Prey Preference of the Stone Crab Platyxanthus Crenulatus (Decapoda: Platyxanthidae) in Laboratory Conditions
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Nauplius 21(1): 17-23, 2013 17 Prey preference of the stone crab Platyxanthus crenulatus (Decapoda: Platyxanthidae) in laboratory conditions María Victoria Laitano*, Nahuel Emiliano Farías and Maximiliano Cledón (MVL) IIMyC, CONICET - Universidad Nacional de Mar del Plata, Deán Funes 3350, Mar del Plata. E-mail: [email protected]. (NEF) Laboratorio de invertebrados, Departamento de Biología, Universidad Nacional de Mar del Plata, Deán Funes 3350, Mar del Plata. E-mail: [email protected] (MC) IIMyC, CONICET - Universidad Nacional de Mar del Plata, Deán Funes 3350, Mar del Plata. E-mail: [email protected]. *Corresponding author ABSTRACT - The preference for a particular type of prey implies an active behavioral choice of the predator. The present work deals with the prey preference of the large stone crab Platyxanthus crenulatus among similar sized gastropods and bivalves of different substrates using a recent proposed methodology based on a two-stage approach. In laboratory, crabs received different mollusk species separately to determine the consumption in number of each species when choice was not possible. Further, crabs were offered the same mollusc species but together to record differences of specific consumption when choice was possible. Results were then compared to assess prey preferences. Platyxanthus crenulatus is able to effectively attack and consume the majority of the sympatric mollusk species, although preferred the dominant species Brachidontes rodriguezii over the others. Such preference seems to be driven by shell shape and thickness which may be a common pattern in prey preference among large stone crab species. Key words: bivalves, gastropods, South-western Atlantic, trophic behavior, xanthids INTRODUCTION patterns are influenced by intrinsic features The distribution and abundance of a consumer of prey and its interaction with the predator can be explained largely by the distribution (Underwood et al., 2004) rather than predator and abundance of the resources it uses and choice along. vice versa (Begon et al., 1996). However, In this sense, many different definitions most predators do not consume prey items and experimental designs have been suggested in proportion to their abundance, but select and used to test preference (Johnson, 1980; particular sizes and/or types of prey (Micheli, Newman et al., 2010; Saunders et al., 2012), 1995), generating non random associations. but there is certainly a consensus that it is Such associations are often explained essential to clarify the used definition before as a direct consequence of the preference for starting to work (Taplin, 2007). A relatively a particular prey (Underwood et al., 2004). new point of view has been developed by Nevertheless the concept of preference implies some authors (Singer, 2000; Underwood et al., the existence of active behavioral choice 2004). Singer (2000) stated that preference (Singer, 2000). Furthermore, consumption depends exclusively on predator and implies c Published by Brazilian Crustacean Society, Ribeirão Preto, Brazil - July 2013 18 Laitano et al.: Stone crab prey preference some predator own behavior. Underwood Juanes, 1992; Smallegange and van der Meer, and Clarke (2005) include this point in the 2003), works dealing with preference among definition: “When an animal is confronted different prey species are contrastingly rare with choices of resource, preference would be a (Ebersole and Kennedy, 1995; Mascaró and behavior that results in use not predicted from Seed, 2001) probably due to the complexity behavior when no choice is available” and this of such systems, in which different factors concept was the approach used in the present interact at the same time. study. In this work we investigate the prey Large crabs, particularly those which preference of P. crenulatus among different prey on mollusks, were successfully used as mollusk species. For this, laboratory model to test general hypothesis on prey experiments were carried out (1) to determine selection and optimal foraging (e.g. Cancer prey preference among preys of differing shell sp. - Yamada and Boulding, 1998; Carcinus shape and (2) to test for differences in the sp.- Smallegange et al., 2008; Menippe sp.- pattern of preference in different habitats. Aronhime and Brown, 2009). Platyxanthus crenulatus A. Milne-Edwards, 1879, is one of four edible crab species belonging to the South MATERIAL AND METHODS American family Platyxanthidae Guinot, 1977 Species and sampling (Ng et al., 2008). The species is restricted to In December 2008 and January 2009, 80 south western Atlantic coasts, inhabiting the individuals of Platyxanthus crenulatus were intertidal and shallow subtidal waters of north manually collected from the rocky shore of Patagonia (Boschi, 1964). Adults are more adjacent to Nágera Biological Coastal Station, common in subtidal rocky reefs, where are Mar del Plata (38°09’S / 57°38’W) and by the most conspicuous invertebrate predators. diving from Mar del Plata port and La Restinga In Mar del Plata these crabs are often seen reef. Simultaneously, the most abundant feeding on small mussels and sporadically mollusks cohabiting with these crabs (Scelzo on other invertebrates such as ascidians and et al., 2002; Farías, 2011) were collected polychaetes. However, their claws show strong from hard and soft bottoms. Hard bottom positive allometry in both sexes, with marked species were the gastropod Tegula patagonica heteroquely and laterality (i.e. dimorphism (d’Orbigny, 1835), the mussels Brachidontes between chelae and the tendency to present rodriguezii (d’Orbigny, 1842) and Mytilus them in a particular side of the body); both edulis Linnaeus, 1758, which were collected traits often associated to durophagous habits by hand from the intertidal area of Waikiki (Seed and Hughes, 1995). The general beach, Mar del Plata (38°04’S / 57°32’W). The morphology of the species is the classical of soft bottom species were the clam Mesodesma the large stone crabs, i.e. conspicuous animals mactroides Reeve, 1854 which was collected with robust bodies that reach large sizes and manually at Punta Mogotes beach, Mar del strong claws. Such crabs are usually high- Plata (38º03’S / 57º 32’W) and the gastropod level predators (Gerhart and Bert, 2008) and Buccinanops monilifer (Kiener, 1834) which dominant over other competitor crab species was collected in shallow waters off that beach (Brown et al., 2005). using a dredge (100 cm opening, 30 cm height Mollusk species differ in life history and 1 cm mesh size) at 5 m depth. traits, morphology and shell thickness among Only crabs ≥ 50 mm in carapace width others. Moreover, they often inhabit different (adults) with undamaged chelae and in the late substrates and consequently present distinctive inter-moult stage were used in the experiments defenses, such as burying and/or clustering to avoid any interference of the life cycle or behaviors, which may decrease encounter moult stages in the results. The commonest rates or increase handling times. In turn, prey sizes found in the field varied between 25 the different shell morphologies can affect to 30 mm in T. patagonica, 20 to 30 mm in B. selection patterns (Bourdeau and O’Connor, rodriguezii, 35 to 45 mm in M. edulis platensis, 2003; Addison, 2009). Whilst the prey size 25 to 40 mm in B. monilifer and 35 to 45 preference has received much attention (see mm of M. mactroides. Animals were kept in Nauplius 21(1): 17-23, 2013 19 500 l tanks with flow-through seawater. Prior when they were on their life position, crabs to experiments, crabs were starved during 72 were placed in the tanks. Due to the small size hours to standardize hunger levels. of B. rodriguezii, it was placed in groups of a similar volume to one individual of the other Experimental design and statistical analysis prey species. Each group was considered as a Experiments were carried out under unique individual in statistical analysis. 12L:12D light conditions in 100 l close system sea water aquaria and containing a 10 cm sand Stage 2: Consumption with choice bottom to ensure that M. mactroides could In the stage 2, crabs were allowed to reach its usual life depth (Cledón and Nuñez, choose among the prey species. It consisted 2010). To provide substrate to preys and refuge on three experiments (E1-E3) where each to crabs, rocks were placed at the center of crab was offered: three individuals (or groups) each tank simulating the rocky outcrops found of each hard bottom species (E1), three in the natural habitat. Water was continuously individuals of each soft bottom species (E2) oxygenated through air injection pumps and and three individuals of all prey items (E3), replaced each 48 hours. simultaneously (Fig. 1). Ten replicates of each The experimental design and the experiment were done and after 48 hours statistical analysis were performed according the number of individuals consumed of each to Underwood and Clarke (2005) based on prey species when choice was possible was the definition of preference exposed in the registered. All trails were carried out within a previous section. The experiments consisted in six-week period, with different individuals of two stages as follows: in the first one, individual P. crenulatus and different individuals of preys crabs were presented with a single prey species; in order to avoid autocorrelation of data. in stage 2, individuals were offered all prey types simultaneously (Fig. 1). Preference Test Some difference in the consumed Stage 1: Consumption without choice proportions between stages 1 and 2 for at least In this stage we first determined if P. one prey species would be indicating preference crenulatus is able to consume the offered prey by P. crenulatus, since its feeding behavior species. Further, for the prey species they would differ according to the possibility of can consume, we registered the number of choice or not. Thus, the consumed numbers consumed individuals of each species when of each prey species were compared with the choice is not available.