Crinoids are suspension-feeding animals that capture food with their tube feet as water flows through their multi-branched arms.
Crinoids are considered basal to the other living Echinodermata (i.e, they are the most "primitive" and show closer relationship to older extinct echinoderms).
Today, most crinoids occur as one of two forms. The more basal or "primitive" of modern crinoids are those with a stalk. These can be seen as fossils (many from the Paleozoic) or living today, mostly in the deep-sea. They can move (this is the subject of a post next week) but generally do not. They look like this:
Modern crinoids occur primarily as unstalked forms, aka "feather stars"-the Comatulida, that can occur in the deep-sea but also in tropical shallow-water habitats. They are mobile and can be very brightly colored.
but then, there's THESE weirdos.
These are weird beasties in the family HOLOPODIDAE (Cyrtocrinida) that are kind of an odd intermediate form. They live in the deep-sea (~100-900 m)and lack a full stalk, with five stout and stocky arms (other crinoids can have many more) but are cemented permanently to the substratum.
A new paper by Donovan & Pawson 2008 describes a new species in the genus Holopus from the tropical Western Atlantic. Their new species, Holopus mikihe is described from a single distinctive specimen with very prominent tubercular ridges on each arm as seen here:
Based on observations from the Florida based submersible the Johnson Sea Link, (operated by Harbor Branch Oceanographic Institute (HBOI) in Ft. Pierce, Florida) several interesting observations of the tropical Atlantic species, Holopus rangii were made:
*H. rangii lives upside down on rocky overhangs.
*Arms seldom open, but when they did, they formed a funnel-like array, taking 0.8 to 1.0 seconds in response to stimuli (either the bow wave or possibly the lights from the submersible). As a model think of it as a living crinoid fist! Opening and then closing!
*It has been suggested that these things feed on active prey!! (as opposed to most crinoids that passively pick food out of the water)
WARNING: COMPLICATED SCIENTIFIC SIMULATION OF HOLOPODID FEEDING MECHANISM (hypothetical!!) TO FOLLOW: