CRITTER CONFIDENTIAL
Cotton-spinner
Holothuria ( Panningothuria ) forskali Echinoderm engineer or prickly slug ? Paul Naylor sings the praises of a misunderstood critter of UK shores
When I come across the instantly familiar ( but let ’ s face it , not instantly attractive ) form of a cotton-spinner , ‘ Ecosystem Engineer ’ is not the first term that springs to mind . But perhaps it should , as we shall see .
What looks like a fat , prickly slug is a type of sea cucumber that belongs to the same group of animals as starfish , brittlestars and sea urchins . The protective conical spikes on the cotton-spinner ’ s dark body are in fact quite reminiscent of those on a starfish , while the other characteristic echinoderm group feature is the rows of tube feet on its underside . These move the animal across the silt-covered rocks and reefs where it generally lives .
Other tube feet around the cotton-spinner ’ s mouth are different and form short , stumpy tentacles . In contrast to the long feathery tentacles of filter-feeding sea cucumbers , these short tentacles shovel copious quantities of muddy material into their owner ’ s mouth as it crawls very slowly along . After eating and processing all this sediment , the cotton-spinner then leaves behind an incredibly distinctive trail of droppings that looks like a string of beads , or perhaps minute sausages . I think this is one of their charms , but perhaps I ’ m just odd !
Whatever your view , it is this reworking of the sediment and recycling of its nutrients that gives them the ‘ Ecosystem Engineer ’ award . Materials that they readily consume and recycle include faeces from other animals such as fish .
A life spent crawling out in the open rather than jammed in a rocky crevice or buried in the seabed like many other sea cucumbers means that the cotton-spinner needs additional defences . In fact its common name comes from this animal ’ s ability to release a mass of sticky white threads when attacked , which confuses and distracts its assailant . These threads are part of its internal organs but can be regrown , although this must be very resource-intensive . One of its main predators is the strong but slow edible crab , and you can imagine how the sticky threads could do an excellent job of tangling up the crab ’ s claws as it tries to grapple with this sea cucumber . It is thought that in addition to this rather extreme method , cotton-spinners may also release chemicals which warn fish that they are unpalatable .
Cotton-spinners have separate sexes and , as with so many marine animals , they release eggs or sperm into the seawater . It would be interesting to know if they congregate to do this . Cotton-spinners spawn in the spring , possibly in response to rising sea temperature , and tend to climb up to the higher points of reefs to maximise dispersion . Planktonic larvae result from the fertilisation and youngsters settle on the seabed after some development but , interestingly , very few juvenile cotton-spinners are seen . Presumably , they stay hidden under stones or in holes until they are large enough to be in full view without facing too much danger . �
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Cotton-spinner under rock overhang showing yellow underside which is a common feature