Wednesday, January 16, 2008

Things that live in sand, Part 2. Feathers help.

Yesterday I wrote about Anniella pulchra, a sand-dwelling, legless lizard, an elongate, cylindrical animal almost devoid of external features. Today, a very different psammobiont (word of the day, means sand-living organism, from the Greek psammo- for sand), an intertidal valviferan isopod, Chiridotea caeca.

You are probably more familiar with isopods than you know. Turn over any log or patio stone, and the "wood bugs" or "wood lice" or "pill bugs" you find are isopods, which also means they are crustaceans. There are vastly more aquatic and marine species, and many of these live in intertidal zones. Chiridotea, a genus of the Atlantic coast of North America, is specialized for living within the top layer of marine sand, and is surprisingly hirsute for something that lives in a particulate environment. Most noticeably,the pereiopods, which are the limbs of the pereion (thorax, more or less), bear long, feather-like plumose setae extending from the trailing edge of each leg.

Scanning electron micrograph of the plumose sete on the ischium of a pereiopod.

Underside of Chiridotea, showing arrangement of appendages. Note how they project outward in a graded change of direction around the periphery of the body. One of the uropods (tail valves) is flipped open to show the flap-like pleopods (swimming feet) that flutter and send a jet of water backward, adding forward thrust to that provided by the legs.

Scanning electron micrograph of underside of pereion, showing legs and setae. Each leg moves back and forth beneath the sheltering roof of setae extending from the leg in front. The setae keep sand grains from falling between the legs and allow them to draw forward on the recovery stroke, the necessary setup for the following power (pushing) stroke. The sum of all the pushing actions of the legs, which occur in a back to front sequence, propel the isopod forward through the submerged sand.

The rectangles represent planes of equal area, and show the relative orientations of the planes in which the limbs move. Each slides back and forth beneath the setae of the leg in front.

The spacing of the leg setae is correlated with the coarseness of the sand the animals inhabit. If moved to a finer substrate, they become bogged down. Reducing water content in the sand, as happens as the tide recedes down a beach, also hinders locomotion. Thus Chiridotea are most easily found by running one's fingers through the sand in shallow pools in the intertidal zone.

Details of the setae on the various appendages. A, C = gnathopod; B = pereiopod; D = uropod.

More about this creature, what it does and how it moves through beach sand, can be found in this publication, for which these images were originally prepared as part of an undergraduate project...a long time ago.

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