The chemical composition of common marine macroalgae from Hong Kong was determined both in the winter (cool and dry) and the summer (hot and wet) of 1994. During the winter, macroalgal diversity was high and variation in chemical composition between the species was great. In contrast, during the summer, algal diversity was reduced and the shores were dominated by only a small number of comparatively nutrient rich encrusting algae. Multivariate analysis indicated that the nutritional value of algal species (in terms of ash, lipid and soluble carbohydrate content) was related to their systematic position and that, overall, the Phaeophyta had the highest levels of nutrients, while the Chlorophyta and the Corallinaceae were of comparatively low nutritional value. The chemical composition of the species varied temporally as did the abundance and diversity of algae on the shore. Overall, however, seasonal differences in algal availability appeared to be more important in determining total nutrient availability than temporal changes in the nutritional value of the species. Local macroalgae are the main food source for a large number of marine herbivores and variation in algal availability and chemical composition are likely to effect the ecology of these grazers and subsequently community organization.

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.

The tropical rocky shore crab Grapsus albolineatus feeds primarily on filamentous algae but eats animal matter whenever it is available. During the summer the crab's diet switches to encrusting algae due to a die-off of filamentous algae. As a result of the switch the nutrients in the diet of the crab vary seasonally and may influence the fitness of the crab. Maintenance, growth, reproductive performance and nutrient storage of crabs were examined under four dietary regimes of increasing nutritional value ranging from low organic to high protein content. The nutritional quality of these diets significantly affected crab survival and moulting. Crabs fed on the nutritionally superior diet of algae and meat exhibited enhanced growth, higher levels of energy in the reproductive organs and stored more energy in the hepatopancreas than did individuals on the shore and crabs fed only on algal diets in the laboratory. Filamentous algae were a better food source than other algae, resulting in fewer deaths and superior levels of maintenance and growth. Growth and maintenance can occur on a pure algal diet, but reproductive performance and nutrient storage require some degree of added nutrients in the form of animal matter in the diet. Crabs fed coralline or foliose algae had higher mortality and fewer successful moults than crabs fed the other two diets. The fitness of G. albolineatus appears to be limited by the amount of extra nutrients obtained from animal matter. The opportunistic consumption of animal material in the form of carrion, or of animals associated with dietary algae, could be a key factor in the reproductive success of this crab.

No abstract

The tropical rocky shore crab Grapsus albolineatus selectively consumes rare filamentous algae over more abundant foliose algae during the wlnter in Hong Kong. Laboratory experiments have shown that growth of G. albolineatus is enhanced and mortality reduced when given a diet of filamentous algae as opposed to fohose algae. In the laboratory, G. albolineatus consumed filamentous algae (Enteromorpha clathrata, Hincksia mitchelliae and Chaetomorpha antennina) in greater amounts than any foliose algae (Dermonema frappieri, Pterocladia tenuis, Porphyra suborbiculata. Ulva fasciata, Endarachne binghamiae) in both multiple choice and no choice experiments. The most energy-rich alga was Pterocladia, while Porphyra had the highest protein content. Filamentous algae had lower overall nutrient contents than foliose algae. Consumption rates for Enteromorpha and Hincksia were, however, sufficiently higher than for Porphyra, which ensured that the net intake of nutrients per day was greater. Even though protein assimilation efficiency was higher for Porphyra than Hincksia, G. alboljneatus assimilated more protein, per day, from Hjncksia. When the confounding effects of morphology and nutrient value were separated, by offenng the crab choices of commercially available algae (Laminaria sp.) cut into different forms (foliose and filamentous), G. albolineatus showed a strong preference for the filamentous form, despite both forms having the same nutrient value. Preference for filamentous forms is likely to be constrained by the morphology of the chelae, which have delicate tips, and appear unable to tear foliose algae. Algal morphology, therefore, appears to be of prime importance and the nutrient content and digestibility of algae of secondary importance in determining the feeding preferences of G. albolineatus. The high consumption rate of filamentous algae outweighs their relative nutrient deficiencies, indicating that they are better suited to meeting the physiological needs of the crab than foliose algae.