Starfish - brutes or bashful
Extracted from The Sea that Never Sleeps by Douglas David Seifert, read the full feature in the DIVE night diving special
The most notorious nocturnal predator is the crown-of-thorns starfish, Acanthaster planci, an out-sized starfish that can reach 60cm in size with 21 arms emanating from a central disc. It lives exclusively on the coral polyps of stony corals – preferring branching or table coral species. It extrudes its stomach over the structure of the coral, subjecting the soft-bodied polyps to an enzyme that liquefies them and they are completely absorbed by the starfish’s stomach.
Fortunately, coral have an ally in their siege war with crown-of-thorns starfish: the large gastropod mollusc known at Triton’s trumpet (Charonia tritonis). Once a Triton’s trumpet detects the coral-eating echinoderm, the crown-of-thorns starfish doesn’t really have a chance. The trumpet is large, strong, fast and has enough mass that it can pin a starfish to the reef while it injects the starfish with an enzyme of its own, one that liquefies the innards of the starfish and allows the giant snail to literally suck the life out of the starfish.
Another fascinating member of the starfish family – the basket star – is benign to corals, other than perhaps crawling over them, and standing atop them for the duration of the darkness. By day, the basket star, like its smaller cousins the brittle stars, is difficult to locate. It hides in any dark recess it can find – a vase sponge or deep reef crevice – and balls up tightly, compressed into as small as space as it can manage.
With the fall of night, the basket star crawls from its sanctuary and ascends to an unobstructed promontory of the reef where it can unfurl its feathered branching arms, each supporting further refined branches of feathered arms, and so on and so on, each arrayed with small sticky hooks to snag plankton from the current.
The basket star has five basal arms branching from the central disc and each of these arms may branch into a further twenty arms. A naturalist named John Winthrop, then Governor of Connecticut, in 1670, counted all the branched arms from one dead specimen of basket star from the Gulf of St Lawrence and counted ‘81,920 small sprouts or threads’.
Basket stars may live as long as 35 years and often have species of Periclimenes shrimp living among their arms. There are approximately 170 species of basket stars and the largest species can achieve an arm length of 70cm, thus making the reach of the fan from arm tip to arm tip more than 140cm.
The sight of a basket star feeding at night is ethereal: when all the arms are fully deployed and arrayed around the central disc of the basket star and slowly sieving the current, the appearance is that of a static firework or a radar dish constructed of the most delicate lace. Basket stars are extraordinarily light averse – casting a beam of light across its unfurled body feeding on the reef at night can trigger an immediate withdrawal of its arms towards its body and the animal crawling away towards any darkness it can find.