Most marine snails of the families Muricidae and Thaididae produce in their hypobranchial gland (mucus gland) a colorless secretion containing minute amounts of chromogens, which develop under the influence of light and oxygen into a pigment known as “Tyrian Purple.” The hypobranchial gland of Plicopurpura pansa (Gould, 1853) is an exception among the muricids, because it is so active the snails can be stimulated periodically to expulse the secretion without harming the animals. In view of reported drastic declines of the populations of P. pansa because of the Tyrian Purple exploitation, in this laboratory study the effect of periodically “milking” of P. pansa on the frequency of expulsion and on the mortality of the snails was determined. At the beginning of the experiment using 110 animals (55 males and 55 females) only 30% expelled secretion. No relation was found between the occurrence of expulsion and the size or sex of the animals. In contrast to the laboratory snails the proportion of expulsions from free-living animals was 56%. Also here no differences were found between the occurrence of expulsion and the size or sex of the animals. For a period of 98 days it was tried in the laboratory to obtain daily secretions from 46 snails (23 males and 23 females). The frequency of expulsion declined drastically and at the end of the experiment no expulsions at all could be obtained. No difference could be noted between the sex and size of the snails and the decline of secretions, however, the total number of secretions during the test period was nearly double with the females, than with the males. The survival rate of the male snails was 83, of the females 87%. Handling mistakes during cleaning and feeding were the main reason for the mortalities. The attempt to milk the snails weekly caused, as during the daily milking experiment, a decline in the frequency of expulsion, and after 13 wk only 13% of the animals expulsed. To milk the animals every 2 wk caused a decline in the number of snails that expulsed, and only 18% of the animals secreted. No decline in the rate of expulsions was found by milking the animals every 3 and every 4 wk, and all animals survived. Frequent attempts to milk the animals has no impact on the survival rate, however, it affects the occurrence of expulsions. From the results reported here the decline of natural snail populations after milking could be explained by the environmental conditions of the intertidal zone. After milking, the animals don't adhere fast enough to the rocks and they are washed away to unsuitable areas by the high wave action. For this reason, P. pansa should never be removed from the rocks, and the collection of the secretion should be allowed only in exceptional cases.
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Vol. 24 • No. 1