Boring Mechanism of Polydora Websteri Inhabiting Crassostrea Virginica

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1969

Boring Mechanism of Polydora websteri Inhabiting Crassostrea virginica

Sarah A. Haigler

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Part of the Marine Biology Commons AM. ZOOLOGIST, 9:821-828 (1969).

Boring Mechanism of Polydora websteri Inhabiting Crassostrea virginica

SARAH A. HAIGLER

Virginia Institute of Marine Science, Gloucester Point, Virginia 23062

SYNOPSIS. The boring mechanisms of species of polydorid polychaetes are little

understood due to lack of experimental evidence and direct observations. In the Downloaded from https://academic.oup.com/icb/article-abstract/9/3/821/224229 by guest on 24 June 2020 present studies the boring mechanism of adults and metamorphosing larvae of Polydora websteri was investigated by (1) inducing adults and larvae to settle against test substrates, (2) observing behavior in natural burrows and in "artificial blisters" composed of transparent "Pliobond" films surrounding Iceland spar substrates, (3) removing the giant setae of worms prior to tests of boring, (4) applying the giant setae to substrates, and by (5) testing for production of acid. All the layers of oyster shell, including conchiolin, were bored. Calcareous substrates and Iceland spar were penetrated rapidly by adults without the assistance of the giant setae. Nor were these organs essential to the boring of a larva. A characteristic type of behavior involving close contact with the substrate during backwards and forwards movements and periods of immobility always preceded boring. The worms produced acid, probably some common product of metabolism, which can account for these results.

The "mud worm", Polydora websteri, is giant setae located on the fifth setiger a small polychaete borer which lives in the (Fig. 1), and chemical action by a secre- shells of oysters and other molluscs. Lar- tion. The evidence supporting the numer- val worms settle on the outer surfaces of ous hypotheses is almost entirely circum- oysters, but adult worms are responsible stantial and subject to conflicting interpre- for the two types of unsightly "mud blis- tation. Most studies merely confirm chosen ters" which lower the quality of raw bar viewpoints and fail to account for incon- oysters and interfere with shucking. Most venient evidence. The experimental evi- "mud blisters" are formed by oysters in dence and direct observations needed to response to adult worms that bore into the resolve the long controversy are difficult to shell cavity. Blisters of another type, com- obtain, for boring activity in nature is in- mon in heavily infested oysters in the late termittent and visibility is obscured by the summer, result when adults crawl between opaque shells. the mantle and inner shell surfaces. The oysters secrete conchiolin followed by cal- IDENTITY OF SPECIES cite layers to wall out the intruders. Con- currently, the worms fill the pockets with Polydora species have often been mis- loose mud, and then compact it, leaving U- identified. The Virginia specimens of shaped channels leading to the environ- adult P. websteri used in this study corre- ment. Continued boring can be detected sponded to the description of Hartman soon after the blisters are formed. (1943) except that the ventral hooded The boring mechanisms of Polydora spe- hooks commence on the seventh instead of cies in calcareous substrates are unknown the eighth setiger. For rapid sorting, P. or disputed despite a century of study. websteri were distinguished from the ever- Varying emphasis has been placed on present P. ligni by the fine longitudinal mechanical abrasion by modified chitinous lines of black pigment on the palps as setae, in most species by the characteristic opposed to the diffuse, brown lines on the palps of P. ligni. Contribution Xo. 323 from the Virginia Institute Characteristics for distinguishing larval of Marine Science, Gloucester Point, Virginia 23062. P. websteri from other polydorids in the 821 822 SARAH A. HAIGLER

predictable and observable. Adult P. websteri removed from their burrows and placed on test substrates constructed opaque tubes from any loose materials available. Because the tubes were only loosely attached to the substrates, the worms did not bore. When instead, tube- building materials were withheld, occasional worms formed short mucous tubes directly against the substrates and bored. The Downloaded from https://academic.oup.com/icb/article-abstract/9/3/821/224229 by guest on 24 June 2020 posterior segments of these worms were usually amputated to prevent the curling movements which broke the mucus connec- tions to the substrates. Since worms vacated tubes formed against conchiolin substrates, they were allowed to settle on adjacent or overlying calcareous regions. This tech- nique was primarily used to demonstrate which substrates were bored and how rapidly, but was not applicable to worms whose giant setae were removed; hence artificial blisters (Fig. 2) were constructed to hold them. These consisted of pieces of FIG. 1. Ventral view of the fifth setiger setae of P. Iceland spar (pure calcite) completely sur- websteri, including giant setae interspersed by com- rounded by films prepared from the house- panion setae, and a small ventral fascicle. Wear is hold adhesive trademarked "Pliobond" evident on the oldest giant setae and companion setae. X288 (Goodyear Tire and Rubber Co.). "Plio- bond" was dropped on fresh water to region were established after extensive form thin, transparent layers which were comparison and rearing of larvae. The lifted out on and sealed around the test only species with which a confusion is con- ceivable is P. ligni which bears a similar series of black dorsal bars and stellate chromatophores beginning on the third setiger. P. luebsteri were readily distinguished by weak ventral pigmentation not confined in chromatophores, a single dorso-lateral bar between the first and second setiger, the absence of anal papillae, heavier black pigmentation dorsally on the pygidium, and a pair o£ transverse black bars on the most distal region of the lips. In addition, late larvae were distinguished by setal characters and 5 pairs of dorsal bars although the fifth pair often has some stellate characteristics.

MATERIALS AND METHODS New and improved techniques depend- FIG. 2. P. websteri within an artificial blister on an Iceland spar substrate. The posterior segments ent upon the limitation of tube-building have been removed to prevent the discharge of materials were required to make boring eggs. X7 BORING MECHANISM OF Polydora 823 substrates. Rinsing in fresh water removed Larvae were obtained from capsules laid in the adhesive solvent, methyl ethyl ketone, nature during die summer spawning sea- and sea water caused the coatings to swell. son, or laid in the laboratory out of season. Openings were then cut in the "Pliobond" The capsules of P. websteri which are laid layers without marring the substrates, and in the dwellings of parent worms contain the worms were inserted within. Septa of two types of spawn requiring different compacted mud from natural blisters were rearing techniques, but yielding larvae provided for traction, but loose tube- identical in appearance. In one type the building materials were withheld to pro- eggs develop at similar rates and the larvae

mote direct contact with the substrates. are released at the 3-setiger stage Downloaded from https://academic.oup.com/icb/article-abstract/9/3/821/224229 by guest on 24 June 2020 The preparations were kept at approx- (Hopkins, 1958) without feeding upon imately 28°C in standing membrane- undeveloped eggs which are occasionally filtered sea water without the addition of present. Capsules of this type were general- food. ly produced in the laboratory out of season Correct spacing was essential for demon- when shells containing P. websteri were strating boring within artificial blisters, for lightly cleaned of fouling organisms and worms rejected blisters with insufficient kept in membrane-filtered sea water at 25° space and did not bore the substrates of C. At this temperature the P. Ugni, which loose preparations. Since "Pliobond" is can never be completely removed by clean- elastic and the septa are compressible, ing the shells, were prevented from spawn- some adjustment of this factor was pos- ing. During the few months prior to a new sible. Because tube-building materials were spawning season, when the eggs degen- withheld and both the films and sub- erate within the bodies of female worms in strates were transparent, direct observa- nature, shells over-wintered at about 20° C tions of behavior were possible. were used for production of larvae. In experiments to determine whether Whether obtained from nature or pro- the giant setae are essential for boring, the duced in the laboratory, these larvae were setae were extirpated with small-bore syr- reared to swimming-crawling stage on al- inge needles. After anesthetization with 5% gae: Dunaliella, Phaeodactylum, and Mon- magnesium chloride on glass slides, most ochrysis. Faster growth was obtained when of the liquid was pipetted off, and the larvae were kept with shells during the worms were bent to each side to expose the younger stages. setal fascicles of the fifth setiger. Incisions The second, unreported, type of spawn were made on each side without damaging was easier to rear. The eggs develop at the medial organs. Returned to sea water, variable rates and, initially, undeveloped the worms recovered rapidly and the eggs are always present. The larvae are wounds healed quickly. usually brooded until some reach a very Fascicles of giant setae and single setae advanced stage, when the undeveloped or were brushed against Iceland spar and the nurse eggs are disintegrated into a brei and various natural substrates to determine consumed as food. Since no pelagic ex- their capacity to remove substrate parti- istence was required for the oldest larvae cles. Whole fascicles were held in fine for- within such capsules, they were maintained ceps. Individual giant setae were teased on the internal food source. from fascicles, cleaned of muscle fibers, P. websteri larvae selected calcareous dried and mounted with "Duco Cement" substrates in the presence of weak water (E. I. duPont de Nemours & Co.) on fine currents produced by organisms or an ap- steel pins mounted in turn within glass paratus. Occasional worms which settled capillary tubes. on non-calcareous substrates repeatedly va- Considerable preparation was necessary cated their tubes to form new ones. The before the boring of metamorphosing lar- larvae selected any crevices available and vae could be studied in the laboratory. often settled between the test substrates 824 SARAH A. HAIGLER and the glass culture dishes. Therefore, to observe settlement behavior, larvae in the swimming-crawling stage were provided with carefully selected and prepared substrates. Cleaned dried substrates without crevices were soaked to remove trapped air and mounted, still wet, in "Pliobond" on pieces of glass. The weak water currents required for settlement were provided by an apparatus based on the design of Han- Downloaded from https://academic.oup.com/icb/article-abstract/9/3/821/224229 by guest on 24 June 2020 nerz (1952-1953). The cultures were maintained at about 28°C and both food and tube-building materials were withheld to increase visibility. Despite the small size and fragility of the larvae, it was also possible to extirpate their giant setae, but the operations had to be performed very rapidly to avoid desic- FIC. 3. Section of channel bored in Iceland spar by cation and death of the larvae. P. websteri after three weeks with an artificial blister. x>00 The acid production of P. websteri and of non-boring species of worms was deter- Boring was also demonstrated within ar- mined by a 1% agar-sea water medium con- tificial blisters on Iceland spar substrates. taining phenol red as an indicator and The first signs of etching were detected antibiotics to prevent bacterial growth. after 4 days, but several weeks were re- Adults were forced into the soft medium, quired for channels to form (Fig. 3). Tn while larvae were maintained in a few ml one experiment to determine whether the of sea water above the medium. When the giant setae are essential for boring, these color of the medium changed from red to setae were removed from two sets of yellow, the worms were removed and worms. One set was allowed to regenerate rhombohedrons of Iceland spar were intro- new unworn setae; the regenerated setal duced to test the etching capacity of the sacs of the other set were removed before secretions. new setae could be secreted. A third set of worms served as controls. Since the worms RESULTS AND CONCLUSIONS without giant setae produced borings iden- P. websteri penetrate all layers of the tical to those produced by the two sets of oyster shell, including prismatic, calcite- worms with giant setae, these organs are ostracum, hypostracum, periostracum, and not essential for boring in this hard substrate. Further, no participate products internal conchiolin layers of visible thick- were found; these would have resulted if ness, as determined by examinations of the giant setae had assisted in substrate natural burrows and duplicated in direct removal. No signs of wear were detected tests against fresh substrates. Worms in- on the new giant setae, although the frag- duced to settle directly on test substrates at ile, regenerated, companion setae were room temperature bored chalky deposits frayed. Apparently, all the calcite was re- within 24 hr, calcite-ostracum and hypos- moved by dissolution, even when the giant tracum within 1 week, and conchiolin lay- setae were retained. Preliminary results ers within a month. This evidence, in ad- from other experiments with artificial blis- dition to that readily obtained by observ- ters show that the giant setae are not es- ing the extension of natural burrows, re- sential for the penetration of either chalky futes the common supposition that boring deposits or calcite-ostracum. is a slow process. BORING MECHANISM OF Polydora 825 In the many direct observations of adults which the giant setae were often seen vigor- within artificial blisters, the giant setae ously removing substrate particles during seldom contacted the substrates. For long settlement and in the presence of mucous periods, however, the worms lay motionless secretions. This behavior is more likely with their ventrums appressed to the sub- related to attaining firm attachment ot the strates, and sometimes slowly moved back- primary tubes, than to boring. As soon as wards and forwards, still maintaining close the larvae had constructed primary tubes, contact with the substrates. At such times however, and long before metamorphosis

the worms were especially unresponsive to was completed, they began to more back- Downloaded from https://academic.oup.com/icb/article-abstract/9/3/821/224229 by guest on 24 June 2020 tactile stimuli. Since this characteristic type wards and forwards very rapidly, with the of behavior always preceded the detection ventrums of their bodies closely appressed of boring, whether observed in natural or to the substrates. These conspicuous mo- artificial burrows, or after direct settlement tions alternated with periods of quies- by adults or larvae, it is believed to be the cence. No particles were removed and the true boring behavior. giant setae were not applied to the sub- Attempts to abrade substrates with iso- strates. Unlike the adults, metamorphosing lated giant setae and setal fascicles were larvae responded to tactile stimulation by mostly ineffective. Brushed against unetch- vacating the primary tubes. ed substrates with light pressure, the tips The fragility of the giant setae and the of the setae wore down or broke off. Only ability of adults and larvae to bore without those substrates which offered slight resist- them proves that a chemical agent is in- ance, such as the prismatic layers and volved in boring. This chemical is not con- chalky deposits, were affected. Certainly chiolinase, for Iceland spar is composed of setal action alone cannot account for the pure calcite and lacks the conchiolin ma- rapid boring of such hard substrates as trix which binds calcium carbonate crystals calcite-ostracum, which comprises the ma- together in most oyster shell layers. The jor portion of oyster shell. On the other adults and larvae of P. websteri did pro- hand, the setae were very effective against duce acid in sea water-agar medium with etched substrates, including Iceland spar, phenol red indicator, and pieces of Iceland although they were not used to remove spar introduced into the medium after the substrate in the experiments with artificial worms were removed were etched in a blisters. The only situation in which the manner indistinguishable from etched ar- giant setae of adult worms removed sub- eas in artificial blisters. Non-boring species strate was during settlement on chalky de- of worms, including adult and larval P. posits and prismatic calcite. Most of the ligni, also produced acid capable of simi- particles were lifted and trapped by mu- larly etching Iceland spar. This suggests cous strands, but some were accidentally that an acid product of metabolism was caught on the giant setae and other setae responsible for boring. If the chemical during locomotory movements. All the agent in boring is not specific, the paniculate calcite was incorporated into manifestation of boring potential in P. the mucous tubes, including that which passed through the gut and was expelled as websteri probably depends upon behavior- feces. After tubes had been constructed, al adaptations, such as the observed capac- the same behavior occurring in artificial ity to maintain intimate contact with sub- blisters was observed. strates for extended periods.

The giant setae were not essential to the DISCUSSION boring of a metamorphosing larva of 16 setigers which was induced to settle satis- By means of new techniques and careful factorily and bore into a cube derived from observations, it has been shown that P. a chalk deposit. Its behavior differed in -websteri bores rapidly into calcareous sub- one respect from that of normal larvae in strates. As recently as 1967, Landers stated 826 SARAH A. HAIGLER that P. websteri apparently does not bore. Unless the setae are seen to remove sub- Actually, the boring species he used was strate, the only valid conclusion is that probably P. websteri, since Pettibone worn setae have been in contact with hard (pers. comm.) has preferred including P. materials. websteri under the name P. ciliata. Several direct observations of so-called Artificial blisters, composed of cover mechanical boring (Wilson, 1928; Han- glasses cemented to oyster shell substrates, nerz, 1956) involve metamorphosing larvae whose behavior during settlement can be were used by Mortensen (1945) who at- Downloaded from https://academic.oup.com/icb/article-abstract/9/3/821/224229 by guest on 24 June 2020 tempted to determine the boring mechan- interpreted quite differently. Wilson has ism of a species she called "P. ligni." When emphasized the brief duration of the be- the worms did not bore after 7 weeks in havior. Soderstrom (1920) observed the these substitute dwellings, she concluded boring of an adult P. ciliata in a thin- that boring was an extremely slow process walled burrow. His observations of the probably effected by carbonic acid result- same sliding motions described in this ing from respiration. Although these par- study have been practically ignored. He ticular experiments were probably con- also saw strokes of the giant setae in what ducted with P. websteri rather than P. lig- seemed to be mechanical action against the ni (which does not penetrate calcareous substrate, though no definite scratch or substrates or stimulate formation of blis- removed substrate was detected. In a later ters), any number of other factors could paper (1923) Soderstrom placed far less have prevented boring. Tube-building ma- emphasis on the mechanical component of terials were added instead of limited, the boring, stating that the giant setae func- spacing may have been unfavorable, and tion primarily as organs for support and an illustration shows a tube formed against adhesion, as during ventilation or feeding the glass cover instead of the test sub- movements. Although vigorous action of strate. the giant setae has been observed by Dor- Assumptions that Polydora species pene- sett (1961), and especially Hempel (1957) trate calcareous substrates mechanically during the penetration of predominantly have been based on weak evidence which clay substrates, such evidence does not re- does not detract from my own conclusion quire mechanical and preclude chemical that the boring mechanism of P. websteri action with respect to predominantly cal- is largely chemical. Although Hempel careous substrates. Without direct observa- (1957) assumed that calcite would be tions, Hempel assumed that P. ciliata used more easily penetrated by mechanical than their giant setae in the same way against by chemical means, P. websteri rapidly clay and calcite, but did not explain why bored hard Iceland spar without using the P. ligni did not do likewise. The "argilla- giant setae. Theoretically, giant setae are ceous difficulty" has probably been over- capable of assisting in the removal of soft emphasized, and in any case, presents and softened calcareous substrates, but evi- no great problem to the understanding of dence is lacking to prove this function. how P. websteri bores, for this species has Impressive as the wear on the giant setae not been reported from clays, shales, sand- may be, this fact in itself speaks equally stones, or from soft wood. Even in the well to the ineffectiveness of the setae occasional penetration of conchiolin lay- against hard substrates. The giant setae of ers, the chemical may be of considerable P. ligni dwelling in mud tubes constructed importance. One often finds such layers against oyster shell are worn, yet they do detached from the floors of the channels not penetrate the substrate. The same spe- and the underlying calcium carbonate cies, collected in intertidal sand has severe- etched. Whether such layers are removed ly frayed and worn setae, but no one by setal action or by prolonged chemical would conclude that therefore the worms action is not known. have the capacity to penetrate sand grains. Paniculate calcareous products clearly BORING MECHANISM OF Polydora 827

the result of mechanical boring have not which seemed too short and narrow to been observed. The few granules found in hold the anterior end. The calcite- the septa of P. websteri burrows could ostracum floor beneath was bored, al- have been detached by mucus or gathered though the podial glands in the posterior from outside the shell. The argument that setigers are rudimentary. Dorsett (1961) such particles would necessarily be dis- has suggested that a chelating agent linked solved (Prell, 1926; Hempel, 1957) if to the biochemistry of mucus produced by chemical means were employed, is opposed other glands is the chemical agent. In P.

by the argument (Mortensen, 1945; Dor- websteri very little mucus is present at the Downloaded from https://academic.oup.com/icb/article-abstract/9/3/821/224229 by guest on 24 June 2020 sett, 1961) which requires direct applica- boring sites except during periods of settle- tion of the chemical to avoid dilution and ment and tube-building. Although several neutralization by currents of sea water deep channels in Iceland spar were even- passing through the burrows. tually lined with mucus tubes similar to The over-all contours of the burrows are those observed in P. ciliata borings, even smooth. Scratches in calcareous substrates with P. ciliata these linings are absent in which might be attributed to the giant areas where boring is probably occurring setae (Lamy and Andre, 1937; Hempel, (Dorsett, 1961). Hannerz (1956) suggested 1957) have seldom been detected. The few that the acidic contents of a set of glands seen on the walls of P. websteri channels associated with the giant setae of metamor- were associated with soft etched areas of phosing larva aid boring at that stage, yet calcite-ostracum. The roughness of the a larva of P. websteri bored after those walls (which may appear smooth in regions were removed. reflected light) can be duplicated without In light of the results and conclusions setal action. reached in this study, the boring mechan- The main weakness of the chemical hy- isms of other Polydora species should be pothesis is that no acid other than mucus carefully re-examined. has been demonstrated before. Lankester (1868a, b), an early proponent of a chemi- REFERENCES cal-mechanical hypothesis believed carbon- Dorsett, D. A. 1961. The behavior of Polydora ic or sulfuric acids might be involved in ciliata (Johnst.) . Tube-building and burrowing. boring. Although he obtained an acid J. Mar. Biol. Assoc. U. K. 41:577-590. reaction in a crude test of P. ciliata against Elliott, W. T., and B. Lindsay, 1912. Remarks on litmus paper, this result could not be du- some of the boring Mollusca. Rep. Brit. Assoc. Advan. Sci. Meeting 81 (1911) :433. plicated by others (Mclntosh, 1868; Elliott Hannerz, L. 1952-1953. A simple apparatus for and Lindsay, 1912; Soderstrom, 1920), and producing weak water-currents in culture dishes. he withdrew his hypothesis (Mclntosh, Oikos 4:148-150. 1868). Hannerz, L. 1956. Larval development of the polychaete families Spionidae Sars, Disomidae Mesnil, Many glands have been shown to have and Poecilochaetidae n. fam. in the Gullmar acidic contents. Although the podial Fjord (Sweden). Zool. Bidrag Uppsala 31:1-204. glands most often cited have been thought Hartman, O. 1943. In V. L. Loosanoff and J. B. by some to produce sulfuric acid (Lankes- Engle. Polydora in oysters suspended in the water. Biol. Bull. 85:70-72. ter, 18686; Soderstrom, 1920), according to Hempel, C. 1957. ttber den Rohrenbau und die Hannerz (1956) and Dorsett (1961) they Nahrungsaufnahme einiger Spioniden (Polychae- produce mucus, which the latter author ta sedentaria) der deutschen Kiisten. Helgoland- says is secreted to line the tubes. A single er Wiss. Meeresunters. 6:100-135. Hopkins, S. H. 1958. The planktonic larvae of observation casts doubt upon the idea that Polydora websteri Hartman (Annelida, Polychae- the chemical is confined to the podial ta) and their settling on oysters. Bull. Mar. Sci. glands. In one artificial blister, a P. webste- Gulf Carribbean 8:268-277. ri constructed a tube with a short dead-end Lamy, £., and M. Andre. 1937. Annelides per- forants les coquilles de mollusques. 12 Congr. offshoot. Only the posterior end of the Int. Zool. (Lisbonne, 1935), C. R. 2:946-968. worm was observed within this section, Landers, W. S. 1967. Infestation of the hard clam, 828 SARAH A. HAICLER

Mercenaria mercenaria, by the boring polychaete D. C. 106 p. worm, Polydora ciliata. Nat. Shellfish. Assoc, Prell, H. 1926. Fossile Wurmrohren. Beitriige zur Proc. 57 (for 1966) : 63-66. palaobiologischen Beurteilung der Polydorinen- Lankester, E. R. 1868a. On the boring of limestones Horizonte. Neues Jahrb. Mineral. Geol. Palaontol., by certain annelids. Rep. Brit. Assoc. Advan. B, 53:325-396. Sci., Trans. Meeting 37 (1867) :85. Soderstiom, A. 1920. Studien iiber die Polychaelen- Lankester, E. R. 1868b. On lithodomous annelids. familie Spionidae. Dissertation. 286 p. Almquist Ann. Mag. Natur. Hist., Ser. 4, 1:233-238. and Wickells, Uppsala, Sweden. Mclntosh, W. C. 1868. On the borings of certain annelids. Ann. Mag. Natur. Hist., Ser. 4, Soderstrom, A. 1923. Uber das Bohren der Po- 2:276-295. lydora ciliata. Zool. Bidrag Uppsala 8:319-326. Mortensen, E. E. 1945. The morphology, behavior Wilson, D. P. 1928. The larvae of Polydora ciliata and habits of Polydora ligni Webster. Ph.D. Johnston and Polydora hoplura Claparede. J. Downloaded from https://academic.oup.com/icb/article-abstract/9/3/821/224229 by guest on 24 June 2020 Thesis. George Washington Univ., Washington, Mar. Biol. Assoc. U. K. 15:567-604.