EVOLUTION IN THE NOTOSTRACA

Abstract

The Notostraca are phyllopod Crustacea in which the carapace is shield-shaped and covers the anterior part of the body; together with other phyllopods, they are considered to be relatively unevolved Crustacea-there is a high, but irregular, number of appendages (forty to sixty pairs are usual) and the segmentation of the body is anameristic. It is characteristic of phyllopods that they occur in temporary rain pools, generally in arid or steppe country where such pools are common, and are unable to maintain themselves in the face of competition from higher Crustacea or Insecta (Ard6, 1947). The survival of such primitive and vulnerable animals poses an interesting problem-the key to which may lie in their main adaptation to a temporary habitat, the drought-resistant egg. The possession of such an egg, from which hatching normally occurs only after desiccation, is an obvious prerequisite for the exploitation of a temporary freshwater habitat by a group of animals which do not have the capacities of aerial migration which the Insects possess. These drought-resistant eggs of phyllopods are the only possible means of dispersal for the group, for the pools in which the animals occur are isolated from one another, and are only rarely part of a stream system along which dispersal of adults would be possible. The eggs are small, light in weight because of their dry alveolar shells, and are sticky when laid in at least one genus (Triops, Notostraca); further, the eggs of some Anostraca are known to retain viability after passing through the guts of Amphibia (Mathias, 1937). These facts suggest several possible means of dispersal-in or upon larger animals, attached to dry pieces of wind-blown vegetation, or airborne singly in wind currents (a desert dust-devil will whirl particles much denser than the eggs of phyllopods high into the air). That 'these passive means of dispersal are effective is quite clearly shown by the ubiquitous occurrence of phyllopods in suitable pools wherever these occur. Notostraca occur on such isolated oceanic islands as the Hawaiian group, and in the few suitable pools that the rainy English climate permits. The phyllopods show another characteristic of passively distributed invertebrates-that of very widely spread, sometimes almost cosmopolitan species; in a recent study of the Notostraca for a systematic revision I found that Triops longicaudatus, which Linder (1952) showed was the only species of the genus in the Americas, extends from the West Indies through Central America and right across the Pacific to Japan by way of the Galapagoes, Oahu and New Caledonia. In all this area there is virtually no morphological change (Longhurst, 1955). These facts of dispersal and geographical distribution seem to me to explain the survival of the phyllopods. The initial arthropod invasion of temporary pools may well have been performed by the primitive fresh-water Crustacean fauna and has radiated into the forms of phyllopods we see today-the carnivorous Notostraca, the pelagic Anostraca, and the burrowing, detritus-feeding Conchostraca. This early fauna would have spread and occupied at least a high proportion of the suitable pools, and such a cosmopolitan occupation would have effectively prevented any geographically iso-