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An Undulate Ray takes camouflage to the next level!
Thursday 22 July 2010
The Shark Trust has recently received some intriguing images from conservationist and photographer Steve Trewhella, who discovered a very unusual find on the seabed of Swanage Bay in Dorset this July.
Steve Trewhella, from the Seahorse Trust, has previously been in touch with the Shark Trust to inform us that Swanage Bay, whilst abundant in seahorses, also appeared to be a nursing ground for Undulate Rays (Raja undulata).
While out surveying the Bay at the beginning of the month, Steve spotted a juvenile Undulate Ray in an eelgrass bed at a depth of two metres. Not a particularly unusual sight, but on closer observation this particular Undulate Ray appeared to have seaweed growing from its back.
Steve felt that the Shark Trust would be interested in these pictures and sent them to the office to see if Trust staff could tell him any more about it. These pictures have now been seen by many marine scientists at various institutions around the UK but as yet, no definitive theory has been proposed as to how the seaweed became so firmly attached to this particular animal.
As far as the Shark Trust can establish, there have been no previous cases of this having been seen on a living specimen. Seaweed normally requires a solid or very slow moving base on which to attach itself, a fish as small and active as a juvenile ray would not normally provide a suitable habitat.
If you have seen something similar to this in the past, or have your own theory as to how the seaweed got so well established, please contact the Shark Trust at: enquiries@sharktrust.org.
 The Seahorse Trust
Comments:
Marc Smith from Dorset Wildlife Trust wrote:
'I have just seen the picture of the undulate ray with seaweed attached to its back and I have a theory. Spider crabs use seaweed, sponges and other organisms to camouflage themselves. The crabs attach these organisms to their carapace with hooked setae (hair like bristles). These bristles can securely hold seaweed in place for some time. It is possible that the ray you have in this photograph has become entangled in seaweed somehow and the weed has attached itself to the midline thorns found on this species.'
Doug Herdson from Marine Fish Information Services wrote:
'The first question is why don't elasmobranchs normally get epizoitic algae growing on them?
I suspect it is a twofold answer. Firstly, the dermal denticles are small, smooth, and regularly move in relation to each other, giving any spores little chance to settle and develop. Secondly, and probably more importantly most elasmobranchs, and particularly rays, have a covering of mucus some of which is constantly being swept off by the flow of water and abrasion by the substrate, thus carrying away any parasites or epizoites before they can become established.
So why have these mechanisms not prevented the algae from growing on the young ray in this case?
The algae, which appear to be, at least predominantly, one species of brown algae, appear to be only attached to the larger thorns. So the defence mechanism is working over the fleshy parts of the body and wings. Why do the thorns normally stay clean but not here? They probably do not have as much mucus, so maybe this individual has less mucus production than normal and consequently hardly any on the thorns, or perhaps at some time it rubbed the thorns clear of the protective layer giving the algal spores a chance of getting attached.
All just speculation, but I hope it helps.'
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