SLIP BENEATH THE WATER AND SWIM DOWN to almost any reef in the western Pacific and you will see them. Bright splashes of colour - yellow, red, orange, tan and green. Flowers in the coral garden, festooning the tops and sides of the coral heads. And around these flowers, like butterflies, schools of rainbow-hued reef fish dance in the sunlight.
The illusion is almost perfect. But the flowers are not flowers at all, they are crinoids, animals whose closest relatives include the starfish, brittlestars, sea urchins and sea cucumbers - the echinoderms.
And an investigation of crevices within the reef would reveal even more, similarly colourful crinoids.
Exclusively marine, crinoids are found in both shallow and deep waters, from tropical to polar seas, but their numbers and diversity increase as one moves toward the Equator and the western side of the Atlantic or Pacific oceans. They are most abundant in the south-west Pacific, along the northern part of Australia's Great Barrier Reef and in Papua New Guinea and Indonesia.
The crinoid is characterised by an oval-shaped central body from which as few as five or as many as 200 arms extend upwards. Each arm bears a system of grooves that converge on the mouth, called the ambulacrum. Smaller structures called pinnules branch from the arms; these carry extensions of the ambulacral system and are equipped with tube feet that are the primary means of gathering food.
Body, arms and pinnules are supported by calcareous plates. With their pinnules, each arm has a distinct feather-like appearance. The crinoid's central body is small compared to its total mass, most of which is devoted to food-gathering. The mouth and anus are located on the upper surface of the body, and are connected by a simple gut. The gonads are located in the arms; fertilisation takes place in open sea water during mass spawnings.
in the groove
Two distinct body plans are found among living crinoids. The stalked form, or sea lily, is found only at depths of 100m or more. The more mobile form, the featherstar, is the crinoid that divers encounter on shallow reefs.
In sea lilies, the body and arms are supported above the substrate by a stalk composed of disc-shaped plates, called columnals. Featherstars, or comatulids, lack a stalk in the adult stage and are usually anchored much closer to the substrate by claw-like cirri that radiate from the centrodorsal plate.
The relationship between sea lilies and featherstars was not realised until the late 1880s, when it was discovered that juvenile featherstars passed through a stalk-bearing stage.
Crinoids are filter-feeders, capturing microscopic organisms such as foraminiferans, larvae, algae, diatoms, small crustaceans and radiolarians, as well as organic detritus. The tube feet, arranged in groups of three on the pinnules, are coated with mucus that ensnares the prey. The food is then transferred to a ciliated food groove on the middle of the pinnule and conveyed down the ambulacral grooves to the mouth.
In feeding, the arms of crinoids can be arranged in several ways. Most species have a preferred style, the most common being a planar semi-circular arrangement in which the arms form an arc of 95-150¡.
The side of the arm bearing the food groove is oriented down-current, allowing food to be placed in it by the tube feet. As crinoids usually remain in one place, they rotate their arms when the flow of water reverses, to keep the food grooves positioned downcurrent.
Large, bushy crinoids have too many arms to be able to form a planar fan. They use a multi-layered hemispherical arrangement that operates like a thicket of thorn bushes, eventually snagging organisms that penetrate it.
Crinoids are among the most conspicuous echinoderms on reefs in the Indo-Pacific region, often perched high on coral, sponges or alcyonarians. Being so exposed to predators, one wonders how they survive, but predators such as fish seem to avoid them.
With a high ratio of skeleton to soft tissue, perhaps crinoids have insufficient nutritional value to be worth the effort.
There have been few observations of attacks on crinoids, but on the Australian Great Barrier Reef, biologists have found that species living in exposed positions frequently seem to have lost their visceral mass, which contains their entire digestive and other major internal systems. There are also reports of crinoids found in the stomachs of certain kinds of fish, such as clown triggerfish.
stomach for survival
Analysis of crinoids on the Great Barrier Reef has revealed the presence of polyketide sulphates, and these deter feeding by certain fish and can even be toxic. Also, many species of Indo-Pacific featherstars that live in exposed positions have developed enlarged, stiffened, even spike-like pinnules in the region surrounding the visceral mass.
Other exposed species have a dense thicket of arms, and pinnules equipped with tiny spines around the visceral mass. These might be for protection but it is clear that, like sea-urchin spines, they are not completely effective in preventing attacks. If a crinoid does lose its gut to a predator, however, it can survive by regenerating the lost parts, a process that takes no more than three weeks.
The night-feeding habit of many reef-dwelling crinoids could be a further means of avoiding predators. By day, the extensive fire-coral reefs along Red Sea shores are devoid of crinoids. At night, these same reefs are covered with masses of featherstars, mostly Lamprometra klunzingeri, emerging from nooks and crannies to take up exposed feeding stations.
It has been suggested that this nocturnal behaviour allows featherstars to avoid reef fish which at dusk and dawn feed on the mucus that adheres to the crinoids' arms.
The ability of some crinoids to swim could also be helpful in escaping from invertebrate predators. In British Columbia, Florometra serratissima responds, when touched by the large predatory sunflower seastar, by swimming away.
Their relative inedibility and dense network of arms and pinnules makes crinoids attractive to commensals - organisms which benefit from close association with a host - and other small animals that wish to hide from predators.
Commensals found on and around crinoids include clingfish, shrimp, crabs, copepods, isopods, myzostomes (annelid worms found almost exclusively on crinoids) and brittlestars.
The clingfish are small, typically about 20mm long, and their colour matches that of the crinoid. They are usually found near the base of the arms or among the stiff oral pinnules. The most common arthropod commensals are small shrimp of the genus Periclimenes, which match their hosts' colour. They move around and are often seen on crinoids' arms. Pairs of a snapping shrimp are also sometimes found.
Brittlestars are the largest commensals found on crinoids; they intertwine themselves around some portion of their bodies and appear to feed on their faeces.
Many small fish, including juvenile fairy basslets and damselfish, retreat into crinoid arms when bigger fish appear.
Ghost pipefish also frequent crinoids, both for shelter and to feed on tiny crustaceans and fish larvae that might be hiding there. They apparently seek out crinoids that approximate to their own coloration. Many fish can change colour, even if only slowly, and it is possible that ghost pipefish change their coloration when moving from one crinoid to the next.
One of the more unusual fish associated with crinoids is Merlet's scorpionfish. Some biologists believe it is a crinoid mimic, as its markings of wavy lines with clear panels on the fins make it look very much like a tangle of crinoid arms.
Its preferred habitat, along the edges of reef drop-offs, is also a favourite of crinoids. And there the scorpionfish sits, right out in the open, waiting to gobble down any unsuspecting little fish that swims close enough.
As an underwater photographer, I find crinoids and the animals that live with them attractive and challenging subjects. The ability of the various commensal animals to adapt their coloration to that of their host crinoid is particularly fascinating. And the bright colours of a few crinoids can add just the "sparkle" needed to breathe life into an otherwise monochromatic blue.
the shape and coloration of Merlet's scorpionfish allows it to mimic a crinoid as camouflage while it waits for prey
a harlequin ghost pipefish hangs among the arms of a featherstar, Bali
a crinoid galathaeid crab, Manado
the Ambon shrimp, like this one on Australia's Great Barrier Reef, lives only on crinoids and adapts its coloration to match that of its host
Fairy basslets and sweepers hover over a plate coral decorated with feeding featherstar crinoids in Milne Bay, PNG
Ambon shrimps show off their colour-changing ability on the Great Barrier Reef and in Manado, Indonesia
Featherstar crinoids with red whip gorgonians on the edge of a reef drop-off in Kimbe Bay, PNG.