An original description of the larval stages of Phoronis australis Haswell, 1883 and an analysis of the world fauna of phoronid larvae

Abstract

Phoronida is a phylum of marine invertebrates; they have a worldwide distribution and form huge benthic aggregations in some areas. The taxonomy of Phoronida remains very poorly developed; only 13 species of phoronids have been described worldwide, while approximately 40 varieties of competent larvae are known. Morphological description of phoronid larvae and the search for correspondence between the larval and adult forms make an important contribution to the taxonomy of this enigmatic group. We present an original description of the larval forms of Phoronis australis Haswell, 1883. The larvae and adults of P. australis were collected in the plankton of Nha Trang Bay in the South China Sea. For the first time, the external morphology of larval forms of P. australis was described at the stages of 4, 8, and 20 tentacles. The basic distinguishing features of Phoronis australis larvae were determined: opaque integuments, the presence of a characteristic dark pigmentation of the oral field epidermis, of the unpaired ventral stomach diverticulum, and more than 20 tentacles in the competent larvae. A molecular-genetic analysis based on comparison of the DNA sequences of the 18S rDNA and 28S rDNA nuclear genes confirmed the identity of the phoronid larvae that were found in the planktonic samples from Nhatrang Bay and adult P. australis that were collected in the same area. The problems of Phoronida taxonomy and correspondence of larval and adult forms can be solved only with a complex approach that involves both morphological and molecular-genetic information.