Feeding habits of Car cinus aestuarii RB Chen et al. 10.1046/j.1444-2906.2004.00822.x Original Article430435BEES SGML FISHERIES SCIENCE 2004; 70: 430–435 Feeding habits of an exotic species, the Mediterranean green crab Carcinus aestuarii, in Tokyo Bay Rong Bin CHEN, Seiichi WATANABE* AND Masashi YOKOTA Department of Aquatic Biosciences, Tokyo University of Marine Science and Technology, Minato, Tokyo 108-8477, Japan ABSTRACT: Feeding habits of an exotic species, the Mediterranean green crab Carcinus aestuarii, in Tokyo Bay, Japan, were studied based on the analysis of stomach contents. Monthly samples were taken from May 2000 to October 2001 at stations near the Keihin Canal along the northern shore of Tokyo Bay. Stomach contents of 367 crabs (male n = 200, female n = 167) were examined. Carapace width ranged from 18.50 mm to 60.67 mm. Eleven food categories were identified: Bivalvia (mostly Mytilus galloprovincialis), other Mollusca, Cirripedia, Amphipoda, Brachyura, other Crustacea, Poly- chaeta, Pisces, unidentified animal materials, plant materials, and unidentified materials. The results showed that C. aestuarii is an omnivorous predator and that its diet depends greatly upon the availability of local prey species, especially in intertidal areas. Moreover, the analysis found no significant differences in the feeding habits of crabs of different sizes or sexes. KEY WORDS: Carcinus aestuarii, exotic species, feeding habits, stomach contents, Tokyo Bay. INTRODUCTION that they were scavengers and predators. The bulk of their prey consisted of slow-moving and sessile Many exotic species have established natural invertebrates, with algae comprising only a small populations in Japan in recent years. One such fraction of their diet. Further studies on the conge- invader, the Mediterranean green crab, Carcinus neric species C. maenas indicated that it is an aestuarii, was first collected in Tokyo Bay in active and voracious predator, especially on Mol- 1984,1,2 and has since become widespread and lusca,15–19 and that it also has an impact on Poly- abundant in this area.3 It has been found also in chaete distributions.20 In contrast, nothing is others places in Japan such as the Sagami, known about the feeding habits of C. aestuarii in Osaka and Dokai Bays.4–6 Tokyo Bay. Carcinus aestuarii belongs to the family Por- In this context, the purpose of the present study tunidae and its original habitat is the estuarine and was to investigate the natural diet of this invasive lagoon waters in the Mediterranean.7 In Tokyo Bay species and to consider its influence on the indig- this species inhabits the shallow water areas of the enous species in Tokyo Bay. inner coast. The life history of this species in Tokyo Bay can be summarized as follows: larvae occur from late January to late May, the peak of megalo- MATERIALS AND METHODS pal settlement occurs in March, and juvenile crabs grow up during the summer and autumn. Breeding From May 2000 to October 2001, specimens of occurs from November to May.8 C. aestuarii were collected monthly from the Many studies on the diet of other shallow water Keihin Canal of Tokyo Bay during low tide. Crabs portunid crabs, Scylla serrata,9 Carcinus maenas were caught with traps or by hand. Only adult spec- (L),10,11 Ovalipes catharus,12 Callinectes arcuatus imens were analyzed. All captured specimens were and C. toxotes,13 and Thalamita crenata14 showed preserved in 10% formalin seawater immediately after collection. Carapace width (CW) was mea- sured to the nearest 0.01 mm and sex was recorded *Corresponding author: Tel: 81-3-5463-0535. for each individual. Fax: 81-3-5463-0684. Email: [email protected] A visual estimation of stomach fullness was Received 16 May 2003. Accepted 21 January 2004. made immediately after the stomach was excised Feeding habits of Carcinus aestuarii FISHERIES SCIENCE 431 from the body.21 Stomach fullness was categorized each item and dividing it by the total number as follows: class 0, empty or containing only traces of points for all crabs and all food items in the of food; class 1, medium fullness; and class 2, full sample. The number of points for each food or nearly full (fully distended stomach). All stom- category of each crab was calculated by multi- achs were subsequently opened and the contents plying the relative contribution by a value were washed with 10% formalin seawater into a dependent on the degree of stomach fullness, Petri dish and examined under a binocular micro- for example 0.5 for class 1, and 1 for class 2. scope. Only stomachs belonging to classes 1 and 2 The relative contribution of each food category were used in the analysis. to the total volume of material in each stomach Because of mastication by the mouthparts was subjectively assessed as follows: a category and the progress of digestion, most food representing 95–100% of the stomach content ingested by the crabs had been broken into was awarded 100 points; 65–95%, 75 points; small fragments and, for this reason, was diffi- 35–65%, 50 points; 5–35%, 25 points; £5%, cult to identify at the species level. Thus, food 2.5 points; and empty, 0 points: items were classified into major categories n ranging from the level of phylum to species B according to criteria established in a preliminary, Â ij Percentage points for ith food = j=1 100 short-term laboratory test as follows. For this m n ¥ purpose, live crabs collected in the same site ÂÂ Bij were taken to the laboratory and fasted for i j== 11 5 days. They were subsequently fed with only where B is the number of points of food item i in one food item also collected in the same area, ij the stomach of the jth crab, and n is the number of i.e. Polychaeta, Bryozoa, Mollusca, Crustacea crabs analyzed in the sample, m is the number of (Cirripedia, Isopoda, Amphipoda, Macrura, food items. Brachyura), Ascidiacea, Pisces, and Algae. The The cumulative number of different food taxa stomach contents were excised after 24 h to found in the stomach, the frequency of occurrence, determine their shape, color and other charac- and the percentage points were then calculated as teristics after partial digestion. crabs were added, one at a time, to the sample. Qualitative analysis of the diet of C. aestuarii Sand and unidentified materials were also consid- was carried out using the frequency of occurrence ered a stomach content category, but sand was not and percentage points methods.12 included in further analysis of the diet. The frequency of occurrence method was cal- culated for each food item by dividing the num- ber of crabs with stomachs containing the specific food item by the total number of crabs in RESULTS the sample: A A total of 367 specimens of C. aestuarii were col- %frequency of occurrence for ith food = i ¥100 n lected, for which 199 (54.22%) had stomachs where Ai is the number of crabs with stomachs within classes 1 and 2 and which contained iden- containing the food item i, and n is the num- tifiable contents for analysis. Carapace width ber of crabs analyzed in the sample. The per- ranged from 18.50 mm to 60.67 mm. The range of centage points for each food item were size and the maximum size were larger in male calculated by summing the weighted points for (n = 113) than in female crabs (n = 86; Fig. 1). Fig. 1 Size–frequency distribution of Carcinus aestuarii used for stomach analysis. 432 FISHERIES SCIENCE R B Chen et al. Stomach contents were classified into 11 cate- Plants were recognized from pieces of algae gories: Bivalvia (mostly Mytilus galloprovincialis), (Table 1). other Mollusca, Cirripedia, Amphipoda, Possible differences in feeding habits of Brachyura, other Crustacea, Polychaeta, Pisces, C. aestuarii due to sex and size variation, were unidentified animal materials, plant materials, and analyzed in terms of the percentage frequency of unidentified materials and sand. They belonged to occurrence (Table 2). This analysis showed that seven major food groups: Mollusca, Crustacea, there were no significant effect of sex and size on Polychaeta, Pisces, unidentified animal materials, feeding habits (c2 test; P = 0.56). Thus, data for both plant materials, and unidentified materials sexes and for all size brackets were combined for (Table 1). Mollusca, such as Bivalvia, were recog- further analysis. nized from pieces of shell and tissues. Crustacea, Quantitative results based on the percentage such as Amphipoda and Brachyura, were recog- of points are summarized in Fig. 2. Crustacea, nized from carapace, antenna, appendage, and Mollusca and Polychaeta were the most important tissue fragments. Polychaeta were recognized from food groups. Plant materials constituted only 1.9% jaws, bristles, and more rarely, body walls and tis- of all food items. Pisces were usually present in sues. Pisces were recognized from fins and scales. small quantities. In addition, unidentified animal materials and unidentified materials were indexed. Table 3 shows the frequency of occurrence and Table 1 Summary of main food categories found in the total points for the 11 food categories and sand. stomach contents of Carcinus aestuarii Mollusca and Crustacea were the dominant food Food category Observations of fragment groups. Polychaeta were the next most important Mollusca group. Bivalvia, mostly M. galloprovincialis, were Bivalvia Pieces of shell and tissues the dominant specimen in all food items, account- Other mollusca ing for 132 of the total 199 individuals. Among Crustacea Crustacea, Amphipoda comprised the most Cirripedia Pieces of carapace and tissues important food item and Brachyura were the next Amphipoda Pieces of carapace, antenna, dominant food item. appendage and tissues Seasonal variations in feeding habits were Brachyura Pieces of carapace, appendage observed in terms of the percentage points (Fig.