The Morphology of Fimbria fimbriata (Linne) (Bivalvia Lucinidae)1 J. A. ALLEN and J. F. TURNER2

ABSTRACT : The morphology of the shell and soft parts of Fimbria fimbriata is described and compared with that of other members of the Lucinacea. Particular attention is given to the ligament and hinge of the shell, and to the tissues and organs involved in food collection, sorting and digestion.

IN EARLIER STUDIES . on the Lucinacea (Allen, ing anteriorly. The shell is white, becoming 1958), specimens of the genus Fimbria (= Car­ somewhat pink at either end where the radial bis) were unobtainable. Since then, through the ridges are flattened and form overlapping plates. kindness of Mme. A. Lapelerie, Magenta, Nou­ The shell is heavy and thick (approximately mea, a few preserved specimens have been ob­ 6 mm in thickness in a specimen 78 mm total tained. Therefore, the object of this study is to length) and is extremely large for the rather describe the morphology of this genus and make small body of the animal it contains (Fig. 1). comparisons with other members of the Lu­ The sculpturing of the shell is reticulate; the cinidae (Allen, 1958). concentric ridges are very pronounced but do The generic name Fimbria Megerle 1811 not always extend continuously round the whole takes precedence over Corbis Cuvier 1817, al­ valve. The radial ridges are most pronounced at though in past literature the latter is more either end, and in addition there are short radial commonly used. Fimbria fimbri ata (Linne) indentations between the concentric lines. occurs in coral sand in shallow water of tropical Anterior to the umbo there is a well-defined reefs in the south and central Pacific and north­ lunule which is brown in color, and posteriorly, eastern Australia (Allan, 1950; Ni col, 1950). less well defined, is a long oval escutcheon. Both The very beautiful, large white shell was known are covered with thickened periostracum. There to Victorian conchologists who described and is no posterior radial indentation that corre­ pictured it. However, most accounts appear to sponds in position with the gill axis, such as is have been taken from Reeve (1841) . Informa­ found in other species of the Lucinacea (Allen, tion on the soft parts is scant, and recent de­ 1958) . scriptions are transcripts from those of Cuvier The hinge plate bears two cardinal teeth be­ (181 7) and Reeve (1841). Thus Woodward low the umbo; in addition there are two lateral (18 81 ) records: "Mantle open below, doubly teeth which are widely separate from the car- fringed , siphonal opening single, gills single on each side, thick quadrangular, plaited, united STOMA CH DEMIBRANCH HEA RT behind." The only other studies since then are those of Lamy (1921) and Nicol (1950). Nicol gives details of its taxonomy, geographical dis­ tribution , and shell characters. At the same time, without giving any reasons, he erects a new family, the Fimbriidae.

OBS ERVATIONS Shell Characters The shell of Fimbria is typical of the Lu­ cinacea, being rounded with the umbones point- PALLIAL MANTLE INHALENT BLOOD V ESSELS EDGE APERTURE

1 Manuscript received July 17, 1969. FIG. 1. Fimbria fimbriata (7 cm total length) . 2 Dove Marine Laboratory , University of Newcastle Semidiagrammatic view of the arrangement of organs upon Tyne,Cullercoats, Northumberland, U.K. seen when the left valve is removed. 147 148 PACIFIC SCIENCE, Vol. 24, April 1970 dinals ( Fig. 2). The anterior lateral tooth is have now suggested, from their work on Dreis­ situated below the lunule, and the posterior sena, that the fusion layer reported for many lateral tooth is close to the posterior limit of the groups of bivalves may be an additional layer of hinge plate and posterior to the adductor scar. fused periostracum, which is the result of the Int ernally the valves are delicately colored union of the inner surfaces of the outer mantle pink and yellow-pink along the deeply crenel­ folds, and that union of the outer surface of the late ventral edge and yellow internal to the outer mantl e fold , which would result in the pallial line. The elongate anterior and more production of a fusion layer, has not occurred. rounded posterior adductor muscle scars are Certainly four layers can be distinguished in the large and white. Above the anterior adductor Lucinacea, and Fimb ria is no exception. These scar and merging with it can be seen the much are the inner and outer layers, the periostracum, smaller anterior retractor muscle scar. The pos­ and a layer which lies between the periostracum terior retractor muscle scar is discrete from the and the posterior layer and which would for­ posterior adductor muscle scar, and it lies dorsal merly have been regarded as the fusion layer. to it and underneath the posterior lateral tooth. This fourth layer is particularly thick in Fimb ria Joining the two adductor muscle scars there is a and secondarily extends the ligament posteriorly. white line marking the insertion of the pallial Transverse sections of the mantle tissue und er­ muscle, and interna l to this and diagonally lying the ligament and traversing the posterior across the shell is a paler line that corresponds part of the mantle isthmus and the fused mantle to the position of the large pallial blood vessel edges posterior to the isthmus show that in the which runs across the mantl e. region of the fourth layer there is a pair of The ligament is internal and opisthodetic grooves separated by a slender ridge of tissue in (Fig. 2). It corresponds to the condition in the sagittal plane (Fi g. 3A). This ridge extends most other lucinids and in the genus Codakia in to the posterior limit of the outer layer of the particular (Allen, 1960). In these earlier studies ligament. Below the outer layer, the secretory the ligament was shown to be composed of four epithelium has a single sagittal groove. Ex­ layers-inner and outer layers, fusion layer, and amined by light microscopy, the epithelial cells periostracum. Yonge and Campbell (1968) secreting the outer layer and the fourth layer are

AN TERIO R LATERA L TOOTH

FIG. 2. Fimbria fimb,oiata. Lateral view of ligament and hinge region of the left valve. Morphology of Fimbria fimbria/a- A LLEN AND T URNER 149

MANT L E GLA NDU LAR REGION I

PA LLI AL MUSCLE

BLOOD SPACE

INNE R PAPI LL A

PERIOSTRACAL GLAN DU LAR REG ION GROOVE OF M IDD L E FOLD

FIG. 3. Fimbria fimbriata. A (l eft) , Tr ansverse section throu gh the mantle below the fourth layer of the ligament and immediately posteri or to the prim ary ligament. For further details see text. B ( rigbt) , Trans­ verse section through the mantl e edge on the ventral side. indistinguishable from one another. Th ey are ing the ligament, and we suggest that the term tall narrow columnar cells with elongate central "fusion layer" be retained for this additi onal nuclei and with a granular cytoplasm. layer in Fimbria and in the Lucinacea generally. Above this epithelium are numerous spherules It should be noted that there is no extension of containing granules. The sections indicate that the pallial line from the posterior adductor these are cut off from the tips of the epithelial below this fourth layer. This further supports cells.In addition, in the region of the fourth the contention that this is fusion layer (Owen, layer, strands of a clear material can be seen 1958) . between the spheru les and originating from be­ The free mantle edge is broad and comprises tween the epithelial cells (Fig. 3A). Below the three folds. The outer fold is responsible for outer layer only spherules are present. Both secreting the outer layer of the shell, while the types of material must be in the process of being periostracum is secreted by the cells at the head added to the fourth layer of the ligamen t. This of the well-defined groove between the outer is unlike the typical sheet of periostracum pro­ fold and midd le sensory fold. The middl e fold duced, say, at the ventral edge of the mantle or has a frin ge of fine papillae along the entire that above the fou rth layer. The strands and length of its outer edge. To the inside of these spherules are produced not only in the depths papillae there is a second row of larger papillae of the grooves, but extensively by the general or short tentacles which are best developed in epithelium of the region (Fig. 3A) . It may be the region of the exhalent and inhalent aper­ that the material forming strands is homologous tures. Two such rows are found in many lu­ with periostracum. N everth eless, together with cinids (Allen, 1958) . The inner muscular lobe the spherules it does form a distinct and char­ is extremely well developed, although in the acteristic layer which has the function of extend - preserved specimens it has contracted g reatly. 150 PACIFIC SCIENCE, Vol. 24, April 1970

The histological details of the mantle edge are present in these two species and which give are similar to those of other members of the rise to the term "mantle gill." family (Fig. 3B). The main rejection tract In transverse sections the central area of the along the mantle edge is probably at the outer ridge in Fimbria appears as a network of spaces edge of the shelf formed by the inner boundary outlined by thin cellular walls (Fig. 4A). The of the inner mantle fold. Beneath the epithe­ diameter of the spaces is in the order of 0.2 mm lium of the shelf are numerous mucus-secreting and they extend along the length of the ridge. cells which stain heavily with Aluminium Meth­ The epithelial cells are ciliated, and below, ylene Blue. These cells probably facilitate the parallel to their relatively thick basement mem­ conveyance of rejected material to the posterior brane, there is a thin layer of muscle fibers. In part of the mantle cavity. Other mucus-secreting addition, a few transverse muscle fibers run cells are found below the epithelium of the across the ridge through the inner spongy lower part of the inner fold and also of the tissue. In the region of the groove and below middle fold. Internal to the inner fold there is the muscle layer there is a number of gland cells no additional fold forming a roof to the main with granular contents, from which ducts lead rejection tract, such as is found in Phacoides to the outside between the epithelial cells. In borealis and Codakia orbicularis (Allen, 1958). this respect Fimbria differs from other lucinids, The pallial musculature in Fimbria is very in which few gland cells have been recorded in well developed and muscles extend well into all the mantle gills (Pelseneer, 1911 ; Allen, 1958). three folds (Fig. 3B). Longitudinal fibers The function of the ridges in Fimb ria may be running the length of the mantle edge extend respiratory; they lie adjacent to the inhalent in a broad band at the outer edge of the well­ current, and the blood spaces are extensive and developed inner fold. At right angles and below in close connection with the pallial blood vessel. the longitudinal muscles, radial muscles extend However, their position at the base of the an­ from the pallial line to the three folds. Those to terior adductor muscle suggests that they may the outer arid middle fold run ventral to a large also act in the manner of guides and help in central blood space. In addition, transverse directing the inhalent current, together with its fibers from the outer epithelium and outer lobe contained food particles, around the ventral cross blood spaces and packing tissue between limit of the adductor muscle toward the mouth. the radial muscles to a position below the longi­ It is difficult to assign a function to the central tudinal fibers of the inner fold (Fig. 3B) . groove of the ridges. The extensive development of muscular tissue A pallial blood vessel extends from the ven­ and the high degree of contraction of the pre­ trallimit of the anterior adductor muscle across served specimens suggest that this animal is the mantle to join the axial vessel from the gills capable of maintaining long periods of closure. at a point where it enters the auricle. It is a A pair of elongate, wedge-shaped structures, large vessel, and from transverse sections it is which are extensions of the inner mantle, lie clear that its lumen has a definite epithelial adjacent to the right and left posteroventral lining (Fig. 4B) . Undoubtedly it acts as the margins of the anterior adductor muscle (Fig. main collecting vessel for the mantle haemocoele. 1). Each consists of a ridge of tissue with a In addition, there is a smaller vessel originating groove running the length of the free edge, the midventrally which connects with the main lips of the groove being somewhat convoluted. pallial vessel. Small numbers of muscle fibers Each ridge extends from the base of the ad­ are present along the outside of the wall of the ductor along the line of a pallial blood vessel main vessel, and it is possible that their contrac­ for about 10-15 mm. They are probably ho­ tion assists the passage of blood from the mologous to similar structures described for mantle to the auricles. Codakia orbicularis and Lacina pennsyloanica Apart from the region of the ligament, (Allen, 1958). Fimbria lacks the complex knot mantle fusion is minimal. As in other species of of tissue seen in Codakia orbicularis, nor does the Lucinidae, the whole outer face of the it develop a series of additional ridges, such as anterior adductor muscle is exposed and, pos- Morphology of Fimbria fimbriata-ALLEN AND TURN ER 151

A

MUSCLE FI BERS INNER MANTLE LUMEN OF BLOOD VESSEL EPITHELIUM

B FIG. 4. Fimbria fimbria/a. A, Transverse section through the "mantle gill." B, Transverse section through the pall ial blood vessel. teriorly, fusion between the inhalent aperture imately 12 papillae on each side of the inhalent and the pedal aperture is slight, involving the aperture and 40 on each side of the exhalent innermost pallial muscles (Fig. 1). A double aperture. row of sensory papillae form a border to the Th e Lucinidae typically possess an exhalent apertures. This arrangement is similar to that in siphon and no inhalent siphon. The exhalent another large lucinid, Codakia orbicularis. In siphon is developed from the inner muscular other species, papillae are either absent or lobe of the mantle edge only, and is of a type border only the inhalent aperture where they are unique in the Lamellibranchia (Allen, 1958) . confined to the inner fringe of the sensory lobe Fimbria is no exception, and the exhalent siphon (Allen, 1958). In Fimbria there are approx- when retracted lies inside out with the supra- 152 PACIFIC SCIENCE, Vol. 24, April 1970 branchial cavity at the base of the posterior Alimentary Canal retractor muscle ( Fig. 1). The course of the alimentary canal in Fimbria is simple and differs little from that reported in Gills other Lucinacea ( Allen, 1968). In the preserved As in other species of the Lucinidae, the gills animals studied, the tissues surrounding the of Fimbria consist of the inner demibranchs stomach were found to be very fragile and only. The demibranchs are large and, pos­ dissection was difficult. Therefore, a reconstruc­ teriorly, extend below the visceral mass (Fig. tion of the anterior part of the gut as far as the .1) . In other species of the Lucinidae, the style sac was made from serial sections (Fig. 6) . demibranchs are homorhabd ic and nonpl icate; The oesophagus is unusually long, and its they are also thick in cross section owing to inner lining is longitudinally ridged. The extension of the abfrontal region of the filament stomach is also elongate, with a small dorsal (Allen, 1958) . In Fimb ria the gills are homo­ hood anterior and to the left side. The style rhabd ic but plicate, although this condition is not sac and midgut are combined and leave the immediately noticeable because the plicae are not stomach posteroventrally, and the hind gut thrown into folds. Sections show that the prin­ continu es as a simple loop passing through the cipal filaments occur at approximately 8 to 10 ventricle of the heart and terminating ventral to filament intervals. This arrangement recalls the the posterior adductor muscle (Fig. 1). condition in another lucinacean family, the The form of the stomach is intermediate be­ Diplodontidae (Ridewood, 1903; Allen, 1958). tween that of Loripes lucinalis and Codakia The abfrontal surface of the filaments is not orbicularis (Allen, 1958) . The gastric shield extended in Fimb ria, so that the demibranch is lies to the left side. It has an anterior flange not in any way thickened. extending into the entrance of the dorsal hood, and there is no second lateral flange such as Palps would fit into a left pouch . There is the usual The palps, as in other lucinids, are much well-defined tooth anterior and to the lower side reduced and merely form slight enlargements of of the gastric shield ( Fig. 6). The apertures the outer limit of the lips. The latter are elon­ from the stomach to the digestive diverticula gate ridges extending on either side of the comprise three ducts close together on the right mouth as far as the anterior limit of the ventral hand side, one large duct on the left, and a ciliated groove of the demibranchs (F ig. 5). second large duct ventral and posterior to the

ORAL GROO VE MOUTHA NTE RIOR ADDUCTOR

UPPER PALP

\1. . . s-:- ~ \ LOWER PALP (\~/IJ\, \, \ BODY A \ GI LL 'B

FIG. 5. Fimbria fimbriata. A, Enlarged drawing of the encircled region in B, to show more detail of the palps of the right side. B, Semidiagrammatic drawing of the region of the mouth. Morph ology of Fimbria fimbriata- A LLEN AND T URNER 153

A BCD E FGH

FIG. 6. Fimbria fimbria/a. Transverse sections of the stomach arranged in an overlapping sequence. Th e sections are 101-t in thickness and each is 0.6 mm from the next. A diagrammatic reconstruction of the stomach, which is drawn from 450 or more serial sections, indicates the position of each of the eight selected sections A-H . latter on the floor of the stomach. Of the ducts or heel performs the locomotory function. The on the right side, two lead to diverticula on foot of Fimbria is divided into anterior and the right side and one to the ventral side of the posterior parts ( Fig. 1), but the posterior part stomach. The large left duct, which is probably is less well developed than in many other species homologous with the left caecum, serves the (Allen, 1958) which may indicate that the anterior diverticula of the left side and the species remains in the same position for much duct leading from the floor of the stomach, of its life. . which doubtfully may be homologous with the Th e anterior part of the foot is elongate with right caecum, serves the remaining diverticula a pointed tip that is heavily and uniformly of the left side. Like other lucinids, the stomach ciliated. Below the epithelium layer of the tip is unusually simple in its form, and sections of the foot there is a thin layer of circular show little evidence of sorting areas. Similarly, muscle and, internal to this, a great mass of the digestive diverticula, although extensive muscle fibers running in all directions within and with a finely branched duct system, show no loose connective tissue. Th e central blood space exceptional features to those already described which is crossed by a few muscle fibers has no (Allen, 1958). definite bound ary. The most striking feature of the tip of the foot is the very large number of Th e Foot mucus-secreting gland cells which presumably provide the material that binds together the In the Lucinacea generally the anterior part walls of an anterior inhalent tube ( Allen, of the foot is responsible for building the 1958). These cells are similar in appearance to anterior inhalent tube, while the posterior part those seen in the mantl e edge and "mantle gill." 154 PACIFIC SCIENCE, Vol. 24, April 1970

DISCUSSION AND CONCLUSIONS ALLEN, J. A. 1958. On the basic form and adaptations to habit in the Lucinacea (Eula­ From this study it appears that Fimbria is in mellibranchia). Philosophical Transactions of most respects a typical member of the Lucinidae, the Royal Society of London, ser. B, vol. 241, and there seems to be little reason to erect a pp. 421-484. new family, the Fimbriidae. Th e gill is unusual as compared with other lucinids in that it is --- 1960. The ligament of the Lucinacea plicate and not noticeably thickened abfrontally. (Eulamellibranchia) . Quarterly Journal of The following features should be noted. The Microscopical Science, vol. 101, pp. 25- 36. palps are mere vestiges in comparison with --- 1968. The functional morphology of eulamellibranchs other than the Lucinidae. There Crassinella mac/races (Linsley) (Bivalvia: is no evidence of selection of food particles Astartacea) . Proceedings of the Malacological according to size, and sorting of filtered material Society of London, vol. 38, pp. 27- 40. must be m inimal. Th e presence of a simple CUVIER, G. 1817. Le regne animal distribue "mantle gill" with an associated pallial blood d'apres son organisation. Vol. 24. Paris. vessel might indicate auxiliary respiratory func­ 532 pp. tion, although sensory and directional functions LAMY, E. 1921. Revision des Lucinacea vivants are also possible. The stomach shows typical du Museum d'Histoire Naturelle de Paris. lucinid modifications and probably accepts all Journal de Conchyliologie, vol. 65, pp. 286­ particles whatever their size. Th e foot is sup­ 288. plied with numerous mucus gland cells at the N ICOL, D. 1950. Recent species of the lucinoid tip that presumably are used to build an anterior pelecypod Fimbria. Journal of the Washing­ inhalent tube. All of these features are to be ton Academy of Sciences, vol. 40, pp. 82-87. found in members of the Lucinidae. Fimbria shows great development of muscle OWEN, G. 1958. Shell form, pallial attachment in the mantle edge, with the pallial muscles and the ligament in the Bivalvia. Proceedings some distance in from the edge of the shell. of the Zoological Society of London, vol. Th is indicates that it may be capable of remain­ 131, pp. 637- 648. ing closed for long periods of time with the PELSENEER, P. 1911. Les Lamellibranches de mantle folds retracted. The small size of the I'expedition "Siboga." Partie anatomique. Si­ heel of the foot suggests a fairly stationary boga Expeditie, vol. 53a, pp. 1-125. existence. The persistence of a double row of REEVE, L. 1841. Conchologia Systematica or papillae on the mantle edge around both the complete system of conchology, in which the inhalent and the exhalent siphon has been ob­ lepodes and conchiferous Mollusca are de­ served in one other of the Lu cinidae species­ scribed and classified according to their Codakia orbicularis. Both Fim bria and Codakia natural organization and habits. Vol. 1. Lon­ orbicularis are large, and this feature may be don. 195 pp. little more than a reflection of their size. The RIDEWOOD, W. G. 1903. On the structure of inhalent aperture is not extended as a siphon, the gills of the Lamellibranchia. Philosophical and it is doubtful if it is used other than for Transactions of the Royal Society of London, the disposal of pseudofaeces. Fimb ria is found ser. B, vol. 195, pp. 147-284. in warm tropical waters which may well account WOODWARD, S. P. 1881. A manual of the for the great size and thickness of the shell. Mollusca: or, rudimentary treatise of recent We wish to thank Mrs. Anne Scott and and fossil shells. London. 486 pp. Miss A. M. Twizell for their assistance with YONGE, C. M., and J. I. CAMPBELL. 1968. On cutting sections and preparing the manuscript. the heteromyarian condition in the Bivalvia, with special reference to D reissena poly­ LITERATURE CITED morpb« and certain Mytilacea. Transactions ALLAN, J. 1950. Australian shells. Melbourne. of the Royal Society of Edinburgh, vol. 68, 470 pp. pp. 4- 42.