attached position. The cup-like, completely enclosed nature of the disc permits a strong suction to be created even without the actions of the muscles; this passive adhering ability of the disc was demonstrated in the determination of the adhesive strength of dead fish of both species. In the normal fish it seems that some muscle action is employed at the onset of the adhering process to flatten the disc just before contact is made with the substrate; but most of the energy apparently is expended during the releasing process when the muscles must work to overcome the negative pressure within the chamber of the disc in order to break the seal around the chamber. The use of the muscles in the releasing process was demonstrated in the clingfish, as discussed above, and in the lumpsucker, in which the nerves to the disc were severed; such treated specimens, when attached, were not able to release themselves promptly but had to undulate the body vigorously before finally detaching. Only when in a strong current (or when an experimenter tries to pull the fish off the substrate) must the lumpsucker or clingfish vigorously con• tract its ventral muscles to increase the suction. The significance of this feature of the disc is quite apparent. As was demonstrated in the be• havioral study, the lumpsucker spends the greater part of its time attached to the substrate, while the clingfish almost exclusively remains attached. It would therefore be most advantageous for the species if the adhering apparatus permitted this constant adhesion with the minimum expenditure of energy. -80- The tests with the dead specimens showed a greater decrease in the ad• hesive strength in the lumpsucker than in the clingfish when compared with the live specimens. This indicates that the use of the muscles in the ad• hesive process is more extensive in the former species than in the latter. However the adhesive strength of the dead lumpsucker is still very high, far greater than that of the dead clingfish and even higher than that of the live clingfish. This suggests that although the mere physical structure of the lumpsucker disc accounts for most of the adhesive capability, the action of the muscles is significant in even further increasing the adhesive strength. Similarly in the clingfish, the physical nature of the disc pro• vides for most of the adhesive strength, but in this case the action of the muscles is not as impressive in promoting additional strength. Thus, in addition to the physical features of the lumpsucker disc previously dis• cussed, the more effective use of the muscles in increasing the adhesive strength of the disc presents another feature to account for the greater efficiency of the disc of the lumpsucker. Before the fish settles on the substrate, the disc is made flat so that the volume of the chamber of the disc can be kept minimal. As the disc contacts the substrate the muscles are relaxed and because of the elastic nature of the connective tissues making up the disc, the disc tends to re• turn to its normal, cup-shaped configuration, resulting in the creation of a partial vacuum within the chamber of the disc. If the attached animal is then pulled upwards, in a flexible disc the roof of the chamber will'tend to bulge upwards to a greater extent, bringing about a greater increase in the volume of the chamber. If the seal around the chamber is watertight, -81- this increase in volume will result in a corresponding increase in the partial vacuum within the chamber of the disc. On the other hand, in a disc with a more rigid support, the bulge in the roof of the chamber would not be as great and the increase in volume, and thus the development of the vacuum, should be equally less. In Gobiesox. the negative pressure devel• oped within the chamber of the disc to resist a given upward force is approximately 1.4 times higher than that developed in the lumpsucker disc resisting an equivalent force. This difference indicates that the cling• fish disc has a greater tendency to bulge upwards, thereby increasing the volume within the disc and creating a higher vacuum within the chamber of the disc. Furthermore, as the roof of the chamber is drawn upwards, the walls of the disc are simultaneously drawn inwards, whereupon the surface area of the disc over which the suction acts becomes decreased, thus re• ducing the effectiveness of the disc. As was mentioned earlier this upward bulging also makes the walls of the disc more liable to collapse. The disc of E. orbis, on the other hand, is better able to maintain its original shape because of the support of the roof of the disc chamber by the medial extension of the proximal ends of the pelvic rays. The bulging of the roof of the chamber and the corresponding inward drawing of the rim of the disc, therefore, do not occur as readily when the fish is pulled upward as is the case with the disc of G. maeandricus. Thus the disc of the lumpsucker can maintain its high efficiency while being subjected to a greater range of the lifting force. The watertight seal formed by the rows of papillae and the adjoining membrane appears to be equally efficient in both species as was indicated by the high negative pressures that were recorded within the disc of the two species. -82- Water currents have a greater effect on the lumpsucker than on the clingfish. The presence of dermal tubercles and the globular body form of the lumpsucker result in the creation of a higher body resistance, or drag, in this species than in the clingfish with its flattened and streamlined body form. Consequently in a current the force which the attached lump• sucker must overcome to maintain its position, and thus the suction it must produce to counteract this force, is higher than that confronted by the clingfish. These differences in the body drag and in the resulting suction needed to overcome the drag were clearly shown (cf., Fig. 28, 29 and 30). However in a current much of the force acting on the body is applied hori• zontally and only a portion goes vertically; therefore, the negative pres• sure needed to overcome the total force of the current should not be as high as that needed to overcome the upward force. For this reason the nega-- tive pressure recorded within the disc of the specimen in the current should be somewhat lower than the lines projected from Figures 26 and 2? and drawn in Figures 29 and 30. On the other hand the body drag of the attached fish might be somewhat higher than the values shown in Figure 28, since these values were determined from a fish suspended from its snout and free to swing about, whereas the fish in the first instance was firmly attached at the center of its ventral surface. Also in both species, in higher currents the head was forced upwards by the force of the current; this invariably increased the drag as well as the proportion of the force applied vertically on the disc For these reasons in the graph of E. orbis (Fig. 29)> some of the negative pressure readings at the higher currents are higher than the projected line. The same happens with G. maeandricus but in currents greater -83- than those indicated (Fig. 30). When attached in a current, both species, to a certain degree,, attempt to face directly into the current. However, such an orientation is appar• ently not too critical for the lumpsucker. As was shown in the behavioral study, this species spends nearly an equal amount of time facing in the opposite direction, and more than half as much time facing perpendicularly. This indicates that, at.least in the lumpsucker, there is not an absolute necessity in trying to attain the minimum body resistance by facing directly into the current; at any rate, as was earlier shown, the disc of the lump• sucker is strong enough to easily overcome whatever additional resistance which may occur while the. animal is oriented in another direction. The clingfish, on the other'hand, shows a greater tendency to orient towards the direction of the current and relies more heavily on its streamlined body form to withstand the. effects of the currents. Thus it seems that in G. maeandricus the development of the flattened, streamlined body form had reduced the necessity for a very efficient adhesive disc, or perhaps such a body form was developed at the expense of the efficiency of the disc, while in E. orbis selection proceeded in the opposite direction to favor a very efficient disc with less regard for streamlining. However, it should be noted that perhaps this loss or lack of efficiency in the disc of the clingfish may not be critical under natural conditions since the forces exerted in the determination of the maximum adhesive strength of the disc, as well as the currents to which the specimens were subjected in these experiments, were exceedingly high and probably never encountered by the animals in their natural -environments. -84- Except for the fact that male lumpsuckers tend to have slightly larger discs than the females (Fig. ll), no definite morphological or anatomical reasons have been found that could account for the sex differences in the adhesive ability of the disc. Similar sexual differences detected in the adhesive ability of G. maeandricus are also unexplainable; no differences in the size of the discs were detected between the sexes of this species. Perhaps the differences in the adhesive abilities can be related to differ• ences in the habits of the sexes of the two species. The males of G. maeandricus are responsible for guarding the nest after spawning (personal observation); brood care has not been observed in E. orbis, but it is assumed that this habit is also maintained by the males, as is the case with other cyclopterids such as Cyclopterus lumpus (Mcintosh, 1886) and Cyclopteropsis bergi (Tarasov, 1937» in Lindberg and Legeza, 1955s65)• It may be that the sexual difference in the strength of the disc is related to the brood care; however if this is the case, it would seen more reasonable for the opposite situation to be true—that the males have the stronger disc than the females, since, in addition to the preparation for spawning, which should more likely weaken the female, the conditions of the inshore areas where the eggs are laid would place a higher premium for a stronger disc in the parent that guards these eggs. Thus the answer cannot be found in the reproductive habits of the two species. Whatever the reason, it seems that, at least in the lumpsuckers, the males compensate for the lesser strength, if indeed the difference is real, by having slightly larger discs than the females. -85- The bending action of the fin-rays caused by the sliding of one lepido• trich over the other has not been previously reported in the literature. The actual distance one lepidotrich slides over the other is very slight; however in G. maeandricus, because of the closely segmented nature of each lepidotrich of the pelvicrays,the bending of the lepidotrichs resulting from this slight movement is greatly magnified. This mechanism, then, provides a subtle means of transmitting and magnifying the motion of the ray from the proximal end, where the action of the muscle is applied, to the shaft and distal end, where most of the opposing force is encountered, e„g., as in the pressing of the rim of the disc onto the substrate during the adhesive process. Bending of the pelvic rays does not occur in E. orbis since the rays in this species are almost entirely ossified. Examination of the fin-rays of other teleosts revealed that most rays are, in fact, able to slide and bend, suggesting that this feature may be an inherent property of all soft fin-rays. In the typical fin-ray, however, the segmentation is not,as close as it is with the pelvic rays of Gobiesox, and therefore, the bending of the lepidotrichs of the typical ray is not as pronounced. The value of this.sliding and bending property is quite obvious. In Gobiesox it increases the force with which the shafts and distal ends of the pelvic rays, and therefore the rim of the disc, can be applied to the sub• strate during the adhesion process, thus permitting a tighter seal to be formed around the inner.chamber of the disc. In other fishes the bending action curves the ray into the direction of the movement of the ray to counteract the opposing pressure of the surrounding water and to prevent -86- the distal end of the ray from being bent back in the opposite direction. An empirical demonstration of the significance of this sliding and bending property of the fin-rays, in the locomotive function of the fins of teleosts has not been made and could possibly be the object of some future research project. SUMMARY The ventral adhesive discs of the lumpsucker Eumicrotremus orbis and the clingfish Gobiesox maeandricus were compared in regards to structure and functional efficiency. The external features, osteology and myology of the adhesive discs were described and illustrated. Both discs have in common certain fundamental structures associated with the adhesive function. These include an inner chamber, the roof of which can be flattened or arched to vary the volume; an outer rim of coarse dermal papillae, which forms a water-tight seal around the inner chamber; and a thin, flap-like membrane around the rim to reinforce the seal. The lumpsucker disc is a single, compact structure with modified and almost completely ossified pelvic fin rays supporting the entire disc. The disc of the clingfish, on the other hand, is divided into two halves separated by two lateral clefts. The flexible pelvic rays are hinged on the lateral edges of the pelvis and support only the rim of the disc, making the disc less firm and the rim more susceptible to the inward force created during adhesion. The disc of E. orbis was found to have a greater adhesive strength than that of G. maeandricus. This difference was concluded to be the result of -87 the firmer and more compact nature of the lumpsucker disc. The creation of a negative pressure between the disc and the substrate during adhesion was empirically demonstrated in both species for the first time. The negative pressure produced in the clingfish disc was found to be higher than that of the lumpsucker. The more flexible nature of the Gobiesox disc permitting a greater tendency for the disc to bulge upwards to increase the volume of the disc chamber as the fish was being pulled up• wards accounts for this higher negative pressure. Tests with dead specimens showed that the mere physical construction of the disc accounts for most of the adhesive qualities in both species. However in the lumpsucker, muscle action furthers the adhesive ability to a greater degree than in the clingfish. This more effective use of the muscles represents another reason for the more efficient function of the disc of the lumpsucker. The pectoral fins were shown to play a greater role in the adhesive function of the disc in G. maeandricus than in E. orbis. Removal of the four ventralmost rays of the pectoral fins completely disrupted the adhe• sive capability of the disc of the clingfish, while the removal of up to 12 of the ventralmost pectoral rays only slightly reduced the adhesive strength of the disc of E. orbis. The role of the pectoral fins in the detaching process in G„ maeandricus was also demonstrated by disrupting the nervous supply to the fins. In a current, the body form of the lumpsucker created a greater re• sistance than did the streamlined body form of the clingfish. For this reason,.in a current, the lumpsucker must exert a greater suction to avoid ~88~ being swept away, and as a result a higher negative pressure was recorded under the disc of the lumpsucker., A slight sexual difference in the adhesive strength was detected in both species but no conclusive reasons could be presented to account for the difference. In relation to body forms, it was concluded that the development of an efficient adhesive disc in E. orbis would be a distinct advantage in view of the globular, unstreamlined body shape of this species; whereas in (J. maeandricus, with its flattened, streamlined body, such a highly efficient structure would not be of absolute necessity. A new mechanism, described as a sliding and bending property of the lepidotrichs of the fin-rays, was presented for the first time. In the function of the adhesive disc in Gf„ maeandricus, this mechanism provides for a firmer seal to be formed by the rim of the disc around the inner chamber. In the locomotive function of the fins in other teleosts, it was suggested that this mechanism may increase the firmness of the individual rays to prevent a backward bending caused by the resistance of the fluid medium, thereby increasing the efficiency of the fins. -89» LITERATURE CITED Bertin, L. 1958. Modifications des nageoires. Ins Traite de Zoologie. P.P. 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