J. Mar. Biol. Ass. U.K. (2000), 80, 95^102 Printed in the United Kingdom Feeding of the spider crab Maja squinado in rocky subtidal areas of the R|¨a de Arousa (north-west Spain) C. Berna¨ rdez, J. Freire* and E. Gonza¨ lez-Gurriara¨ n Departamento de Biolox|¨a Animal, Biolox|¨aVexetal e Ecolox|¨a, Universidade da Corun¬ a, Campus da Zapateira s/n, 15071 A Corun¬ a, Spain. *E-mail: [email protected] The diet of the spider crab, Maja squinado, was studied in the rocky subtidal areas of the R|¨a de Arousa (Galicia, north-west Spain), by analysing the gut contents of crabs caught in the summer and winter of 1992. The highly diverse diet was made up primarily of macroalgae and benthic invertebrates that were either sessile or had little mobility. The most important prey were the seaweeds Laminariaceae (43% of the frequency of occurrence and 15% of the food dry weight), Corallina spp. (38% and 3%), molluscs [the chiton Acanthochitona crinitus (15% and 1%), the gastropods Bittium sp. (30% and 2%),Trochiidae and others and the bivalve Mytilus sp. (32% and 12%)], echinoderms [the holothurian Aslia lefevrei (32% and 18%) and the echi- noid Paracentrotus lividus (16% and 7%)] and solitary ascidians (18% and 6%). The variability in diet composition was determined by the season (Laminariaceae, Corallina spp., P. lividus, Mytilus sp., gastropods and chitons appeared in greater frequency in winter, while the solitary ascidians and A. lefevrei were consumed to a greater extent in summer) in addition to sexual maturity (prey such as Bittium sp. orTrochiidae were more common in juveniles). Moreover, the changes in the food consumption rate were linked primarily to the moult stage. Feeding activity plummeted during the phases immediately preceding and following ecdysis (stages D0^D3^4 and A), and the diet was less diverse during these phases. No feeding di¡erences were found that could be linked to sex. The composition of the diet of Maja squinado appears to be determined by the seasonal abundance of the di¡erent prey in subtidal rocky areas and by their availability (depending on their behavioural and anatomical characteristics, mainly mobility and the presence of hard external struc- tures). Moreover, life history factors have little importance in the variability of the diet composition and only the moult cycle has a considerable e¡ect on feeding rate. INTRODUCTION ¢eld (Carlisle, 1957; Kergariou, 1974). Laboratory experi- ments done by Stevcic (1967) indicate that the spider crab The spider crab Maja squinado (Herbst, 1788) (Crustacea: has an omnivorous diet. Data from studies based on gut Decapoda: Majidae) is a species of great commercial impor- contents are scarce (Brosnan, 1981; Kergariou, 1974; tance which is distributed along the north-west Atlantic Stevcic, 1967, 1968), but they also point to an omnivorous coast and in the Mediterranean, from the subtidal level down diet based on the consumption of prey common in the to depths of 90 m (Kergariou, 1984). Due to the importance habitat. to ¢sheries of this species in the waters o¡ the coast of Galicia (north-west Spain), studies focusing on reproduction, growth, migration and ¢sheries have been carried out to date (Gonza¨ lez-Gurriara¨ n et al., 1993, 1995, 1998; Gonza¨ lez- MATERIALS AND METHODS Gurriara¨ n & Freire, 1994; Hines et al., 1995; Freire & Sampling area Gonza¨ lez-Gurriara¨ n,1998; Freire et al.,1998). In the R|¨a de Arousa juveniles of the spider crab The crabs were caught in the R|¨a de Arousa, on the inhabit shallow areas (up to 15 m deep), with mixed rocky north coast of the O Grove Peninsula (428290N 098560W, and sandy bottoms and their movements are non Galicia, north-west Spain). The capture areas were directional and slow (less than about 10 m per day). shallow waters (510 m deep), with rocky vegetated Immediately after the onset of sexual maturity and the bottoms alternating with sandy bottoms. This type of terminal moult, these animals begin moving somewhat habitat is typical of juveniles and adults that have just faster. From one to three months after the terminal moult, undergone the pubertal moult (Gonza¨ lez-Gurriara¨ n & they begin to migrate towards deeper waters (Gonza¨ lez- Freire, 1994; unpublished data). The crabs were caught Gurriara¨ n & Freire, 1994; Hines et al., 1995; Freire & only on rocky bottoms. Gonza¨ lez-Gurriara¨ n, 1998). This paper examines diet Two samplings were carried out in summer (20 July and composition and gut fullness (as an indicator of feeding 7 August 1992) and one in winter (22 December 1992) in rate) of individuals caught in shallow areas, that is, juve- which 132, 158 and 97 specimens of Maja squinado were niles and adults that have just undergone the pubertal caught respectively. The glass-box, which allows direct moult. observation of shallow bottoms (510 m), was used to locate There is little information available on the feeding the crabs, that were caught from the vessel with a boat- habits of M. squinado based on direct observations in the hook. Journal of the Marine Biological Association of the United Kingdom (2000) 96 C. Berna¨ rdez et al. Spider crabfeeding After the crabs were caught, the carapace length (CL, where ni is the number of occurrences (number of guts in in mm) was measured in all specimens and the right which the prey i is present) and N is the total number of cheliped length in males. The relationship between guts containing food. (ii) Dry weight index (DWI): cheliped size and carapace length was used to determine percentage of standardized dry weight of the gut contents morphometric maturity in males by means of a discrimi- corresponding to each prey. To calculate this index, it is nant equation that was calculated for the population in necessary to have previously estimated the dry weight of the sampling area (Sampedro et al., 1999). Also deter- each prey in each gut (DWij), depending on its estimated mined were abdomen morphology, gonad development importance determined by the point method: stage (Gonza¨ lez-Gurriara¨ n et al., 1993, 1998) and the presence of a brood in females. Sexual maturity in PIij females was determined by following morphological DW DW (3) ij st;j  PI criteria: £at abdomen in juveniles and domed abdomen j with developed pleopods in adults. The stage of the intermoult cycle was determined by where PIij are the points assigned to prey i in gut j, and examining the endite of the second maxilla according to PIj are the total points assigned to gut j, and DWst, j is the the criteria of Gonza¨ lez-Gurriara¨ n et al. (1995) [adapted standardized dry weight of contents of gut j. from Drach & Tchernigovtze¡ (1967)], and using the nomenclature of Moriyasu & Mallet (1986) (immediate n postecdysis, A; postmoult, B; intermoult, C; premoult, DWIi DWi/ DWst 100 (4) D0, D1', D1'', D1''', D3^4). j 1 !  The cardiac stomach was extracted from each X X specimen and ¢xed in a solution of 4% formaldehyde in seawater. After 48^72 h they were changed to 70% This index provides information on the importance of a alcohol in which the gut was preserved until analysed. particular prey in the diet in terms of biomass. There could be an overestimation of prey having hard parts, which are slower to evacuate, than softer prey. However Analysis of gut contents this bias would be partially compensated because the Gut fullness was determined by visually estimating the contribution of each gut to the index increases with the percentage of gut volume occupied by the contents (the fullness level. The guts having a greater fullness are those points method) (Williams, 1981; Freire & Gonza¨ lez- from crabs which had ingested food most recently, and Gurriara¨ n, 1995; Freire, 1996). Each prey was determined are, therefore, more representative of the true proportions at the lowest possible taxonomic level. Next, each one was of the prey in the diet. assigned a score in terms of its relative importance, esti- The analysis of seasonal and life history (sex, maturity mated visually, as compared to the total level of fullness. and moult stage) variations in the consumption of the The fragmented state in which the prey are found in the di¡erent prey was carried out by means of analysis of gut contents of the decapods makes them impossible to variance (ANOVA, using the DWI) and ¢tting of contin- sort, which would be necessary to obtain other quantita- gency tables using log-linear models (using frequency of tive indices such as weight (Williams, 1981). The dry occurrence). Only guts with food were used. In the cases weight of the contents of each gut was found (at 608C for where there were signi¢cant di¡erences in the consump- 24^48 h). tion of a prey category between the di¡erent stages of the moult cycle (the only factor having more than two cate- gories), paired comparisons between these categories were Data analysis carried out a posteriori by means of Tukey tests of multiple For data analysis, the 100 types of prey ¢rst determined comparisons (Day & Quinn, 1989). were grouped into 30 categories exclusive of each other. Diet diversity was estimated by the Shannon^Wiener (H') index: The standardized food dry weight (DWst, which is equivalent to the weight of the gut contents of an indivi- dual with the same fullness and carapace length identical n to the sample average, 115.2 mm.) was used to measure H0 pi log2 pi (5) i 1 gut fullness. The food dry weight (DW) was standardized X in terms of body size (CL), based on the regression relating the food dry weight (DW) to the CL: where pi is the proportion of prey i in the diet, and n the total number of prey categories present in the group of crabs analysed.