I find myself wanting to know more of the biology of the deep as we watch the high definition images from the ROV – what is that? Is it a sponge? A crinoid? What kind of fish is that? Some groups are new to me, but starfish are one species of deep-sea denizens that I am familiar with.
I remember being admonished in college to call them sea stars, since they are invertebrates and not fish, but let’s face it – most people call them starfish anyway. There are an estimated 2000 described species of starfish. They are echinoderms (like the urchins discussed earlier) and most typically have five rays or arms that radiate from a central disk.
Starfish eat a range of food from clams and snails to coral polyps to decomposing plant or animal tissue. Some have a most unusual dining habit: they push their stomach out through their mouth (located on the underside) and consume their prey before bringing it back inside. I’m glad we aren’t capable of that, although it does allow for eating things larger than your mouth!
On our first night the ROV grabbed two starfish and we were able to take a close look at their amazing structure. The top is characterized by large numbers of bumps and pores. One feature that stands out is the madreporite. It is located slightly off center (to the lower left of the central disk in the photo) and is a darkened porous plate. Water flows in through the madreporite to the animal’s water vascular system – an internal set of channels and tube feet that aids in locomotion.
I’m not sure what the other bumps and pores do, but some probably relate to defense, water exchange or sensory information. The underside (or oral surface) has less topography. There are five deep grooves one along the midline of each arm. These ambulacral grooves contain the rows of tube feet.
A close-up shows rows of long, flattened movable spines on each side of the ambulacral groove.
It looks like an ad for teeth whiteners of the deep, but they are actually spines that are used to cover the groove to protect the tube feet. The word ambulacrum is Latin for “covered way”, an apt name for this protective structure.
One thing I did not look for are the eyespots, small light-sensitive structures located at the terminal end of each arm. They are supposedly slightly pigmented so I will take a closer look if we bring up more specimens.