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Underwater Photo of the Week - July 16, 2018

Crinoid on a Fijian Wall

POTW July 16, 2018

 

Hi Gang!

This week’s POTW is a seascape that features a crinoid on a wall in Fiji. Closely related to sea stars, brittle stars, basket stars, sea cucumbers, sea urchins, heart urchins and sand dollars, crinoids are invertebrates described in the phylum Echinodermata. An ancient group of animals, echinoderms are well known from fossil records. There are approximately 13,000 extinct species and 7,000 described currently living species. These numbers give the phylum the most members of all phyla that do not have any freshwater or terrestrial forms. With only a few exceptions that inhabit brackish waters, echinoderms occur in the marine environment. The phylum is well represented in all oceans, but most species occur in temperate seas. 

 

Crinoids add splashes of color in many tropical reef communities, especially in the Indo-Pacific region. Their colors vary greatly, and include bright red, orange, yellow, green hues. Others are almost pure white or black.  

Members of the class Crinoidea, crinoids occur in a wide array of colors and color combinations. Crinoids are characterized by (1) a mouth that is located on the top of the body, and (2) a number of long arms that surround the mouth. Various species of crinoids are equipped with anywhere between five and two hundred arms. Also known by the terms sea lilies and feather stars, crinoids inhabit waters close to the surface down to depths of at least feet 29, 409 feet (9049 meters). 

Fossil records indicate that crinoids are among the oldest of all sea creatures. Ancient crinoids were immobile being attached to the sea floor by large stalks. It is believed that over time crinoids had to become more mobile to catch prey and escape predation, and most lost their stalk. However, some ancient species still flourish today, and scientists sometimes refer to them as “living fossils”.

Crinoids are suspension feeders using the tiny tube feet located on their arms to capture small food particles from the water column. The arms of crinoids are branched into small structures called pinnules, and each pinnule contains a number of tube feet. The entire pinnule is covered with secretions that trap food particles that, in turn, are grabbed by the tube feet. Once laden with food the crinoid’s arm is curled inward to its mouth where the food is taken in. 

Crinoids are equipped with claw-like legs know as cirri that are used to grip the substrate. Crinoids can change position by walking, and some species swim for short distances by undulating their bodies. Many species tend to be reclusive during the day, but at night crinoids often perch themselves high atop sea fans and coral heads where they gain the best access to food-bearing currents. 

When photographing crinoids, I often try to get below them and shoot at an upward angle while composing a frame that makes the crinoid standout in contrast against the water around them. That is what I did to create this week’s POTW, and I think that technique worked well for this image.

I hope you enjoy the splash of color provided by this week’s POTW, and that it brightens your week.

See you next week,

Marty 

See you next week,