by Matthew Norton
Dog whelks (Nucella lapillus) are small sea snails, but they are also very ambitious predators who will attack barnacles, mussels and other shellfish that are bigger than they are. They cannot swallow their prey whole (and then spit out the inedible parts), or prise the shell apart with powerful claws. Instead they use the less common method of sliding onto the shell and then drilling into it using their radula, a conveyor belt of teeth that dog whelks have modified for drilling. To soften up the shell, making it easier to drill through, they release a chemical mixture of acids, toxins and ‘sticky’ proteins to (slightly) dissolve the shell material. Once they break through the shell dog whelks get really nasty by releasing more chemicals that turns the animal into a mushy soup that the they can suck up.
Despite these formidable weapons, dog whelks still face many problems while on the hunt, many of which are caused by the fact that it takes hours, if not days for them to break through each shell. So if they want to survive, they have to include some strategic thinking into their hunting activities. For example the long drilling time can leave dog whelks exposed to the harsh seashore environment and their own predators. When conditions are especially harsh, or their predators are close, dog whelks are more likely to hide away in rocky crevices, unless they are very hungry and the meal they desperately need is worth the risk.
Dog whelks also have to be careful in how they handle their prey. Firstly, they need the amount of food from a particular mussel (or other shellfish) to be worth the monumental effort they put into drilling through the shell. Some studies have shown that dog whelks prefer mussels of a particular size and focus drilling on the thinner parts of the shell, although this takes time to learn for inexperienced mussel hunters. Secondly, their prey sometimes fight back. In particular should a single dog whelk get too close to several mussels they may use their byssal threads to tie them down, leaving the dog whelk unable to move, or attack the mussels. If they cannot break free from this trap the dog whelk will die from starvation, or prolonged air exposure. It unsurprising that most dog whelks will hunt at the edges of dense mussel beds with only the occasional fool venturing further in.
Dog whelks are one of those creatures that show, in gruesome detail, how nasty nature can be. However, this does not make them mindless eating machines, they still consider their options and potential risks while looking for their next meal. With their brains, or what passes for a brain in this little snail, they are unlikely to ‘think’ as much as we do, but dog whelks are still effective hunters, combining powerful weaponry with strategic ‘thinking’.
From a human perspective
Unlike most creatures in the sea, animals on the seashore have been within our reach for thousands (probably millions) of years and so have had more potential to influence human history and culture. In the case of dog whelks, and other closely related species, we having been using their flesh to make purple and violet dyes for centuries. Clothing dyed in “Tyrian purple” was particularly valuable in ancient cultures, often worth its weight in silver and worn as a status symbol. Archaeological finds have dated its use to at least the 13th century B.C. in the Mediterranean region and pre-roman times for much of Europe.
Dog whelks would have been killed and crushed in vast numbers to satisfy the demand for this dye, which may sound excessively destructive, but this is nothing compared our more recent coastal activities. In particular our boats, both large and small, once had their hulls painted with a chemical called tributyltin (TBT) to prevent sea creatures from growing on it. Unfortunately, this had unexpected effects on dog whelks, causing females to grow male body parts by interfering with their hormones. This phenomenon is called ‘imposex’ and can make reproduction impossible for them by blocking up the tubes through which they eject their egg capsules, a blockage which can kill them as the capsules accumulate in their body.
Dog whelks were hit hard by TBT exposure during the 1970’s and 1980’s, especially near harbours and other boating hotspots, with populations being completely wiped out in some areas. Eventually, this issue was addressed with many countries bringing in partial bans on the use of TBT in the 1990’s, followed by a full worldwide ban in 2008. Since this action was taken TBT levels have been coming down, although many habitats will remain contaminated for a long time. To measure this decline we have been using the number of dog whelks that are still suffering from imposex as an indicator. Some have kept groups of dog whelks in cages in rivers and estuaries for several months while others have monitored natural populations on the shore for many years.
It is a sad fact that our actions have very destructive effects on sea life, whether intentional or not, and we are often slow to do anything about it. In the case of TBT its purpose was to be harmful to sea creatures, but we probably didn’t realise how effective it was, or that it could leak away from the hulls of boats. However, the actions we took (eventually) to reduce TBT contamination in the seas shows that we can learn from our mistakes.
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All other images are public domain and do not require attribution