Learn About Seastars

Seastar diversity on the Pacific coast of North America is the highest of anywhere in the world.  On a single SCUBA dive one can expect to see a dozen or more species.  Diversity of intertidal forms is less, but number of individuals, for example, ochre stars Pisaster ochraceus, may be great.

photograph of multi-coloured ochre stars Pisaster ochraceus in Barkley Sound, British ColumbiaNOTE “seastar” or “starfish”?  According to the Oxford English Dictionary, the first literature reference to asteroids was in 1538 to a type of  “sterrefyshe”, and the designation “sea star” did not appear until 1672.  Contemporary invertebrate textbooks refer to asteroids as “sea stars” or "seastars", and both terms will be used in the ODYSSEY.  The first part of the name reminds us that echinoderms live only in the sea.  As for the pentaradiate symmetry implied by the second part, consider this. Of 37 species of asteroids in British Columbia, 30 have 5 arms (a classic "star"-shape), and 7 have more than 5, so the “star” designation seems mostly applicable.  The sunflower star Pycnopodia helianthoides has up to 40 arms and, at sizes up to 1.3m across, is the largest-diameter asteroid in the world.  Larger in mass than Pycnopodia, however, is the pink seastar Pisaster brevispinus, and this species is the largest-mass 5-armed sea star in the world (60cm dia)

NOTE the idea that P. ochraceus exists in 2 subspecies, ochraceus and segnis, appears to have been put to rest from results of a genetics investigation by a consortium of Mexican researchers. The researchers analyse tube-feet tissue of specimens from 17 localities spanning British Columbia to Baja California and find no significant genetic differentiation. Pisaster ochraceus apparently exists as an homogeneous species. Frontana-Uribe et al., 2010 p. 187 In, Echinoderms: Durham (Harris et al., eds.) Taylor & Francis Group, London

Ochre stars Pisaster ochraceus sheltering in a crevice on the west coast of Vancouver Island, British Columbia during low tide 0.3X

schematic showing 2 possible hypotheses for echinoderm phylogenyPast morphological and recent molecular phylogenetic studies have created 2 hypotheses for echinoderm evolution named Asterozoa (G. “star” “animal”) and Cryptosyringida (see schematic). At the morphological level the basis of the asterozoan hypothesis is the similarity of the 5-rayed body plan of the ophiuroids and asteroids, placing them in their a sister clade. The second hypothesis is supported mainly from characters relating to comparative anatomy and embryology, most notably an apparent homology of the ophiopluteus and echinopluteus larvae, and similarities in certain aspects of embryonic development, placing the ophiuroids, echinoids, and holothuroids in a sister clade. Past molecular phylogenetic analyses have bounced back and forth between the 2 hypotheses, with some supporting asterozoa and some cryptosyringida, but other researchers have suggested that the specific phylogenies derived in these studies may be sensitive to the particular analytical method being used. Most recent phylogenetic analyses, however, have favoured the asterozoa plan and, with strong supporting data from a recent phylogenomic analysis done by a consortium of British, French, and American workers, the final nail may indeed have been driven into the Cryptosyringida coffin. Telford et al. 2014 Proc R Soc B 281: 2014479.

NOTE the name cryptosyringida (G. “hidden” “pipe”) refers to the fact that in holothuroids, echinoids, and ophiuroids the radial water canals and radial nerves become covered during development, and these 3 classes are combined into the Super-class Cryptosyringida by the author who created the concept (A.B. Smith 1984 Palaeontology 27: 431. As noted above, the idea has fallen into disfavour

NOTE the analysis involves cDNA library construction using embryonic stages and some adult material from freshly collected specimens, with supporting nucleotide sequences obtained from online databases

ANIMATION of snail meeting SEA STAR
© 2010 Thomas Carefoot

drawing of map of snail being used in its Osyssey

To learn about west-coast SEA STARS: select a topic from the Echinodermata > Sea stars menu at the top of the page

OR: play the ANIMATION of the snail meeting the SEA STAR

OR, if you want to see other animations: follow the snail on its ODYSSEY by CLICKING on any X-marked invertebrate on the map

Phylum Echinodermata (lit. “spiny skin” G.) including sea lilies, sea stars, sea urchins, sea cucumbers, and brittle stars

Class Asteroidea (lit. “like a star” G.), including sea stars

NOTE some recent name changes in west-coast asteroids include: bat stars changed from Patiria miniata to Asterina miniata and then back again.  Blood stars Henricia spp. remain a “puzzling complex”; however, whatever species name used by an author will be the one used in the ODYSSEY. Mah 2007 p. 927 and 922, respectively, In The Light and Smith Manual Intertidal invertebrates from central California to Oregon (Carlton, ed.) U Cal Press, Berkely.