Boring Mechanism of Polydora Websteri Inhabiting Crassostrea Virginica
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W&M ScholarWorks VIMS Articles Virginia Institute of Marine Science 1969 Boring Mechanism of Polydora websteri Inhabiting Crassostrea virginica Sarah A. Haigler Follow this and additional works at: https://scholarworks.wm.edu/vimsarticles 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, occasion- al worms formed short mucous tubes direct- ly 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 rap- idly, but was not applicable to worms whose giant setae were removed; hence ar- tificial 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 dis- tinguished by weak ventral pigmentation not confined in chromatophores, a single dorso-lateral bar between the first and sec- ond 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.