c Observations on the Distribution and Dispersal of the Gastropod Crepidula perforans (Valenciennes, 1846) Amy V. Alpert Hopkins Marine Station of Stanford University, Pacific Grove, California The marine gastropod Crepidula perforans (Valenciennes, 1846) lives inside Tegula funebralis shells inhabited by her- mit crabs (Pagurus species.) (See figure + 1.) It has also been reported as living in rocks by Smith and Gordon (1948) and by Coe (1949). While Crepidula adunca Sowerby, a commen- sal which lives on the outside of Tegula funebralis shells, has been the subject of many studies (Coe, 1949; Moritz, 1938, 1939; Putnam, 1964; etc.,) little has been published a- bout C. perforans. The present study was undertaken at the Hopkins Marine Station of Stanford University, Pacific Grove, California, in April and May, 1965, to determine the distri- bution of the mollusc and to investigate some aspects of the relationship between the mollusc and the crustacean. DISTRIBUTION AND NUMBERS The survey of the Crepidula perforans population was done in conjunction with a study of the Pagurus population of Mussel Point, Pacific Grove, California by Robert Belknap and John Markham. Thirteen samples were collected from the profiles (see figure + 2) surveyed by Belknap and Markham (see Belknap and Markham, 1965.) Each hermit crab shell was measured and examined for presence of Crepidula perforans. The sex and spe- cies of the crab, size and number of Crepidula, type of shell, and position of C. perforans on the shell were also noted. The geographic distribution of Crepidula perforans was -2- found to vary markedly in closely associated areas. In hermit crab shells large enough to contain C. perforans the infestation usse was quite high on one side of Point of Hopkins Marine Station, as it also was at nearby Point Pinos. On the other side of Point the number of infected crabs was much lower. (See figure + 2.) The size of the host shell has great bearing on the pre- sence of C. perforans. No molluscs were found in Tegula fun- ebralis shells less than O.25 inches in greatest diameter. Other types of shells, such as Thais emarginata, were considered to be C. perforans-bearing size if they did not pass through 0.25 inch screening in the sizing process of the Pagurus sur- vey/ (See Belknap and Markham, 1965.) Of 120 crabs smaller than the minimum size, only one, a Calliostoma shell, had a Crepidula perforans on the outside. In crabs with shells great- er than 0.25 inches in diameter infestation was quite high. (See figure + 3.) Crepidula perforans is not exclusively found in Tegula funebralis shells. The 350 crabs of C. perforans-bearing size inhabited 284 T. funebralis shells and 66 non-T. funebralis shells (including 23 Tegula brunea shells.) Of the latter, 22 or 33% contained C. perforans. No C. perforans were found in unoccupied shells or in rocks in the course of this study. Infestation by C. perforans of different species of Pag- urus was also quite variable. 68% of Pagurus hemphilii (n-33) -3- and 45.4% of Pagurus samuelis (n-156) contained C. perforans in contrast with Pagurus granosimanus (n-144) which had 17.5% infection and Pagurus hirsutiusculus (n-17) which had 10.6% infection (where n-total number of crabs examined.) The low infestation of P. hirsutiusculus is due in large part to the smaller size of the species at maturity. Within the host shell, distribution of C. perforans depends on several factors. The number of molluscs per shell depends on crab size. The larger the crab, the more C. perforans it is likely to possess. (See figure + 4.) The number of C. per- forans in the shell depends also in large part on the size of molluscs present in the shell. It is unusual to find two C. perforans of egg-bearing size in a single shell, while a large number of small ones are often found together or in association with a large one. (See figure + 5.) The small number of large molluscs may be due to mortality, to lack of room in the shell, or toinhibiting factor produced by the large C. per- forans. The mollusc is situated most often on the outside wall of the bottom-most whorl of the Tegula shell near its opening. (See figure + 6.) In 40 animals examined, the head of the mol- lusc always pointed toward the opening. This position of the mollusc in the shell may have a great deal to do with its fil- ter-feeding mechanism. The current which it creates for feed- ing goes from left to right and at- 4- tachk to the side rather than the bottom of the shell may facilitate feeding by causing water to flow from the top of the shell to the bottom. (See figure + 7.) This position in the shell might also be enforced by the hermit crab. Further investigation of the currents of the C. perforans as they in- teract with the currents of the hermit crab may clarify the commensal relation of the two. DISPERSAL The uneven distribution of Crepidula perforans in the in- tertidal may be due in part to complex physical factors which were not investigated during this study. Instead this study concentrated on biological aspects of the mollusc which may account for the observation that two nearby areas may have wide differences in C. perforans population. The behavior of the young and the attraction behavior of C. perforans to hermit crabs were examined to gain knowledge about the dispersal of the molluscs. In Crepidula adunca the young do not undergo a pelagic stage. Upon hatching they are unable to attach to the shell and are pushed off by the mother. Presumably the young live somewhere in the intertidal until they are strong enough to hold onto the shell/ (Putnam, 1964/), In Crepidula perforans, which also does not have a pelagic stage, the behavior is some- what different. To study the behavior of the young the mother -5- was removed from the shell exposing the egg capsule which was torn open to hatch the young. The young were immediately able to attach to the shell and remain on it. (See figure + 8.) Their clinging ability may explain the uneven distribution of the mollusc. If the young are able to stay in the shell, dis- persal will not be effected in the same manner as in C. adunca. (See above.) Perhaps the C. perforans rarely leave the shell they were born in or do so at a more advanced age than C. adunea. Certainly the distribution of the mollusc indicates a dispersal mechanism which is effective only over a small range. If in- deed under natural situations the young are able to cling to the shell some discrepancy is found in comparing the number of eggs per mother and the number of C. perforans per shell. The largest number of molluscs found on a single shell was 14, while the average number of eggs per mother was 150. Somehow the large excess must be gotten rid of, either by the crab or im the mother. The attraction of C. perforans to members of its own spe- cies or to Pagurus is probably a very important factor in dis- persal of the molluscs. Initial attempts to establish chemo- taxis in C. perforans using glass dishes nine inches in diam- eter with shells and/or hermit crabs placed at intervals along the outer perimeter and C. perforans in the center were unsuc- cessful due to the immobility of the mollusc. This very char- acteristic may account in part for the animal's uneven distri- bution. Certainly the mollusc with its small foot and contoured shell could not traverse great distances to lodge in a hermit crab, so experiments were set up to see if attraction could be established over small distances. First ten C. perforans were placed on the bottom of a small (4inch by 4 inch) plastic dish inwwhich was also placed a crab or an empty shell. Be- coming attached to the crab or shell was considered evidence of attraction. In three instances out of seven trials C. per- forans were able to attach toethe outside of a hermit crab mov- ing about in the dish. In these instances the method by which the C. perforans became attached to the shell was not observed so a new set of experiments was devised. Small C. perforans were placed 5 mm from stationary Pagurus and were observed for two hour periods for signs of attraction. In two instances out of six the mollusc was able to lift itself onto the out- side of the shell and crawl to the inside. Attraction of small C. perforans males to large females was also examined for in- sight into dispersal. In C. adunca the young males are attracted primarily to females rather than shells. (See Putnam, 1964,). It was thought that this might also be true for C. perforans. To test this hypothesis small males were placed 5 mm from a large female and were observed for two hours for evidence of attraction. No marked attraction of young males toward large females was observed. The attraction of the small C. perforans -7- to Pagurids was much more than the attraction to females, even though in the field many times young were seen on top of females. It thenet thet thes spssies of Cpeptdee is p- tendie het peres it dess pet gu Be . C. perforans did not seem to be attracted at all to uninhabited Tegula funebralis shells. In no case were they observed attached to uninhabited shells. In addition to thetattraction of C. perforans to Pagurus interactions among hermit crabs were investigated as possible means of dispersal of C. perforans. A tank was set up contain- ing ten unmarked hermit crabs with C. perforans and ten marked Pagurus without them. Appearance of C. perforans in the marked shell was looked for. Transfer of C. perforans from a hermit crab bearing them to another hermit crab not hav- ing them occurred once during a three week period. The means of transfer is not known. However, the hermit crab might be effective in dispersing the molluscs by carrying them on its body when changing shells, by close contact occurring when Pagurus are mating, or by close contact occurring when inact- ive crabs are clustered together. SUMMARY Crepidula perforans (Valencienes, 1846), a gastropod liv- ing inside hermit crab shells, is unevenly distributed in the area of China Point at Hopkins Marine Station. The mollusc is not found in shells less than 0.25 inches in diameter. It is most often, though not always, found in Tegula funebralis shells. The distribution of the mollusc in different species of Pagurus is varied. The animal is situated in the outermost whorl of the shell with its head fading the opening. The larger the crab, the more C. perforans it is likely to have. The larger the Crepidula inhabiting the shell, the fewer found in the shell. The young molluscs are able to cling to the shell upon hatching. C. perforans is attracted to Pagurus but not to mem- bers of its own species or T. funebralis shells. The molluse is able to move from one hermit crab to another. -9. LITERATURE CITEL Belknap, Robert and John Markham The intertidal and subtidal distribution of 1965. Unpublished manuscript on file at Hopkins Mar- Pagurus. ine Station, Pacific Grove, California. Wesley R. Coe, Divergent methods of development in morphologi- 1949. cally similar species of Prosobranch Gasteropods. Journ. Morph. 84: 383-399. Putnam, Deborah A. The dispersal of young of the commensal Crepidula 1964. from its host, Tegula funebralis. The Veliger. adunca 6 (supplement): 63-66; 4 text figures. Moritz, Charles The anatomy of the gasteropod Crepidula adunca 1938 Univ. Calif. Publ. Zool. 43: 83-92; 6 text Sowerby. figures. Organogenesis in the gasteropod Crepidula adunca 1939. Sowerby. Univ. Calif. Publ. Zool. 43: 217-248; 22 text figures. Smith, Allyn G. and Mackenzie Gordon, Jr. The marine mollusks and brachiopods of Monterey 1948. Bay, California, and vicinity. Proc. Calif. Acad. Sciences. 25 (+ 8): 147-245. pa COTTOM AND SIDE VIEWS OF Tequla funebmalis SUEUS SHOWING POSTTION OF Crepidula perforans Fi. 11 S g L V sia. Ja 333 325 9 4 3 E 2 L 27 P Fr 8 6 3 . 3 ? 152 : : ..315 . . * 4 . ..: 8 : 1 3 : .. . . *3 . : : 8 katatatavakatatavatatatataa- P Fe O 8 10 11» -7 Tog Veu O Sel Moie PoSfTors n unien 104 MONVIDUNS VERE. FOUND Fio 46. E aer CREPIDULA CURRENTS VITAMN TAE TEEUA SHEU FIG.E 1 C NURGER LLINGINE 8421 13 10 1a 3— ABLIT OFYOUNG TO CLING TO SHEU UP HATCH INE — Group a —— ORoup 12. ...... Gp (Voune) TUAE IN HOURS 3 TIC 28