A New Phoronid Species, Phoronis savinkini sp. n., from the South China Sea and an Analysis of the Taxonomic Diversity of Phoronida

Abstract

Phoronida is a phylum of marine invertebrates with a worldwide distribution. Currently, there are 14 species of phoronids in the world fauna, but this number may increase, based on the latest research. In the present study, the morphology and microanatomy of a new phoronid species, Phoronis savinkini sp. n., from the Gulf of Tonkin in the South China Sea are studied. The new phoronid species forms highly dense populations on rocky ground covered with soft sediment at a depth of 2 m. Since the epidermal collar at the base of the tentacles is absent, we refer this species to the genus Phoronis. The lophophore is a one-coil spiral in shape. There are 154 (160) tentacles. Special reproductive glands in the lophophoral cavity are absent. Longitudinal muscle bundles are of the bushy type. The maximum number of bundles is 50. Each metanephridium has a strongly curved excretory canal and two funnels: anal (small) and oral (large). There are two giant nerves of 2–5 μm in diameter. A comparative analysis of the morphological characters used in phoronid taxonomy shows that the morphology of the new phoronid species from the South China Sea seems to be the most similar to that of Phoronis hippocrepia Wright 1856, which has two morphs, one burrowing in hard ground and the other living in soft substrates. However, there are significant morphological differences between the two species: a fundamentally different type of lophophore organization, which is the main taxonomic character in phoronids, a different type of organization of excretory organs (including the structure of the nephridial funnels), and the absence of reproductive glands in the crown of tentacles in the new species. The cladistics analysis of phoronid taxonomic diversity shows that the ecological division of phoronids into two groups (burrowing and living in soft substrates) is meaningful taxonomically as well. Burrowing and digging phoronids might have diverged long ago. The muscle type and the number of branches of the nephridial canal can be considered as the main differences between the two groups. In the present paper, the monophyly of the genus Phoronopsis and the paraphyly of the genus Phoronis are shown. Secondary simplification of Phoronis ovalis is assumed. A separate clade that includes Phoronis pallida and Phoronis embryolabi is suggested.