Sea lemon nudibranch laying egg ribbon rosette on a leg of Swanage Pier
Apologies for the awful pun , just one of many possible on the egg theme ! This is the time of year when if a fellow diver asks me ‘ what was that strange thing I ’ ve just seen ?’ the answer is often an egg mass of some kind . Thinking about which examples of characteristic eggs would be good to include in this ‘ Critter ’ then made me realise how so many of them are the calling cards of our wonderfully varied molluscs .
Indeed , when searching for attractive small nudibranchs , their white egg strings and coils attached to seaweed and hydroids are often easier to spot than the adults responsible for them . Unless I can see one of the small sea slugs actually laying , I find it difficult to know which species the eggs belong to ; however , in some larger species the eggs are easily identified . There is the coiled ribbon rosette of the sea lemon and the dead men ’ s finger sea slug ’ s twisted string that is attached to the seabed with a transparent membrane . Highly distinctive egg masses are also laid by non-nudibranch slugs including the highland dancer ’ s large spiral and the sea hare ’ s ( often pink ) ‘ spaghetti ’.
As well as varying in their appearance , eggs from different mollusc species can develop in contrasting ways , even when they are quite closely related . Many produce larvae that drift with the plankton , while others hatch straight into miniature versions of adults . An example of the latter is those laid by the common whelk . Their large egg clumps can be spotted attached to rocks or other structures and may apparently represent the work of several females , although I have only ever seen a single owner engaged in that painstaking laying process . Each of the many capsules in the clump contains up to a thousand individual eggs , but the vast majority don ’ t hatch and are eaten by the fortunate few survivors . The whelk ’ s egg masses are actually best known when they are washed up and empty , looking like bunches of Rice Krispies and sometimes called ‘ sea wash balls ’, as they were historically used by sailors to clean their hands .
The dog-whelk , the deadly ‘ driller-killer ’ of rocky shores , lays distinctive creamcoloured bottle-shaped egg capsules in crevices . As with the whelk , the eggs develop into crawling youngsters and this , along with an adult dog-whelk ’ s reluctance to roam when prey is plentiful , can result in what I find intriguing local evolution . Populations in different areas develop different shell shapes , for example , depending on whether it is more important for them to resist predatory crabs by having a small shell opening , or cling to wave-battered rocks with a strong foot that needs a larger shell aperture .
One sea snail , another ‘ driller killer ’, is even named after its eggs . The necklace shell ’ s collar-shaped egg mass can be found on sandy seabeds , sometimes near its other calling card , a small open clam shell with a neat hole that indicates it was a necklace shell victim .
Cuttlefish and squid , those highly cultured cousins of slugs and snails , produce easily spotted and identified eggs too . The ‘ sea grapes ’ so meticulously attached to seagrass and seaweed by female cuttlefish have what I think is the best story of the lot . Although their cases are dyed black with ink , the eggs become more transparent with time so the baby cuttlefish can observe the outside world . Experiments have shown that newly hatched cuttlefish are more likely to hunt prey that they have seen from inside the egg and this was the first known example of visual learning by embryos . Now that really is eggsciting ! �
Dead men ’ s finger sea slug with egg string , among plenty of its soft coral food
Whelk laying a clump of the characteristic egg capsules
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