BULLETIN OF MARINE SCIENCE, 65(2): 391–405, 1999 SHELL UTILIZATION BY THE HERMIT CRABS DIOGENES PUGILATOR (ROUX, 1829), PAGURISTES EREMITA (LINNAEUS, 1767) AND PAGURUS FORBESII BELL, 1845 (CRUSTACEA: DECAPODA: ANOMURA), IN A SHALLOW-WATER COMMUNITY FROM SOUTHERN SPAIN M. E. Manjón-Cabeza and J. E. García Raso ABSTRACT Gastropod shells used by the three dominant hermit crabs, Diogenes pugilator (Roux, 1829), Paguristes eremita (Linnaeus, 1767), and Pagurus forbesii Bell, 1845, of the detritic littoral bottoms from Barbate Bay (Cadiz, Spain) were analyzed. The study showed that these hermit crabs species have different patterns of gastropod shell use. Paguristes eremita, the largest and strongest species, inhabits heavier gastropod shell species with wider aperture (belonging to the Muricidae family), while, Diogenes pugilator and Pagurus forbesii, inhabit smaller and lighter shells (mainly those belonging to the family Turridae). Diogenes pugilator, despite being clearly the more abundant species, does not use the most abundant species of the gastropod community (Turritella turbona), which instead is used by Pagurus forbesii. However, no morphological relationships between these her- mit crabs and the diameter of shell aperture have been found, either in relation with the whole gastropod shells used or in relation with the more specifically used shells. On the other hand, specimens of D. pugilator with cephalothoracic shield widths larger than the shell aperture have been found, however, this result has not been found in P. forbesii or in Paguristes eremita. Also, in these three species no differences in shell use by sexes exist. These and other data indicate that D. pugilator does not make a strong shell selection, perhaps in part, due to a competition with P. forbesii and a scarcity of available useful shells in the area, which are a fundamental limiting factor. On the contrary, P. eremita seems to use adequate shells, a point that allows us to speculate that this species makes (with or without competition with the other hermit crabs) a real selection. Finally, the data about relative growth of the cheliped of D. pugilator in different areas, in which they use different shells, are similar. These data contrasts with the stunting hypothesis. Studies related to the utilization of gastropod shells by hermit crab populations have been based on direct observation in natural conditions (Bertness, 1980, 1981a, 1982; Blackstone, 1986a, 1989; Gherardi and Vannini, 1989; Lancaster, 1990; Pessani et al., 1990; Gherardi, 1991; Pessani and Premoli, 1993) or through laboratory studies (McLay, 1985; Walters and Griffiths, 1987; Lively, 1988; Lowery and Nelson, 1988; Hazlett, 1989, 1990,1992; Bertness, 1981b; Asakura, 1992). Direct observations are basically made on intertidal species and they permit the de- scriptions of interspecific and intraspecific competition for the available gastropod shells in this environment. On the other hand, laboratory experiments are only partially useful to learn the relationships among the species and their surroundings. Gastropod shell utilization by different species of hermit crabs in natural intertidal habitat studies are scarce because there are many difficulties and factors without control; however, they allow us to know more about the real behavior of the species in their natural habitats and the reactions to different conditions (Bach, et al., 1976; Gherardi, 1991; Gherardi and Vannini, 1989; Lowery and Nelson, 1988). 391 392 BULLETIN OF MARINE SCIENCE, VOL. 65, NO. 2, 1999 The authors tried to analyze how the allometry between the chelipeds and cephalotho- rax shield length can be altered if an individual of a hermit crab species uses a particular shell over a period of time (Wilber, 1990). There are two potential explanations that have made in some interesting papers which will allow us to discuss it. First, “the molding hypothesis” suggests hermit crabs may conform to the shape of the shell aperture when their exoskeleton is malleable during the postmolt period (Golsdschmidt, 1940; Wolff, 1961). The second hypothesis, “stunting hypothesis”, states that hermit crabs whose overall growth is stunted from occuping poorly fitting shells (Markham, 1968; Fotheringham, 1976a,b; Bertness, 1981a,c) acquire relatively large chelipeds. This paper seeks to describe and analyze the utilization of the shells inhabited by the dominant species of hermit crabs of shallow detritic bottoms in relation to the living community in the study area. Furthermore, it tries to explain the adaptation of the hermit crabs in the gastropod shells, the influence of this fitness and in aspects such as inter- and intra-specific compe- tition and the utilization of the largest cheliped as a cover of the shell aperture. MATERIAL AND METHODS The study area. (Fig. 1) is located in Barbate Bay, province of Cadiz, in the neighborhood of the Straits of Gibraltar, between 36°08.73'–36°09.71'N and 05°55.19'–05°53.59'W. The analyzed benthos are situated between 15 and 24 m deep and we selected for sampling two transects separated by 2.5 km, one in front of the village of Barbate (B) and the other one further east, in front of Retín (R), taking in each one two samples at 15–18 m (B1, R1) and at 24 m (B2, R2), respectively. Samples were taken from October 1993 to December 1994. For sampling, a small heavy dredge similar to a rock-dredge, with a rectangular frame of 42 × 22 cm, and a double net were used; the inside mesh size was 4.5 mm. In the laboratory, sediment was washed over a sieve column with a mesh size between 1 cm and 1 mm. The fauna were separated, and the hermit crabs and their gastropod shells were identified based on the works of Zariquiey Álvarez, 1968; Ingle, 1993; Sabelli et al., 1990, 1992a,b; Pope and Goto, 1991. The sediment is mostly detritic composed of coarse sand, fine gravel and Amphioxus sand, under bottom currents, with abundant bioclastic remains (bivalve and gastropod shells). The tidal cur- rents locally increased by the existence of flagstones that guided them, and the main one by the proximity of Punta del Tajo and Trafalgar Cape. For relative growth, the anatomical structures were analyzed with a VID V computer program that processed stereoscopic microscope images taken by a video camera, with a measurement error of 0.001 mm. Four dimensions were measured: cephalothoracic shield length (SL): maximum length, from rostrum to posterior midpoint of shield; cephalothoracic shield width (SW): maximum width; cheliped length (CHL): from the distal part of propodal prolongation to the basal part of propodus; and cheliped width (CHW): maximum propodal width. The maximum diameter of the gastropod shell aperture (SAW) was measured with a caliper (measurement error: ± 0.05 mm). To determine the relationships between morphological parameters of hermit crabs species and gastropod shell aperture widths, the correlation coefficients were computed. Kendall (t) or Pearson (r) was applied depending on whether the parameters showed a non-normal or normal distribution following the significance of the Kolmogorov-Smirnoff test. MANJÓN-CABEZA AND GARCÍA RASO: SHELL UTILIZATION BY HERMIT CRABS FROM SOUTHERN SPAIN 393 Figure 1. Location of the study area showing sampling stations (R1, B1, R2 and B2). Additionally, simple (linear, logarithmic, quadratic, potential and exponential) and multiple re- gression analyses were carried out on shells used, and on dominant gastropod shell species use by hermit crabs: D. pugilator, P. eremita and P. forbesii. Since the abundance of D. pugilator was large, it was possible to analyze shell-utilization by sex. For this, a χ2 test (Siegel, 1972) was applied. Also, on the principally used shells, a percentage comparison test was carried out (Lamotte, 1988) between sexes and between ovigerous and non- ovigerous females. RESULTS In the study area, 42 species of living gastropods were found (the pateliform species, such as Calyptraea chinensis Linnaeus, 1758, were excluded from the study). The species 394 BULLETIN OF MARINE SCIENCE, VOL. 65, NO. 2, 1999 Figure 2. Composition of living gastropod community in Barbate Bay. MANJÓN-CABEZA AND GARCÍA RASO: SHELL UTILIZATION BY HERMIT CRABS FROM SOUTHERN SPAIN 395 distribution in the study area revealed the dominance of T. turbona Monterosato, 1877, Gibbula magus Linnaeus, 1758 and Mesalia varia (Kiener, 1887) (Fig. 2). DIOGENES PUGILATOR.—D. pugilator occupied shells of 27 gastropod species (Fig. 3); shells of the remaining 15 species were of very small sizes and they only represented 11.59% of the total number of living specimens of the gastropod community. Also, there were occupied shell of other eight species imported from elsewhere, but they only repre- sented 4.68% of the total shells species used. Other mollusk shells occupied by D. pugilator were two species of Scaphopoda (Den- talium novemcostatum Dautzenberg, 1891 and Dentalium vulgare Da Costa, 1778), but with a very small occupation percentage (Table 1). The utilization of Dentalium shells has been documented elsewhere. An uncommon finding was the 0.1% use of Polychaeta tubes. We have found this kind of occupation in other samples but were not included in this study. The shell species used by D. pugilator (Fig. 3) in the area were Mesalia varia, followed by Turritella communis Risso, 1826, Nassarius reticulatus (Linnaeus, 1758), Nassarius mutabilis (Linnaeus, 1758), T. turbona and G. magus, which represented 76.35% of the shells used by this hermit crab. These species were dominant in the living gastropod community, but in a different order (Fig. 2). Of gastropod families, Turritellidae species were used by D. pugilator (54.56%) fol- lowed by Muricidae (28.89%) (the same sequence as in the living gastropod community). D. pugilator showed a clear preference for Naticidae species over Trochidae, even though the latter family was more abundant in the living gastropod community. A general analysis of the use of shells (all species) by males and females shows that there was no significant difference (test χ2 at 99.95% of confidence level); both sexes have the same pattern of shell-use but, significantly different percentages were found in G.