Isostichopus fuscus

The brown sea cucumber, Isostichopus fuscus, belongs to the family Stichopodidae and lives in the eastern Pacific. German biologist Hubert Ludwig first described it in 1875. Its thick, spineless skin makes it easy to eat, which has driven unsustainable fishing for export to East Asia, leading the IUCN to classify it as Critically Endangered.

Distribution and habitat

This species ranges along the Pacific coast from Mexico (including the Gulf of California) through Central America down to northern Peru, and also around eastern Pacific islands like the Galápagos. It prefers shallow waters between 3 and 29 meters deep, though it can be found from 0 to 40 meters, typically on rocky reefs with sandy patches. It is most common where Ulva seaweed grows, and it is more active at night, hiding during the day to avoid predators.

Feeding

Like other sea cucumbers, I. fuscus feeds by ingesting deposits from the seafloor, and it survives better on soft substrates like sand or clay. Recent research suggests a mix of P. durvillaei and S. ecuadoreanum seaweeds is an optimal diet, though it can eat other types.

Morphology

These sea cucumbers are small, averaging 23 cm in length and 386 grams. The largest recorded specimens reach 40 cm and 830 grams, though weight changes seasonally with reproduction and maturity. Growth is allometric—body length and weight increase together. The species is dioecious, with gonads only distinguishable under a microscope; there is no visible difference between sexes unless the gonads are ripe, at which point males have white gonads and females have orange ones.

The body is elongated, soft, and gelatinous, curved like a half circle, dark brown with orange papillae (small bumps). On the underside, ambulacral extensions help with movement. In 2013, the first albino individuals were found—one in Loreto, Mexico, and another in Los Angeles Bay.

Sexual reproduction

Spawning occurs from July to September, with post-spawning gonads appearing in October. Gametogenesis (development of sex cells) happens from January to July, with oogenesis starting about a month before spermatogenesis. The species is an episodic spawner, with reproduction peaks followed by sharp declines, and water temperature likely acts as a trigger. The reproductive cycle has five stages: undetermined, gametogenesis, maturity, spawning, and post-spawning, completed annually.