Comparative Studies on the Life History of the Grapsid Crabs (Crustacea, Brachyura) Inhabiting Intertidal Cobble and Boulder Shores

Abstract

Growth, survivorship and reproduction of seven species of grapsid crabs inhabiting ntertidal cobble and boulder shores were investigated in Shirahama, Wakayama Prefecture, Japan. In general, growth rate decreased with age, was similar between the sexes for the immature crabs, and was lower in mature females than in mature males within a species. Growth rate showed considerable variations among species, which seemed to be mainly due to the interspecific differences in intermolt period. Survival rate was similar between the sexes, and tended to be lower in mature crabs than in immature crabs within a species. Survival rate varied considerably among species, and an apparent trade-off relationship to an index of reproductive activity existed. This brood index was calculated by multiplying the ratio of brood weight per body weight by the number of broods per year. Egg size and the ratio of brood weight per body weight were constant within a species regardless of body size of parent female crabs, whereas the number of eggs per brood was nearly proportional to the third power of their carapace width. Among species, mean number of eggs per brood and size at maturity were correlated with mean adult body size of the species, whereas egg size, the number of broods per year, brood index and age at maturity were not. A model which shows the relationships between the expected value of the number of eggs laid per life per female and some life history traits was made on the basis of the general rules followed by the present grapsid crabs in the intraspecific and interspecific variations in life history traits. Using this model, the factors influencing the evolution of age at maturity and brood index were analyzed. Delayed maturity and smaller brood index were predicted to be favored as survival rate in immature period increases and/or relative growth rate to maximum body size decreases. Recently the information on the life history traits of brachyuran crabs has been accumulated using laboratory and field studies. Since brachyuran crabs grow through molts, some studies have analyzed their growth rate from molt increment and intermolt period (Hartnoll, 1982). Molting makes it almost impossible to mark an individual crab for a long time in order to follow its growth under natural conditions. Probably for the same reason, there are few studies on survival rate of individual branchyuran crabs. Likewise, the number of broods per year and age at maturity are relatively difficult to estimate, thereby being studied for a few species (Pillay & Ono, 1978; Seiple, 1979; McDonald, 1982). In contrast, some reproductive traits, e.g., egg size, the number of eggs per brood and reproductive cycle, have been studied for many species (e.g., Knudsen, 1964; Pillay & Nair, 1971; Hines, Publ. Seto Mar. Biol. Lab. , 33(4/6), 121-162, 1988. (Article 6)