Synaptula spinifera

Synaptula spinifera is a sea cucumber in the family Synaptidae, the group known as worm-like sea cucumbers. It was first discovered in the anchialine lagoon of Kakaban Island, part of Indonesia’s Derawan Islands, and is believed to live only in that isolated body of water.

Description

This is a small holothurian: the largest preserved specimen measured 85 millimeters long and 8 millimeters in diameter. Living individuals are brown-red with very fine white longitudinal lines and small white dots. At the front end are 13 tentacles, each with 18 to 22 finger-like projections joined by a web. Like others in its order, it uses these tentacles to feed, primarily as a suspension feeder.

Inside the body, a very large cartilaginous ring surrounds the white calcareous ring. Its ossicles come in a few forms—anchors, anchor-plates, and miliary granules (though granules are often absent)—and are spread through the body wall but absent from the tentacles. The species name spinifera refers to the long spine on the bridge of the anchor-plate.

Habitat

Synaptula spinifera lives in Kakaban’s lagoon, which is completely encircled by a densely vegetated limestone ridge. The lagoon is anchialine, meaning it connects to the sea only through submarine passages in the porous carbonate rock and small fissures.

The island was once an atoll but was uplifted 40–60 meters above current sea level, isolating the lagoon’s ecosystem. The lagoon’s salinity is lower than seawater, at 26–28 psu. Echinoderms, especially holothurians, are sensitive to low salinity, so it is thought that the species now living there gradually adapted to the changing conditions after the uplift and are remnants of a once more diverse fauna.

This sea cucumber occurs throughout the lagoon, which measures 1.5 by 2.6 kilometers. It is often found between the thalli of the dominant Halimeda algae or on Enteromorpha clathrata, which frequently grows on Halimeda.

It lives at depths from 0.25 to 8 meters, with a density of 20–34 individuals per square meter in the Halimeda meadows. It may also inhabit the roots of Rhizophora mangroves fringing the lagoon, which are often overgrown with epiphytic algae and invertebrates like encrusting sponges. Extensive surveys of the outer reef of Kakaban atoll have not found Synaptula spinifera, supporting the idea that it is endemic to the isolated lagoon.