Autumn gales throw up on our western beaches heaps of rotting kelp and other seaweeds wrenched from their rocky substrates. Such was the scene at Balchladich (NC0230) on 8th November 2018, when I went to prospect the beach for material for a workshop.
The major component on this occasion, as usual, was cuvie Laminaria hyperborea, the most robust of the kelps which grow in forests at or below low tide mark. It has thick rough stipes (stalks) which hold the fronds upright in life, facilitating photosynthesis in a wave-battered environment. These stipes acquire in their lifetime a rich covering of smaller plant and animal life, epiphytes and epizooites.
One of the stipes, picked out at the workshop, was host to such an epizooite. It took the form of a pale-brown fuzz, extending up to 7cm from the surface of the lowest 30cm of the stipe, to which it was securely attached. Examined under a hand lens, this resolved itself into the much-branched skeleton of a hydroid.
Hydroids are solitary or colonial animals distantly related to jellyfish, with alternating generations, usually between a sessile stage, as in this case, and a mobile medusoid (jellyfish-like) stage, which is their dispersal phase. There is however one large colonial hydroid that is free-floating in both stages, the by-the-wind-sailor Velella velella, wrecks of which sometimes occur on our shores (By-the-wind-sailor on the north-west coast).
Over 140 species of hydroids occur in British waters and their identification is not easy, requiring access to a low-power microscope and mastering the detailed terminology of their anatomy. However, a good look down the microscope, fixed in the mind with some sketches, and then a slow scan through the illustrations in the Handbook of the Marine Fauna of North-West Europe (Hayward, P.J. and Ryland, J.S., 2012), enabled me to put a name to this one. It is Amphisbetia operculata, whose preferred habitat is, indeed, the stipes of cuvie. Although the living parts have long since disappeared, the delicate skeleton is a beautiful object, with numerous, paired, sharp-cusped hydrothecae, which once housed feeding polyps, and occasional flask-shaped gonothecae, in which medusae were produced. To give some idea of size, the gonothecae are about 1.6mm long.
Elsewhere on the beach, I found, amongst the kelp, something rather bigger. It was the largest mermaid’s-purse I have ever seen, measuring 18 x 14 cm. Black all over when collected, it acquired as it dried some finely-fringed blonde highlights.
Mermaid’s-purses are the eggcases of cartilaginous fishes such as small sharks, skates and rays. This was that of the Flapper Skate Dipturus cf. intermedia, which can reach a size of almost 3m and has eggcases to match. They are apparently ‘extremely rare finds around the UK apart from a few locations in which populations still exist’, which was gratifying. Coincidentally, Vickii Campen of CALL had found another example a little further up the coast at Raffin (NC0131) on 8thAugust, so perhaps we have a breeding population in The Minch.
Ian M. Evans