File:Lytechinus variegatus (variegated sea urchin) (Cayo Costa Island, Florida, USA) 2 (24269739502).jpg

Lytechinus variegatus (Lamarck, 1816) - variegated sea urchin (oral side) in Florida, USA (January 2016).

The echinoids are a group of echinoderms - they are starfish relatives. On modern Earth and in the fossil record, echinoids are exclusively marine. Two broad groups exist - the regular echinoids (sea urchins) and the irregular echinoids (heart urchins and sea biscuits and sand dollars).

Sea urchins have subglobular to slightly flattened, radially symmetrical skeletons (tests) composed of calcite (CaCO3 - calcium carbonate). The sea urchin test is covered with spines - these readily detach after death. Sea urchins have a mouth with a pentaradial structure called an "Aristotle's lantern" (see the center of the urchin in the above photo) that is used to graze on algae and other biofilms on hard substrates.

Irregular echinoids have moderately flattened to significantly flattened, bilaterally symmetrically tests of calcite that are covered with small, short, almost hair-like spines. Most irregular echinoids are infaunal and deposit feeders.

The dead sea urchin shown above is Lytechinus variegatus, the variegated sea urchin, or short-spined sea urchin. The specimen sits on a marine beach having abundant bivalve shells, some gastropod shells, fragmented sand dollar tests, and other organisms, plus siliciclastic-dominated sand.

Classification: Animalia, Echinodermata, Echinoidea, Temnopleuroida, Toxopneustidae

Locality: marine beach at the southern tip of Cayo Costa Island, Gulf of Mexico coast of southwestern Florida, USA (vicinity of 26° 36' 48.74" North latitude, 82° 13' 19.91" West longitude)

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