Trophodiscus almus

Lore & Background

Trophodiscus almus was first described by the American zoologist Walter K. Fisher in 1917 from material collected by the marine research vessel USS Albatross. It was thought to be endemic to the Sea of Okhotsk, but has more recently been discovered in the Sea of Japan and waters around Hokkaido. Its depth range is 150 to 300 m, and it is fairly uncommon. In the Sea of Japan it is found on muddy sand bottoms where the water temperature is 1.5 °C.

The starfish has a five-armed pentagonal shape, growing to about 33 mm in diameter, with a wide disc and short, bluntly pointed arms. The aboral surface is covered in paxillae, umbrella-shaped structures with spiny margins. A notable feature is the 13 to 20 large calcareous plates extending around the margins of each arm. Two color forms exist: some individuals have a bright red or orange-red aboral surface and a paler reddish-brown oral surface; others have a yellowish-green aboral surface and yellowish-brown oral surface. In both forms, the superomarginal plates are paler and contrast with the rest.

Juveniles are brooded on the aboral surface of the female between the paxillae. Up to sixteen young have been found on one adult, and the juveniles are either reddish-orange or greyish-green. The color of the offspring does not always match that of the parent; greyish-green young have been found on red adults. This brooding location is very unusual among starfish, shared only with the sister species Trophodiscus uber, Leptychaster kerguelenensis, and Ctenodiscus australis.

Reader's Guide

Trophodiscus almus is significant primarily for its unusual reproductive strategy: brooding its young on the aboral surface between the paxillae. This behavior is extremely rare among starfish, with only three other species known to do so: its sister species Trophodiscus uber from the Sea of Okhotsk, Leptychaster kerguelenensis from the extreme south of the Indian Ocean, and Ctenodiscus australis from the Southern Ocean. The species was first described in 1917 by Walter K. Fisher from material collected by the USS Albatross, and was initially thought to be endemic to the Sea of Okhotsk. Its range has since been extended to the Sea of Japan and waters around Hokkaido. The starfish exhibits two distinct color forms—red/orange-red and yellowish-green—with the superomarginal plates paler in both. The offspring colors do not always match the parent, suggesting genetic or environmental variation. Its depth range of 150 to 300 m and preference for muddy sand bottoms at cold temperatures (1.5 °C) indicate a specialized deep-water habitat. The species is fairly uncommon, and its Japanese common name is Komochi-momiji.

Did You Know?

• Up to sixteen juveniles have been found brooded on a single adult.
• The species has two color forms: bright red/orange-red or yellowish-green.
• It was first described in 1917 by American zoologist Walter K. Fisher from material collected by the USS Albatross.