Short Papers and Notes 299

TABLE 2. Effect of maturing Limulus eggs on the Gastrotricha subcommunity of meiofauna. Species number, species diversity and percerit composition were taken from raw data-composites; nested ANOVA tests were based on log~o (n + I) transformatioi~s of data from individual samples.

Parameter value

Limulus eggs Limulus eggs Results of Mixed Model absent present Nested ANOVA tests

18 June 29 J une 18 June 29 June eggs dates

Species number 7 6 6 3 H' lpecms diversity (bits)* 1.17 1.46 0.32 0.23 J ' evenness* * 0.42 0.56 0.12 0.15 Percent composition

Aspidiophorus mediterraneus 74.1 28.9 95.7 96.7 Turbanella ambronensis 19.4 60.8 2.1 2.9

Geometric mean of individuals/' I0 cm ~ sediment. Aspidiophorus mediterraneus 2.25 3.14 13.83 78.48 Turbanella ambronensis 0.67 4.94 0.45 1.85

m

m

m

m

.005 > P incr. n.s. n . s . n . s .

* Lloyd el al., 1968. ** Pielou 1969.

that such interactions be considered when problems of community structure or energetics are studied in littoral, or for that matter in sublittoral, ecosystems.

We thank B. A. Brenum, J. J. Fleming, R. W. Jones, Jr.. E. I. Bauereis and R. K. Sasaki for their participa- tion in the overall project. The work was done in alliance with a summer-session course in Ecology of Benthic Micrometazoa, University of Delaware, taught by W. D. Hummon and assisted by J. Fleeger. Analysis of data and preparation of the manuscript were completed during the tenure ot" the senior author at DAFS Marine Laboratory, Aberdeen, Scotland, under a NAT O Postdoctoral Fel- lowship.

LITERATURE Crl El)

HCMMO.,,', W. D. 1974. S~,: a similarity index based on shared species diversity, used to assess temporal and spatial relations among intertidal marine Gastrotricha. Oecologia ( Berl. ) 17:203- 220.

I,LO'CD, M., J. t l . ZAR, AND J. R. KARR. 1968. On the calculation of information-theoretical measures of diversity. Amer. Midl. Natur. 79:257 272.

P[ELOU, E. C. 1969. An Introduction to Mathematical Ecology. 286 p. [nterscicnce.

POI,I,OCK, L. W., AND W. D. HUMMON. 1971. Cyclic changes in interstitial water content, atmospheric exposure and temperature in a marine beach. Limnol. Oceanogr. 16:522 535.

WILLIAM D. HUMMON JOHN W. FLEEGER 1 MARGAREr R. HUMMON

Department of Zoology and Microbiology Ohio University Athens, Ohio 45701

and College of Marine Studies Field Station University o f Delaware Lewes, Delaware 19958

L Present address: Department of Biology, University of South Carolina, Columbia, S.C. 29208.

Predation of the Razor Clam Ensis directus by the Nemertean Worm Cerebratulus lacteus z

ABSTRACT: The nemertean worm Cerebratulus lac- teus feeds on the razor clam Ensis directus by entering the burrow from below and engulfing its anterior end. The clam is forced to project much of its body above the surface (sometimes leaving the burrow), thus becoming subject to surface predators and desiccation. This preda- tion was obserced from New Jersey to North Carolina.

~ Supported in part by National Science Foundation grants G-11448 and G-22068.

Cerebratulus lacteus (Leidy, 1851) (class Anopla, order t teteronemertea) is the most conspicuous nemer- tean of littoral sediments from Maine to Florida because of its large size and its frequent nocturnal swimming. It is a flesh-colored, carnivorous worm with a very long proboscis and a longitudinally oriented, slit-like mouth on the ventral side of the head. The food and feeding habits of Cerebratulus are poorly known, although it is believed generally that polychaetes are its main natural prey (Coe 1943; Hyman 1951). This paper presents field

Chesapeake Science Vol. 17, No. 4, December, 1976

3 0 0 Short Papers and Notes

observations on the behavior of the razor clam Ensis direetus Conrad 1843, when being preyed upon by Cerebratulus.

On July 6, 196l, on an exposed tidal flat inside of Hereford Inlet, New Jersey (Grassy Sound Channel, lat. 39001.72 ' north and long. 74~ ' west) I observed two large razor clams (130 and 132 mm in length) protruding from the sand. The posterior ends of each were 4 to 5 cm above the surface. They were alive but when stimulated did not descend into their burrows in the usual manner. One of the clams was removed carefully with a spade. Its anterior end was covered with a copious clear mucus. The other clam was removed in the same manner but with a swifter movement of the spade. Attached to the anterior end of this clam was the cut anterior end of the nemertean Cerebratulus. It had about 4 cm of the clam in its digestive tract, enclosing it like a stocking (Fig. 1). The same thick mucus seen on the first clam remained on the valves of this clam after the nemertean was removed.

Further confirmatory observations on this predator- prey relationship were made at Hereford Inlet during the same year (August, 1961) and in 1962, 1963, 1967, and 1969. The predation was seen from July through Novem- ber which coincided with my activity in the region, and is not meant to imply a seasonal trend. The same phenome- non was noted on July 8, 1967, on exposed sand flats along the south side of Oregon Inlet, North Carolina. More recently, Drs. Charles E. Epifanio and John N.

Fig. 1. Semidiagrammatic vertical sections through the burrows of Ensis directus, showing a clam being attacked by Cerebratulus (left), and a clam in its normal position with the foot poised for retreat (right). Based on a clam collected 10-21-67 in New Jerscy. Scale line = 5 cm.

Kraeuter observed this relationship on the sand flats inside of Cape Henlopen, Delaware (pers. commun.). It was also the aberrant position of the living clams that originally attracted their attention.

As the exposed clams are being dug, the nemerteans sometimes remain attached (as described above) but usually they release their hold on the clam and are either seen nearby or are not recovered. Each clam, however, has the characteristic mucus covering the anterior part of the shell which is evidence that it was being attacked.

On one occasion while walking on the exposed sand flat, I observed a large Ensis force itself out of bottom to expose approximately 5 cm of its posterior end. I immediately inserted a spade under the clam, removed it, and examined its anterior end. There was no mucus and the nemertean was not attached, but a detached probos- cis of Cerebratulus was in the burrow. I interpret this as the clam's initial response to a nemertean attack. Per- haps the movement of the worm into the burrow or contact with the worm before being engulfed elicited this upward retreat. For reasons discussed below, it is unlikely that this clam could have moved completely out of its burrow on the dry flat.

On another occasion, three clams were observed at the New Jersey site during an incoming tide in about 10 cm of water. One clam protruded about a third of its length (4 cm) above the substratum; another was at least half way (6-7 cm) out of the bottom; and another clam was completely out of the substratum but a long string of the nemertean mucus extended from its valves into the burrow. The second clam was apparently beginning to move out of the burrow, and the last had accomplished the feat and dislodged itself from the worm. As discussed below, it is likely that this can be accomplished only if the clams are covered by water and are not too weakened by the nemertean.

1 may summarize the razor clam's response to Cere- bratulus as follows. Stimulated by the invading worm, the clam moves upward and partially out of its burrow and the foot is retracted between the shell valves. As the soft parts are not completely enclosed by the valves they are subjected to the digestive juices of the worm. The "d i lemma" of the razor clam is that moving downward with the foot merely exposes more of its body to digestion. On the other hand, moving upward out of the burrow exposes it to surface predators and desiccation.

Ensis is a thin-shelled burrower adapted for minimal exposure, and normal feeding position involves exposure of the siphons only. The clam when covered by water is poised in the burrow with its foot relaxed, elongated and dilated at the tip (Fig. 1). Tactile, light or other stimuli cause a strong contraction of the foot retractors and a rapid downward movement of the clam (Drew 1907). The usual coordinated cycle of muscular and fluid changes causes further penetration into the burrow. These reflexes are interfered with by the invading nemertean, and the clam moves in the opposite direction by jetting water out of the anterior end, thus placing itself in a precarious and indefensible position with regard to other predators.

Forcing water out around the foot is a normal function of burrowing bivalves which reduces the shear- ing force necessary for penetration (Trueman 1966). If water is available to the clam it could jet itself out of the

burrow and even break the hold of an attacking nemer- tean. The clam could then burrow elsewhere if not consumed by birds, fishes or crabs. On the dry sand flat the clam under attack may jet itself part way out of the burrow with one expulsion of water but can go no farther because there is insufficient water left in the mantle cavity.

On October 14 and 21, 1967, l collected all clams that were projecting above the surface of the Hereford flat. Twenty clams ranged in total length from 32.8 to 140.0 mm (mean 108.0 mm). The smallest clam was a member of the most recent year class, i.e., it had metamorphosed sometime during the summer. The length-frequency distribution of a random preserved sample of 332 clams revealed a sharp mode at approximately 30 mm, rcprc- senting the 1967 year class.

Williams and Porter (1971) suggested that the pres- ence of certain juvenile (postmetamorphal) bivalves in the plankton in North Carolina was a passive phenome- non caused by turbulent substrate-scouring currents. They feel, however, that juveniles of other species (Ensis directus, Solemya velum, Solen viridis and Tagelus divisus) occur in the plankton due to swimming behavior that is a normal seasonal event. While the mechanisms involved in their transport seem clear, the factors that induce the juveniles to migrate out of the bottom, however, are vague. I suggest that aside from physical scouring and browsing by surface predators such as fish, underground predation by Cerebratulus may be a factor contributing to this phenomenon.

Short Papers and Notes 301

ACKNOWLEDGEMENTS

I wish to thank some of my students as well as D. F. Boesch of the Virginia Institute of Marine Science for their help in collecting specimens. I am grateful to J. L. Richardson for reviewing the manuscript.

LITERATURE CITED

CoE, W. R. 1943. Biology of the nemerteans of the Atlantic coast of North America. Trans. Conn. Acad. Arts Sci. 35:129-327.

DREW, G. A. 1907. The habits and movements of the razor-shell clam, Ensis directus, Con. Biol. Bull. 12:127-140.

HYMAN, L. H. 1951. The Inverebrates: Platyhelminthes and Rhynchocoela. The Acoelomate Bilateria. Vol. I1. McGraw Hill Book Company, Inc., N. Y. 550 pp.

TRUEMAN, E. R. 1966. Bivalve mollusks: Fluid dynamics of burrowing. Science 152:523-525.

WILLIAMS, A. B., AND H. J. PORTER. 1971. A ten-year study of meroplaukton in North Carolina estuarics: Occurrence of postmetamorphal bivalves. Chesapeake Sci. 12:26-32.

JOHN J. McDERMOTT

Department of Biology Franklin and Marshall College Lancaster, Pennsylvania 17604

A Relict Population of the Mottled Sculpin, Cottus bairdi, from the Maryland Coastal Plain

ABSTRACT: A population of mottled sculpin, Cottus bairdi, in the central Delmarva Peninsula (Nantieoke drainage) is the only record of this species from the Atlantic Coastal Plain. Examination reveals that speci- mens have characteristics of both C. bairdi and the problematic C. girardi. The presence of other Piedmont indicator-species in this general area suggest a relict Piedmont community.

Collection of stream-dwelling fishes on the Delmarva Peninsula have revealed the presence of Cottus bairdi, a species typically associated with Piedmont and montane streams. On 7 August 1970 and again on 29 April 1972 we collected several series of fishes from Tull Branch, a tributary of Marshy Hope Creek (Nanticoke River), at the U.S. Route 313 bridge, north of Federalsburg, Caroline County, Maryland. Here we found an unusual assemblage of Piedmont species (least brook lamprey, Okketbergia aepyptera; and mottled sculpin, Cottus bairdi) and Coastal Plain species (bridle shiner, Notropis bifrenatus; eastern mudminnow, Umbra pygmaea; and pirate perch, Aphredoderus sayanus). The American eel (Anguilla rostrata) and tesseUatcd darter (Etheostoma olmstedi) were the only other species observed. To the best of our knowledge, sculpins have not been previously reported in the literature from the Delmarva, or from the Atlantic Coastal Plain.

There are few records of sculpins from the Chesa- peake Bay area, but only from the Piedmont. Cottus are reported from the Potomac and Patapsco River systems (Robins 1955, 1961; Savage 1962). We have additional records from Harford County at Winter's Run and Deer Creek; Baltimore County at Gunpowder and Little Gunpowder rivers. All of these collections were made on the Piedmont.

Identi[ication of sculpins from this area is problem- atic. Robins (196l) described the Potomac sculpin, Cot- tus girardi, and suggested that this new form repre- sented a disjunct member of the carolinae group occur- ring as an endemic species in the Potomac River system. He distinguished Cottus girardi from Cottus bairdi based on the presence (in C. girardi) of a strongly mottled chin, preoperculomandibular canals joining to form a single anterior median chin pore and usually 15 (versus 14 in C. bairdi) pectoral rays. Based on re-examination of the types, additional material, and certain behavioral studies, Savage (1962) placed Coitus girardi in the synonymy of Cottus bairdi. Robins (pers. comm.) does not accept Savage's evaluation.

The sculpins from Caroline County arc characterized by having an incomplete lateral line ending slightly before the last ray of the second dorsal fin; fine, even stippling (i.e. not mottled) on the cheeks, chin, and lips; preoperculomandibular canals either joined to form a

Chesapeake Science Vol. 17, No. 4, December, 1976