Trophodiscus almus

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Trophodiscus almus is a small starfish in the family Astropectinidae that lives in fairly deep waters of the Sea of Okhotsk, the Sea of Japan, and around Hokkaido, Japan. It is notable for brooding its young on its upper surface, a rare trait among starfish. In Japanese it is known as Komochi-momiji.

This five-armed starfish grows to about 33 mm across. It has a wide disk and short, blunt arms. The upper (aboral) surface is slightly curved and covered with paxillae—umbrella-shaped plates with spiny edges. Paxillae are larger near the arms and smaller toward the center and tips. The madreporite lies about halfway from the center to the edge, the anus is small, and there are papulae (gill-like structures) between and under the paxillae. A distinctive feature is 13–20 large calcareous plates that run around the margins of each arm.

There are two color forms. Some individuals have a bright red or orange-red upper surface with a paler reddish-brown underside, while others have a yellowish-green upper surface with yellowish-brown underside. In both forms the pale superomarginal plates stand out.

Trophodiscus almus was first described in 1917 by Walter K. Fisher from material collected by the USS Albatross and was long thought to be endemic to the Sea of Okhotsk. It has since been found in the Sea of Japan and around Hokkaido. It lives at depths of 150–300 meters (492–984 feet) on muddy sand bottoms, where the water can be around 1.5 °C.

Juvenile stars are carried on the female’s aboral surface between the paxillae, with up to sixteen young observed on one adult. The young can be reddish-orange or greyish-green, and their color does not always match the parent. Brooding the young in this way is very unusual among starfish; only a few other species do it, including the closely related Trophodiscus uber, Leptychaster kerguelenensis, and Ctenodiscus australis.