Evolution and adaptive radiation in the Macrothricidae (Crustacea: Cladocera): A study in comparative functional morphology and ecology

Abstract

An account is given of the functional morphology, ecology, habits and - in most cases - feeding mechanisms of representatives of 13 genera of anomopod cladocerans of the family Macrothricidae. As in the Chydoridae, habits, which often depend on anatomical specializations of great complexity, are diverse, and permit the exploitation of a wide variety of ecological niches. In general, locomotion is less specialized than in the Ghydoridae and most species neither swim nor creep so efficiently as do chydorids. So far as is known only one species, Macrothrix triseal , has evolved a chydorid-like method of crawling by means of the first trunk limbs but others have acquired means of locomotion peculiar to themselves. In only a few cases is the post-abdomen used for pushing, its lack of such employment and the lack of a joint between it and the trunk being primitive features. In basic structure and arrangement the trunk limbs are similar to those of the Chydoridae and the principles involved in many of the feeding mechanisms are the same in both families. By numerous, sometimes subtle, sometimes profound modifications of the components involved, this basic arrangement has been adapted to widely differing ways of life and different functions. Many macrothricids use the first trunk limbs for food collection rather than the second as in most chydorids. This is interpreted as a primitive feature. Acantholeberis curvirostris (figures 1-6), a frequenter of peaty pools, is primitive in several respects; for example, it lacks a headshield and is unable to push effectively with its post-abdomen, but exhibits several specializations. The antennae, which also serve as props and levers, are the principal means of locomotion, and have massive protopods flexed at about 90°. Progression is by means of intermittent ‘leaps’: steady swimming is not practised. Crawling by use of the first trunk limbs in a manner that foreshadows the efficient crawling of certain anomopods is feebly developed. Food collected by the first trunk limbs is dragged into the chamber made up by these and other trunk limbs (figures 8, 9), both mechanical handling and filtration being involved in its manipulation