Ruditapes Philippinarum*

MARINE ECOLOGY - PROGRESS SERIES Vol. 40: 87-93, 1987 Published October 7 Mar. Ecol. Prog. Ser. 1 I Mechanics of prey size preference in the gastropod Neverita didyma preying on the bivalve Ruditapes philippinarum* C. L. Rodrigues", S. Nojima, T. Kikuchi Amakusa Marine Biological Laboratory. Kyushu University, Tornioka. Reihoku-cho. Arnakusa, Kumarnoto-ken 863-25, Japan ABSTRACT: Laboratory experiments on Neverita didyma (Roding) preying on Ruditapesphilippinarum (Adams & Reeve) indicate distinct prey size preference which is a function of predator size. Prey size limits are determined by foot size, the organ used in captunng and handling prey. When presented with 2 prey patches equal in area but containing different prey sizes, the predator orients itself in the direction of the preferred prey size. N. didyma is unable to assess prey shell thickness and relies solely on prey size, gauged by the ease of 'handling', for selecting prey. Evidence and arguments are presented to show that optimal foraging in N. didyma, and possibly other predators which seize their prey, evolved from passive mechanical selection leading to increased predator fitness. INTRODUCTION expected from the optimal diet model (Hughes & Elner 1979, Hughes & Seed 1981, Jubb et al. 1983). Experi- Drilling by gastropods is a relatively recent form of ments were planned to investigate the prey size prefer- predation which arose during the marine mesozoic ence of Neventa didyma, its relation to predator size revolution wherein the infaunalization of soft-bottom and its relevance to optimal foraging theory. Results of benthic organisms occurred (Vermeij 1977). Note- our experiments are compared with those obtained for worthy among such gastropods are the naticids and other predators which seize their prey. muricids which prey on bivalves. In an earlier report (Rodrigues 1986), predatory habits and functional responses of the naticid gastropod Neverita didyma MATERIALS AND METHODS preying on the bivalve Ruditapes phhppinarum were described. The predators used, Neventa didyma, were collected The present paper deals with the naticid's preference in December 1984 from Tsuyazaki, Fukuoka Prefec- for specific prey sizes. Size selective predation is a ture, in Northern Kyushu, Japan, via SCUBA diving. potentially important factor affecting population struc- They were maintained in the laboratory in plastic tanks ture as it may lead to the depletion of certain prey size with a layer of fine sand and supplied with filtered classes (Ebling et al. 1964, Seed & Brown 1975, Corn- running seawater. Additional collections were made in mito 1982). It has also been employed for testing opti- March 1985. The gastropods were fed an abundant mal foraging theory, which predicts the foraging supply of bivalves Ruditapes philippinarum, collected behaviour of animals by assuming that fitness is a from the intertidal shore facing the Amakusa Marine function of foraging efficiency (Pyke et al. 1977, Biological Laboratory (Amakusa), which served as Hughes 1980, Pyke 1984). However, there are cases prey. As a rule, the duration of each experiment was where predators did not specialize exclusively on opti- 7 d, allowing a sufficient number of prey to be con- mally sized prey when these were abundant as sumed. For standardizing hunger levels, 2 d starvation were found sufficient. Predators were grouped on the Contribution No. 318 from the Amakusa Marine Biological Laboratory basis of operculum length; prey, on the basis of shell ' ' Present address: Department of Marine Science, Goa Uni- length. Size classes were arbitrarily fixed at 5 mm versity, Bambolim, Goa 403 005, India intervals. Additional details are given by Rodrigues O Inter-Research/Printed In F. R. Germany 88 Mar. Ecol. Prog. Ser. 40: 87-93, 1987 (1986). Shell measurements were taken using slide grinding, the degree of reduction was not uniform calipers (accuracy of 0.1 mm); whell thickness at the among all prey and ~twas not possible to estimate the bore site, using dial calipers (accuracy of 0.05 mm). amount of shell material removed without sacrificing Shell size preference. Preference experiments were the prey. Eight each of ground and unground prey carried out in April/May 1985. The predators were were offered to the predators in trials lasting 3 wk. individually placed in separate plastic tanks (38 X 26 X Behavioural experiments. Additional information on 16 cm), arranged in a cascade, containing a 5 cm layer size selection was obtained by recording the movement of fine sand collected from the subtidal region. The of the predator, in the presence of patches of different experimental set-up consisted of 24 tanks placed in 4 sized prey, using an Olympus VX-304 (Japan) colour rows, each row containing similar sized individuals video camera. The predator (25 to 30 mm class) was serving as replicates. The 4 predator classes, based on placed in a large plastic tank (64 X 48 X 14 cm) with a the operculum length, were 20 to 25 mm, 25 to 30 mm, 2 cm layer of artificial sediment (glass beads, 0.4 mm 30 to 35 mm and 35 to 40 mm; the prey classes, on the dia.) to provide better contrast. Four experiments were basis of their shell length, were 15 to 20 mm, 20 to carried out. Experiment 1 sewed as a control and no 25 mm, 25 to 30 mm and 30 to 35 mm. prey were present. The movement of the predator was We conducted 3 types of prey size preference experi- recorded over 24 h and ~tsposition at 2 min intervals ments. (Expenment 1) Six individuals of each prey marked. Two prey sizes (25 to 30 and 15 to 20 mm) class (total of 24) were offered to each predator in the were then presented in patches of roughly equal areas. cascade. (Experiment 2) Numbers of prey were offered Movements of bivalves out of each patch were that roughly presented equal surface areas to the restricted by a wooden ruler. In Experiment 2, the predators; this corresponded to 6, 4, 3 and 2 individuals patches were located in the proximal left and right of the respective prey size groups. (Experiment 3) Nine corners of the tank. This experiment was repeated by individuals of the 15 to 20 mm prey class and 4 indi- interchanging the position of the patches (Experiment viduals of the 30 to 35 mm prey class (to equalize 3). In Experiment 4, the 2 prey patches were presented surface areas), which represented extreme size classes, in the &stal left and proximal right corners of the tank. were offered. In order to quantify prey size selective Prey energy content. The prey length-weight rela- predation, Chesson's alpha index (Chesson 1978, 1983) tion, used to estimate weekly consumption of the was selected among the many preference indices avail- predator, was determined by least squares regression able in the literature (see Lechowicz 1982 for recent (Sokal & Rohlf 1981). Bivalves were left in clean sea review). This index has the advantage of being water for 24 h to clear their digestive tracts, and their unaffected by relative abundance of food types, flesh was dried in an oven at 60°C to constant weight. facilitating comparison of samples. It also allows for Caloric content was then determined using a mi- changing numbers of prey as they are consumed. For m crobomb calorimeter (Ogawa Seik CO, Model O.S.K. prey types, values above l/m indicate preference. The 150, Japan) in a constant temperature room (20°C). All preference index is given by the equation: statistical analyses are adapted from Sokal & Rohlf (1981). RESULTS where i = prey type; n,, = number of prey of type i offered; ri = number of prey type i consumed. Shell size preference Shell valve preference. In addition to studying size selection, the preference for either valve of the prey Results of Experiments l to 3 are listed in Table 1. was studied by noting the valves drilled in the present Progressively larger predators preferred larger prey experiments and in a previous study (Rodrigues 1986). both in Experiment 1 with equal number of each prey Shell thickness preference. We also tested whether size category and in Experiment 2 with equal areas of the predators were able to assess the thickness of the each category. In the latter, preference indices for prey shell or to respond to tactile clues. Prey of the predator sizes 35 to 40 mm could not be calculated as same size (25 to 30 mm) but of different handling costs the predators depleted all prey of sizes 30 to 35 mm. were prepared by artificially grinding the shells at the Results of Experiments 1 and 2 were pooled to reveal umbonal region (which would in turn reduce drilling the general trend of size preference exhibited by time). It was not necessary to grind other regions of the Neventa dldyma (Fig. 1).The smallest predators (20 to shetl since the bivalves are drilled in the vicinity of the 25 mm) did not exhibit strong preferences for a particu- umbo (Rodrigues 1986). Post-grinding mortality was lar prey group, but preference for increasingly larger not observed. Although shell thickness decreased after prey was evident for successively larger predators Rodrigues et al.. Mechanics of prey size preference 89 Table 1. Neventa didyrna preying on Ruditapes philippinarum. Results of preference experiments using 4 prey sizes (15 to 20, 20 to 25, 25 to 30, 30 to 35 mm; denoted by subscripts 1 to 4).The preference index used is Chesson's oc index and, for m prey types, values above 1 m-' indicate preference. Values in brackets Indicate number of prey offered per predator Experiment l Predator size Preference index (mm) W I a:! e4 20-25 0.284 ' 0.256 0.230 0.230

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