MARINE ECOLOGY - PROGRESS SERIES Vol. 6: 91-99, 1981 Published September 15 Mar. Ecol. Prog. Ser. 1 l Natural Diet, Feeding and Predatory Activity of the Crabs Callinectes arcuatus and C. toxotes (Decapoda, Brachyura, Portunidae) R. K. G.Paul Department of Marine Biology. University of Liverpool, Port Erin, Isle of Man, Great Britain and Centro de Ciencias del Mar y Limnologia, Estacion Mazatlan, Unam, A.P. 81 1, Mazatlan, Sinaloa, Mexico ABSTRACT: The natural diet of Callin~ctesarcuatlls Ordway in the Huizache-Caimanero lagoon system (W. Mexico) was detern~~nedand several aspects of its feeding and predatory activity were investigated. Limited observations ware also made on the dlet of C. toxotes Ordway. The diets of the two crabs were similar, but thdt of C. arcuatus appeared more varied; it consistc:d nla~nlyof bivalve molluscs, crabs and fish. D~ffrrencesin the diet of C. arcuatcls in different areas of the lagoon system and at different times of the year reflected differences in prey availability rather than In prey selt>ctlon. Small crabs (below 60 mm breadth) were basically detritivores and scavengers, whilst larger crabs were scavengers and predatory feeders. They were not found to be major preddtors of penacs~dshrimp, and C arcclatus was shown not to 1x1 an elficlent predator of highly rnob~lcan~ni~~ls such as shrimp. Feedlng occurred mainly at night, especially around duyk INTRODUCTION MATERIALS AND METHODS The Huizache-Caimanero lagoon system, like most Analysis of Foregut Contents others on the Pacific Coast of Mexico, supports an important scasonal fishery for penaeid shrimp Random sub-samples were taken from routine (Edwards. 1977, 1978a, b). Crabs have often been biweekly trawl samples (Paul, in press) of Callinectes regarded locally as major predators of shrimp, but no arcuatus Ordway and C. toxotes Ordway from the study known to this author has been conducted to Huizache-Caimanero lagoon system between Febru- investigate thls assun~ption.Paul (in press) described ary 1975 and June 1976 for analysis of foregut contents. the distribution and general ecology of Callinectes All the crabs were killed immediately upon capture arcuatus and C. toxotes in the lagoon system The and preserved in 5-10 % formalin solution. present paper deals with the natural diet and some as- Upon return to the laboratory the foreguts of the pects of feeding behaviour, principally of C. arcuatus, crabs were removed by dissection and their exteriors and evaluates the trophic relationships between crabs washed in water, holding both ends closed with for- and shrimp. ceps. Each foregut was then opened and its contents The lagoon system is located between 22"48'- washed out into a Petri dish with water The gut con- 23"06'N and 106"OO'-106"16'W, 25 km south of the city tents were then identified, separated into taxa and of Mazatlan, in the State of Sinaloa on the Pacific coast dried to constant weight at 80 "C, since Steigenberger of Mexico. Its geography and general conditions have and Larkin (1974) reported that initial hydrolysis of been described by several authors (Edwards, 1977, food in stomachs leads to inaccuracies in wet weights. 1978a; Warburton, 1978, 1979; Paul, in press); its main Only foreguts estimated to be over 50 % full were used features are shown in Fig. 1. in the analysis. O Inter-Research/Printed in F. R. Germany 92 Mar Ecol. Prog. Ser. 6: 91-99, 1981 STATE OF SINALOA Fig. 1. Map of Huizache-cairnan- ero lagoon system. Mean min- imum l~mitsof the lagoons (late dry season) are dotted. Canals are shown by a broken lines. Irn- portant sampling stations: A, Tapo Botadero; B, Tapo Ostial; C, Las Garzas; D, Tapo Pozo de la Hacienda; E, El Tanque Canal: F, Tapo Caimanero; G. Tapo Agua Dulce The gravimetric and frequency of occurrence shrimp abdomen. Of the fed crabs, 1 was fed l g of methods (Hynes, 1950; Windell, 1968) were used to shrimp daily before and throughout the experiment, analyse the gut contents. In the former, the dry weight and 1 was fed daily more shrimp than it could con- of each food item is expressed as a percentage of the sume; the other 2 were not fed after the experiment total dry weight of all food items in the sample; in the had begun. latter, the number of foreguts in which each food item The tanks were observed frequently over a period of occurred is expressed as a percentage of the total 5 d, including several 2 h periods of continuous obser- number of foreguts examined (frequency per 100 fore- vation, and the number of shrimp in each tank was guts). counted each morning when the remains of dead shrimp were removed from the tanks. At night, illumi- nation was provided by a red light bulb so as not to Predation on Live Shrimps and Feeding Activity of affect the behaviour of the crabs and shrimps, since Callinectes arcuatus benthic crustaceans are relatively insensitive to red light (Goldsmith and Fernandez, 1968; Fernandez, Two separate experiments were carried out to 1973) investigate the predation of Callinectes arcuatus on In the second experiment 3 tanks, 1 measuring Penaeus vannamei. In the first experiment 7 glass 116 X 25 X 60cm (height) and 2 measuring tanks, each measuring 70 X 35 X 35cm (height) and 96 X 36 X 46cm (height),were filled with 10 cm of sand containing sand to a depth of 10 cm and lagoon water and diluted seawater (salinity = 34 %O S, temperature (salinity = 30 %OS, temperature = 29 "C k 2 C") to a = 27 "C k 2 CO) to total depths of 55 cm and 30 cm total depth of 30 cm, were used. The water in each tank respectively. Ten crabs (carapace breadths 90-120 was aerated and all tanks were covered with glass lids. mm) and 4 shrimps (total lengths 100-140 mm) were Four shrimps (total lengths 90-130 mm) and 1 crab introduced into the largest tank, and 1 crab and 4 (carapace breadths 64-120 mm) were introduced to shrimps and 1 crab and 2 shrimps, respectively, were each of 6 tanks, and 4 shrimps were placed in a control placed in the other 2 tanks. The crabs were fed regu- tank. The shrimps used were freshly brought from the larly on peeled shrimp abdomen up till the start of the lagoon, but the crabs were acclimated in the laboratory experiment. The tanks were inspected twice daily over under different feeding regimes for 7 d prior to the start a period of 30 d and the number of shrimps in each of the experiment. During the period of acclimation 2 tank counted each morning. Illumination at night was crabs were starved and 4 were fed daily on peeled provided by a red light bulb. Paul: Feeding of Calllnectes 93 Table 1 Callinectes arcuatus and C toxotes. Foregut contents; February 1975 to May 1976 C. arcuatus C toxotes Number of foreguts examined 533 8 % dry weight Frequency per % dry welght Frequency pel- of diet 100 foreguts of dlet 100 foreguts Molluscs. B~valves 26.9 47 6 59.7 87 5 Gastropods 10 10 2 - Total 27 9 59.7 Crustaceans. Crabs 15 2 29.1 Non-brachyuran decapods 5.6 - Other crustaceans 1.3 - Total 22.1 29.1 Fish 16.5 43.9 3.6 25.0 Plant material 2.9 44.9 0 7 50.0 Polychaetes 1.8 26 6 0 3 25.0 Foraminiferans 0.1 10.2 0.1 37.5 Bryozoans 0.1 15.3 0.1 12.5 Sponges 0.1 13.3 Hydrozoans 0.1 40 8 (0.1 12.5 Insects ( 0.1 (0.1 - Nematodes ( 0.1 ( 0.1 - - Un~dentifiableorganlc material 14 5 45.9 2.6 37.5 Sand etc. 13.8 44.9 3.8 50.0 RESULTS less than the total Decapoda. Fish remains consisted mainly of Lile stolifera (sardine), Anchoa panamensis Analysis of Foregut Contents (anchovy),Diapterus peruvianus (mojarra) and Gerres cinereus; those of other fish such as Galeichthys Molluscs were the most important item in the diet of caerulescens (catfish) and Mugil spp. (mullet) were Callinectes arcuatus; they constituted nearly 28 % of less common. Plant material, consisting mainly of Rup- the total dry weight of the foregut contents (Table 1). pia rnaritirna, Salicornia sp., Cladophora sp. and Bivalves occurred nearly 5 times as frequently as gas- Enterornorpha sp., was present in 44.9 % of the fore- tropods and accounted for more than 25 times as much guts examined but only in very small quantities. of the dry weight of food, with Tagelus affinis being Polychaete remains were fairly common in the foreguts the dominant species in the former group, and examined (including Polydora sp.) but constituted less Cerithidea mazatlanica in the latter. Next most abun- than 2 % of the total foregut contents. Foraminiferans, dant in the foreguts were crustaceans, particularly bryozoans, sponges and hydrozoans were all present in crabs, which together formed over 22 % of the total dry a fairly large number of foreguts but they each consti- weight of food. Most of the crab remains were those of tuted 0.1 % or less of the total foregut contents. The Callinectes spp., but remains of Gecarcinus sp. and state of mechanical and chemical breakdown of the Cardisorna crassum were also found in large quan- food is reflected by the fact that nearly half the foreguts tities. Non-brachyuran decapods occurred less fre- examined contained unidentifiable organic debris quently than crabs and constituted a much smaller which, in bulk, approached the importance of identifi- proportion of the mass of food, consisting chiefly of the able remains of fish or crabs.