Descriptions of two azooxanthellate Palythoa species (Subclass Hexacorallia, Order Zoantharia) from the Ryukyu Archipelago, southern Japan.

Yuka Irei,James Davis Reimer,Frederic Sinniger

doi:10.3897/zookeys.478.8512

Abstract

Two new species of zoantharians (Hexacorallia, Zoantharia, Sphenopidae), Palythoa mizigama sp. n. and Palythoa umbrosa sp. n., are described from the Ryukyu Archipelago, southern Japan. Unlike almost all other known Palythoa spp., both species are azooxanthellate and inhabit low-light environments such as floors or sides of caves, crevasses, or hollows of shallow coral reefs. The two species were initially considered to be the same species from their similar habitat environments and highly similar morphological features. However, phylogenetic analyses of nuclear internal transcribed spacer (ITS) ribosomal DNA, mitochondrial 16S ribosomal DNA, and cytochrome oxidase subunit I (COI) sequences revealed that these two species have a genetically distant relationship within the genus Palythoa. Morphological characteristics, including polyp size, tentacle number, external/internal coloration, and types and sizes of cnidae were examined in this study. As a result, only tentacle coloration was found to be useful for the morphological distinction between the two species. Palythoa mizigama possesses white tentacles with black horizontal stripes while Palythoa umbrosa possesses white tentacles without any stripe patterns. Considering their distant phylogenetic relationship, it can be assumed that their unique yet similar morphological and ecological characteristics developed independently in each species as an example of parallel evolution.

Highlights

Zoantharia are sessile marine cnidarians within the class Anthozoa, subclass Hexacorallia
Zoantharians are usually distinguishable by the combination of two rows of tentacles arranged around the oral disc, colonial life form, and the incrustation of hard particles in their column taken from the surrounding habitat
Many macrocnemic zoantharians live in association with other organisms such as sponges (Crocker and Reiswig 1981), hydrozoans (Camillo et al 2010, Sinniger et al 2010), antipatharians (Sinniger et al 2010), or they are epizoic on shells inhabited by hermit crabs (Muirhead et al 1986, Reimer et al 2010, Schejter et al 2011)

Summary

Introduction

Zoantharia are sessile marine cnidarians within the class Anthozoa, subclass Hexacorallia This taxon is often referred to as intermediate in form between hard corals (Scleractinia) and sea anemones (Actiniaria), as most species lack a skeleton and yet are colonial. Some species contain unique chemicals such as palytoxin or norzoanthamine (Kuramoto et al 1997, Moore and Scheuer 1971, Vitor and Vitor 2010) They are important organisms both ecologically and as a source of bioactive compounds, there is a lack of knowledge on their taxonomy and diversity, with both many synonyms and undescribed species existing (Burnett et al 1997, Reimer et al 2004). Brachycnemic zoantharians are abundant on many coral reefs (Sebens 1982, Acosta 2001, Irei et al 2011) and usually have competitive relationships with other benthic organisms (Suchanek and Green 1981)

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