Feeding preferences of the herbivorous crab Grapsus albolineatus: The differential influence of algal nutrient content and morphology

Abstract

The tropical rocky shore crab Grapsus albolineatus selectively consumes rare filamentous algae over more abundant foliose algae during the winter 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 foliose 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. albolineatus assimilated more protein, per day, from Hincksia. When the confounding effects of morphology and nutrient value were separated, by offering 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.