Seasonal patterns of food availability: influences on the reproductive output and body condition of the herbivorous crab Grapsus albolineatus.

Rocky shores in Hong Kong experience marked seasonal differences in climate resulting in seasonal changes in macroalgal assemblages. The tropical rocky shore crab, Grapsus albolineatus, feeds selectively on filamentous algae through the year but the abundance of these algae and foliose algae is greatly reduced during the summer when encrusting algae dominate the shores and the crab's diet. This switch in diet may have implications for the reproductive output of this crab. Standing crop of algae varied greatly through the year, peaking in March and April, when the nutritional quality of the algae was also highest. On the shore, available algal protein and energy were both lowest in July. The crab selected an algal diet rich in nutrients than that available to it on the shore for all months of the year except September to December for protein, and July and August for energy. The input of animal matter considerably increased the protein content of the diet, but made little difference to the energy content. Growth and body condition were greatest during March and June, coinciding with the peak in algal biomass. Storage of nutrients in the hepatopancreas of G. albolineatus commenced in November, coinciding with the increase in biomass and quality of algae on the shores, and then, peaking in May and June, these nutrients were utilised by the reproductive organs during the reproductive season which ran from April to November. The middle of the reproductive season coincided with the period when the standing crop of algae on the shore was at its lowest levels and poorest quality. Cycles of growth, reproduction and storage in G. albolineatus appear to be directly influenced by seasonal patterns of algal food availability. Nutrient storage in the hepatopancreas, during periods when food is nutritionally rich and is most abundant, does not guarantee reproductive success of the crab, but appears to be a prerequisite. Selection of an optimal diet (energy- and protein-rich) in a seasonal environment by ingesting abundant nutrient-rich algal species and the opportunistic consumption of animal matter strongly influences growth and reproductive output of G. albolineatus.