Dog Whelks good in­di­ca­tors of TBT in sea­wa­ter


THE Dog Whelk is a very com­mon sea snail found in tide pools on rocky seashores all around the coast of Ire­land. It is a meat-eater and feeds on bar­na­cles, mus­sels and other marine life that it finds at­tached to the rocky seashores it lives on.

Since its prey is at­tached and can’t es­cape, a hun­gry Dog Whelk leisurely bores into its vic­tim’s shell, in­jects di­ges­tive enzymes and, over time, sucks up the re­sul­tant soup of bro­ken-down meat. Its diet is said to de­ter­mine its shell colour.

Shell colour varies from white to black through a spec­trum of greys, yel­lows, or­anges and browns. And in­stead of be­ing all one colour, shells are of­ten dec­o­rated with bands of any com­bi­na­tion of all of these colours.

Dog Whelks are not ed­i­ble so they are of lit­tle in­ter­est to most peo­ple other than to chil­dren col­lect­ing their empty shells for their at­trac­tive colours.

How­ever, dur­ing the 1970s and 80s sci­en­tists noted a de­cline in Dog Whelk pop­u­la­tions. The cause of the de­cline was not im­me­di­ately ob­vi­ous. These sea snails breed in spring and those who are fa­mil­iar with rocky seashores will know their yel­low eggs about the size of grains of rice at­tached to rocks in groups.

In­ves­ti­ga­tions re­vealed that fe­males were not lay­ing eggs. The rea­son for their in­abil­ity to lay turned out to be caused by they each grow­ing a pe­nis that blocked their oviducts. Though mainly seen in Dog Whelks, the dis­or­der, called im­po­sex, was also found in other sea snails.

The cause of the dis­or­der was be­lieved to be due to marine pol­lu­tants. The cul­prit was dis­cov­ered to be mainly a chem­i­cal called trib­utyl tin. Trib­utyl tin, pop­u­larly known as TBT, is found in anti-foul­ing paints used on boats and port struc­tures. The dis­cov­ery, to­gether with fears for pos­si­ble un­known im­pacts it might have on peo­ple, led to calls for the ban­ning of that chem­i­cal.

A ban on the use of TBT in struc­tures and ves­sels less than 25m long was in­tro­duced in Ire­land in 1987. That re­sulted in an over­all im­prove­ment ex­cept in the vicin­ity of fish­ing and ship­ping ports.

The most re­cent (2015) sur­vey re­ported ‘on­go­ing im­prove­ment’ with lev­els of TBT fall­ing sig­nif­i­cantly. Fur­ther im­prove­ment is ex­pected as it is planned to even­tu­ally phase out TBT al­to­gether.

Fe­male Dog Whelks re­main the most sen­si­tive in­di­ca­tors of TBT in sea­wa­ter and as such act as whis­tle blow­ers and guardians in the quest for clean seas around our shores.

Trib­utyl tin (TBT) has been known to cause in­fer­til­ity in fe­male Dog Whelks.

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