Lesser Antillean Polychaetes, chiefly from brackish water with a Survey and a Bibliography of Fresh and Brackish – Water Polychaetes

The number of euryhaline polychaetes known at present is relatively small, when one considers that this almost exclusively marine order numbers hundreds of species. Only a few families have representatives in fresh water, viz. Nereidae, Nephthydidae, Spionidae, Capitellidae, Sabellidae, and Serpulidae. By far the greater number of fresh-water species belong to the Nereidae (in particular to the subfamily Lycastinae), and to the Sabellidae, and these two families must therefore be regarded as those with the strongest tendency towards euryhalism. It has often been pointed out that, on account of their especial “tenacity”, the nereids of the littoral area can stand the passage from one medium to another better than any other polychaetous family. It is therefore quite natural that the few genuine terrestrial polychaetes known as yet, and by far the greater number of fresh and brackish-water species, belong to the nereids. Incidentally, it may be mentioned that Pflugfelder (1933) showed that the epidermis of the most pronouncedly terrestrial nereids is especially rich in glands. To the best of my knowledge, no histological study of the epidermis of the fresh-water forms has yet been performed. It may be of interest to point out that none of the lacustrine polychaetes differ so much from their marine congeners that the establishment of new families has ever been necessary; the change from marine conditions is only small. As was realized already by Johnson (1903), their affinities are quite clear, but the one feature common to them all is their diminutive size, often only a few mm. The fauna in localities with typical euryhaline conditions most probably represents an impoverished remnant of the fauna of the open sea. It looks as if the nereid type is the type which, of all the polychaete types, has the greatest power of adaptation to different circumstances. In this connection attention is called to the fact that other polychaetes, which have also adopted a mode of life widely different from a “normal” polychaete life, viz., the endoparasitic forms (mainly belonging to the Eunicea), have likewise undergone “only the most trivial changes” (Clark, Ann. Mag. Nat. Hist (12) 9, 1956, p. 432). A fairly large number of papers on fresh and brackish-water polychaetes, each dealing with a single or a few new finds mainly scattered widely over the tropical and subtropical areas, have been published during the last 20-30 years. (The first lacustrine polychaetes were described by Nussbaum, 1900.) Nevertheless our knowledge is still very incomplete. The most exhaustive paper on polychaetes from such localities as those mentioned is Southern’s “Fauna of the Chilka Lake” (1921), but to this category should also be reckoned Horst’slist of Polychaeta from the Zuider Zee, and Stammer’s and Augener’s investigations of some of the brackish-water areas of the Baltic. Each new collection and find of fresh-water polychaetes is of the greatest interest to taxonomists and topographers.