Distribution of Two Species of <I>Dissodactylus</I> (Brachyura
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BULLETIN OF MARINE SCIENCE, 28(4): 651~58, 1978 DISTRIBUTION OF TWO SPECIES OF DISSODACTYLUS (BRACHYURA: PINNOTHERIDAE) AMONG THEIR ECHINOID HOST POPULATIONS IN BARBADOS Malcolm Telford ABSTRACT A contribution is made to the biology and behavior of two species of Dissodactyills. The six-lunuled sand dollar, Mellita sexiesperforata (Leske), is a newly discovered host for Dissodactyills crillitichelis Moreira in Barbados. The fourth species of c1ypeasteroid with which this crab is known to associate, from 300/0 to 600/0 of the sand dollars examined housed crabs, which were normally located in the lunules or the food grooves. The fre- quency distribution of crabs on their hosts indicates a degree of aggregation which is probably attributable to sexually mature pairs remaining together and to preferential settlement of megalopae on more shallow burrowing hosts. On exposed beaches the sand dollars burrow more deeply than on sheltered beaches and larger individuals burrow deeper than smaller ones. Depth of burrowing influences host selection by the crabs but, apart from this, size of host (within the sample range) appears to be unimportant. Labora- tory experiments indicate that D. crillitiche/is leaves its host occasionally and may spend Some time away from it. Meoma velltricosa (Lamarck) is the first host to be found for the previously little-known species, Dissodactyills primitivus Bouvier, and is the first non-c1ypeasteroid host for any species of Dissodactylus. From 800/0 to 1000/0 of the M. ventricosa examined were found to house crabs, mostly externally, close to the mouth but several were found inside the esophagus. There are 14 species of Dissodactylus Very little is known about the biology of listed by Schmitt et aI., 1973. Some of these these interesting crabs. They appear to be are very poorly known, from single speci- generally symbiotic with echinoids, perhaps mens or only a small number of them col- exclusively so. The nature of their relation- lected at widely different times and places: ship to their hosts is not clear. The term for example, D. primitivus Bouvier 1917 was "inquilism" was used by Caullery (1952) for known until recently from only a single situations in which an organism was sheltered specimen collected 100 years ago and D. by and obtained at least part of its food from borradailei Rathbun 1918 was known from the host. Cheng (1967) rejected the term en- four specimens collected singly in Florida tirely, preferring to regard the relationship and Jamaica. Dissodactylus borradailei has as parasitic if there was any metabolic de- now been shown to be a junior synonym of pendence on the host. There is some evi- D. primitivus (Telford, 1978), leaving 13 dence that D. lockingtoni Glassell (GlasseIl, species in the genus at present. Seven of cited by Hyman, 1955) feeds on the feces these species are already known to be of its host, Encope emarginata (Leske), in symbionts of echinoids and in this paper the which case it could be regarded as a com- symbiotic relationship between D. primitivus mensal, although hardly in the terms of and a spatangoid urchin, Meoma ventricosa Cheng (1967) ".... literally eating at the (Lamarck), will be added to the list. Of the same table." The entire family Pinnotheridae remaining five species almost nothing is is often regarded as parasitic or at least semi- known except that D. calmani Rathbun has parasitic. The best known species are cer- been collected more than once in the absence tainly established as parasites, for example of an echinoid host. the oyster crab, Pinnotheres ostreum Say 651 652 BULLETIN OF MARINE SCIENCE, VOL. 28, NO.4, 1978 (Stauber, 1945) which shows several struc- ness to move from host to host, as evidenced tural and behavioral specializations for its in laboratory studies is described. particular life style. The genus Dissodactylus is of special interest in this regard because it MATERIALS AND METHODS shows no clearly parasitic modifications and might well be considered to be in an early Dissodactylus crinitichelis and stage of the evolution of the parasitic habit. Mellita sexiesperforata There is some slight evidence that D. Specimens were collected from four areas crinitichelis Moreira might obtain at least of the coast of Barbados, two of which some of its food directly from the food sup- were chosen as the principal sites for study. ply of its host, in which case it could be re- These were the northern end of Discovery garded as parasitic in Cheng's terms. Bay on the west coast and the eastern end The two species discussed in this paper are of Rockley Beach on the south coast. The D. crinitichelis and D. primitivus. The Discovery Bay site was well sheltered and former was described by Moreira in 1901 usually without noticeable wave action, (Rathbun, 1933) and by Rathbun herself whereas the Rockley Beach site was exposed later in 1901 under the name D. encopei. and subjected to continuous wave action so Both authors found it living with the echinoid that the bottom was always more or less Encope emarginata and suspected that it disturbed. All of the sites had clean sandy lived also with E. michelini Agassiz. It has bottoms. The sand dollars were collected been found with both of these hosts subse- in 3-8 ill of water by free-diving and im- quently (Williams et aI., 1968) and also with mediately placed in individual plastic bags the sea biscuit, Clypeaster subdepressus to avoid loss of the crabs. Diameter of the (Gray) (Wass, 1955). To this list can now be sand dollars was measured to the nearest added the six-lunuled sand dollar, Mellita millimeter along the functional antero- sexiesperforata (Leske), in Barbados. The posterior axis. The depth to which they bur- previously known specimens of D. primitivus rowed was measured to the apex of the test, were taken singly in dredges with no indica- taking care not to move them as they were tion of a possible host. Its rediscovery in located by hand. 1974 as a symbiont of M. ventricosa thus The extent to which the crabs would move provides the first host record. However, from one sand dollar to another was investi- Chesher (1969) reported the occurrence of gated in the laboratory water table. These an unidentified species of Dissodactylus on experiments lasted 4 or 5 days each. In the M. ventricosa. From his description and first, four sand dollars with identifying photographs there is every reason to suppose scratches in the petalloids were placed in a that it was, indeed, D. primitivus. glass dish which allowed limited movement; one sand dollar was carrying six crabs, the The only published data on the distribu- others none. At 24-h intervals the positions tion of any species of Dissodactylus within its of these crabs were noted. The second and host population is that of Gray et aI., (1968). third experiments were conducted with the They examined the activity of a "host factor" host echinoids free to move in the water which attracts D. mellitae Rathbun to the table. In one instance, one of the four sand five-Iunuled sand dollar, Mellita quinques- dollars started with five crabs; in the other perforata (Leske). At the same time they experiment five crabs were released 2.5 ill reported the incidence of the crab-host asso- away from the nearest host. The fourth ciations in a few small collections of sand experiment used nine sand dollars restricted dollars. In this paper the distributions of D. from free movement in individual petri dish crinitichelis and D. primitivus within their lids placed in a 90 em grid located in a slow host populations is examined and their readi- current by the water table outlet. These sand TELFORD: TWO SPECIES OF DISSODACTYLUS 653 dollars were given different numbers of collected was 0.4 and per host which actually symbiotic crabs at the outset, in an attempt had the symbionts, 1.4. The maximum to determine how they would move with number per host found here was three. At respect to the current and the presence of two other sites where smaller numbers of crabs on the other hosts. sand dollars were collected similar degrees of infestations (36% and 50%) were found Dissodactylus primitivus and and the mean number of crabs per infested Meoma ventricosa sand dollar was 1.3. The frequency distribution of D. crinitich- The spatangoid urchins were found in elis on M. sexiesperforata at Discovery Bay areas where a considerable amount of mud gave a poor fit to the Poisson distribution was mixed with sand. They were collected (P < 0.02) and an acceptable fit to the during scuba dives, 15-20 m deep, in negative binomial (P = 0.18). The Rockley Speightstown Harbour on the west coast. population did not fit either distribution well Each specimen was immediately placed in an (P = 0.001 and 0.02 respectively). Ac- individual plastic bag and tied. The actual 10- cording to Bliss (1953) the classes in the cations of the crabs on their hosts was re- tails should be pooled so that no expected corded underwater as they were collected. frequency is less than five. When that was Size of the host was measured to the nearest done the Rockley population gave a good millimeter excluding spines, along the longi- fit to the negative binomial. Too small a tudinal axis. Several specimens of M. ventri- sample was collected at each of the other cosa with their symbionts were maintained in sites to give a reliable fit to these distribu- the water table. tions. Inspection of the data for the Rockley The observed distributions of the crabs site showed that there were almost twice as were fitted to the Poisson distribution fol- many sand dollars housing two crabs as the lowing the method in Stanley (1963) and fitted distribution predicted.