Seastars of a feather flock together

Author(s)

Dr Stephen Keable
Category

Science
Published

02 June 2015
Read time

2 minutes

Feather stars and sea lilies are exclusively marine organisms which together form the class Crinoidea in the invertebrate phylum Echinodermata. Characteristics feather stars share with their echinoderm relatives, such as seastars and sea urchins, include body symmetry, an internal calcareous skeleton and a water vascular system composed of fluid filled canals that are evident as external tube feet. Their most distinguishing feature is a number of jointed arms which are composed of a central ray from which small appendages, known as pinnules, branch and give a feather-like appearance. The pinnules are used for collecting food particles from the water column and for gas exchange. Overall the feather-like arms, which come in an array of colours, provide an elegant and graceful appearance to these intriguing animals. There are about 700 species, distributed through most seas but they are particularly conspicuous in the deep-sea as well as the shallow tropics.

The crinoid Oligometra serripinna — The crinoid *Oligometra serripinna*, with arms spread to catch items in the water column, sitting on a seafan. Image: Lyle Vail & Anne Hoggett
© Australian Museum

Crinoids may be free-swimming or bottom dwelling, they are relatively slow moving. An understanding of the morphology is essential to their classification which is based on the analysis of the features of the skeleton and skeletal parts (ossicles). The crinoid fossil record dates back almost 500 million years to the early Ordovician, with the group undergoing a number of radiation and extinction events with peak diversity reached in the later Paleozoic. Crinoids are susceptible to environmental disturbances and particularly vulnerable to ocean acidification related to climate change. Study of crinoids has much to reveal about diversity, relationships, evolution, biogeography and response to impacts.

During March and April Dr Marc Eléaume of the Muséum national d'Histoire naturelle, Paris, undertook an Australian Museum Research Institute Visiting Collection Fellowship to examine the Australian Museum crinoid collection. During his stay he was able to examine approximately 200 crinoid samples. He focused on Australian endemic species Antedon incommoda, A. loveni and Aporometra wilsoni. Recent study of European Antedon, from which the genus was described, found they form a discrete group whereas the Pacific representatives are separated into a group of Australian and Japanese species and a further cluster from the Indo-west Pacific. The picture became more complicated when Marc and his collaborators realised that the Australian species are not as closely related to each other as they are to species from other genera.

The main goal of Marc’s visit was to revise the description of the species of Antedon from Australia and evaluate species belonging to the same group (i.e. Aporometra wilsoni, Euantedon species, Dorometra species, Toxometra species). For this he made use of the museum scanning electron microscope facilities to examine characteristics of the skeleton of the specimens as well as taking many measurements to compare.

While results are still to be analysed in detail we now have a better understanding of the variability within Antedon incommoda and A. loveni. This has helped clarify the geographical distribution of each species and shows that there is no reason for separating A. incommoda and a subsequently proposed Australian species (A. lacertosa) but possible that sub-species named within A. incommoda are actually two different species, one restricted to the southern more temperate part of Australia and another restricted to the tropical part of Western Australia. To confirm this Marc has some more comparisons to make.Then comes the job of piecing together all the information he has accumulated to make sense of the bigger story of crinoid biology. The data he gathered during his stay at the Australian Museum will be invaluable in this process.