Studies on the functional morphology and ecology of the atyid prawns of Dominica.

Abstract

Six species of atyid prawns, representing five genera, occur in streams on the West Indian island of Dominica (figures 1-6). The ecology and habits of each are described and the relation of features of gross morphology to ways of life noted. Xiphocaris elongata, the most primitive living atyid, is a lightly built prawn whose adult habits are related to life in quiet pools in streams. An agile species and an excellent swimmer, it picks up individual small food particles with specialized chelipeds (figures 18 and 19) that differ from those of all other atyids and manipulates them with mouthparts (figure 77) which, while highly complex, are more primitive than those described for any other member of the family. Atya innocous and A. scabra, representing perhaps the most specialized atyid genus, are very similar in gross morphology and are robustly built ambulatory species. A. innocous is common in a variety of situations: A. scabra is rare and has been found only in fast-flowing water. Both have chelipeds whose three distal segments are extremely specialized (figure 36) and whose propus and dactylus are armed with an exceedingly complex array of long, slender bristles. These can be used either as brushes for collecting finely particulate detritus (figures 58-60) or as filtering fans (figures 68 and 69) which, held passively in flowing water, extract suspended particles. The Atyidae is unique among the Malacostraca in having representatives that filter passively by means of the chelipeds. The bristles (figure 40) are extended (figure 49), not by muscles, of which there are none in the distal parts of the propus and none anywhere in the dactylus, but by hydraulic forces. The return of the bristles to rest is by means of a cuticular spring. Some of the bristles of A. innocous are armed distally with minute denticles (figures 41 and 42) that facilitate scraping and sweeping: no such are present in A. scabra. The difference is related to the relative importance of scraping in the two species: A. innocous scrapes frequently, A. scabra seldom. Finely particulate food is transferred and manipulated by the extremely complex oral machinery (figure 78). One of the most elaborate parts of this is a teaselling device in which components of the maxillae and first maxillipeds participate (figures 80 and 81). The feeding mechanism is described. Morphologically and functionally Micratya poeyi can be regarded as a miniature version of Atya. It can both sweep and filter. Potimirim glabra is rare in Dominica and its habits but little known. Morphologically it is similar to, but more primitive than, Micratya. Its cheliped bristles are clearly specialized for sweeping and show few signs of being used for passive filtration. Jonga serrei occupies a separate and well-defined niche in the quieter parts of streams. For this it shows many morphological specializations and lacks such attributes as stout claws and robust walking legs that are the hallmark of its relatives living in fast-flowing waters.