Phylogeny and Adaptive Radiation

Phylogeny and Adaptive Radiation
Phylogeny
Hyman (1951) grouped Rotifera, Gastrotricha, Kinorhyncha, Nematoda, and Nematomorpha into a single phylum (Aschelminthes). All of these phyla share a certain combination of characteristics. Hyman contended that such evidences of relationships were so concrete and specific that they could not be disregarded. Nevertheless, most authors now consider that differences between the groups are sufficient to merit phylum status for each, although some accept the concept of the Aschelminthes as a superphylum. These phyla may well have been derived originally from the protostome line via an acoelomate common ancestor (Figure 15-22). However, sequence analysis of the gene encoding the small subunit of ribosomal RNA supports a quite different phylogenetic hypothesis.* This evidence suggests that some time after the ancestral deuterostome diverged from the ancestral protostome in the Precambrian, protostomes split again into two large groups (or superphyla): Ecdysozoa, containing phyla that go through a series of molts during development, and Lophotrochozoa, including lophophorate phyla (Lophophorate Animals ) and phyla whose larvae are trochophore-like. Some pseudocoelomates fall into Ecdysozoa; sequences of others place them in Lophotrochozoa, and some are yet to be determined (Table 15-2). If this arrangement is correct, the concept of Aschelminthes is invalid. We will discuss the Ecdysozoa/Lophotrochozoa hypothesis further in Arthropods .

Loriciferans bear some similarity to kinorhynchs, larval Priapulida, larval nematomorphs, rotifers, and tardigrades. Although loriciferans are poorly known, cladistic analysis suggests that they form a sister group to kinorhynchs and that these two phyla together are a sister group of priapulids (Figure 15-22). If so, we would expect sequence analysis to place Loricifera in Ecdysozoa.

Entoprocts were once included with phylum Ectoprocta in a phylum called Bryozoa, but ectoprocts are true coelomate animals, and many zoologists prefer to place them in a separate group. Ectoprocts are still often called bryozoans. Sequence analysis places both entoprocts and ectoprocts among lophotrochozoan phyla.

Acanthocephalans are highly specialized parasites with a unique morphology and have doubtless been so for millions of years. Any ancestral or other related group that would shed a clue to phyletic relationships of Acanthocephala is probably long since extinct. Like cestodes, acanthocephalans have no digestive tract and must absorb all nutrients across their tegument, but the tegument of the two groups is quite different in structure. Also, acanthocephalans are pseudocoelomate and show eutely, as do nematodes, although here, too, structural and developmental differences are great.

Adaptive Radiation
Certainly the most impressive adaptive radiation in this group of phyla is shown by nematodes. They are by far the most numerous in terms of both individuals and species, and they have been able to adapt to almost every habitat available to animal life. Their basic pseudocoelomate body plan, with the cuticle, hydrostatic skeleton, and longitudinal muscles, has proved generalized and plastic enough to adapt to an enormous variety of physical conditions. Free-living lines gave rise to parasitic forms on at least several occasions, and virtually all potential hosts have been exploited. All types of life cycle occur: from simple and direct to complex, with intermediate hosts; from normal dioecious reproduction to parthenogenesis, hermaphroditism, and alternation of free-living and parasitic generations. A major factor contributing to evolutionary opportunism of nematodes has been their extraordinary capacity to survive suboptimal conditions, for example, developmental arrests in many free-living and animal parasitic species and ability to undergo cryptobiosis (survival in harsh conditions by assuming a very low metabolic rate) in many freeliving and plant parasitic species.