Notes on Haliotis tuberculata. I

Extract

1. Eggs may be clearly differentiated in a female specimen of Haliotis before a length of 2 cm. is reached by the animal, and the female gonad visible before 3 cm. is reached. The female may spawn at 5·5 cm. In the male, active sperms with tails may be differentiated before a length of 3 cm. is attained, and the gonad visible before 4 cm. is reached. The male may spawn at 5·5 cm., if not before.

2. It is estimated that specimens up to about 3·5 cm. long are a year old or less. If so, male and female can both spawn at three years old at any rate, more likely at two years.

3. The first week in August certainly lies within the breeding season. The range of that season is not known, but evidence points to its being late summer.

4. The male gonad is typically white or cream-coloured, or with a tinge of green in addition; rarely salmon or pale yellow. The female gonad varies more, various shades of green, grey-green, brownish grey, etc., are found. These data refer to specimens observed in June and July.

5. Of 127 specimens examined for sex, 50 were males, 71 females, 6 young ones not giving decisive evidence.

6. A male and female were seen spawning. The male puffed out successive clouds of sperms. The female puffed out one cloud of eggs through the holes in the shell, perhaps 10,000 of them. They sank rapidly. They were visible to the naked eye, pale green and quite free from one another. Each was enclosed in a transparent shell.

7. Fertilisation took place after the eggs were laid. The young had reached an early trochophore stage at fourteen to fifteen hours after introduction of sperm. At forty-four to forty-six hours many had left the egg shell and were swimming actively, being then at a decidedly more advanced trochophore stage. How long it is before they settle down and end the larval stage is not known.

8. The food supply consists of algae (both coarse ones and confervas, also diatoms), polyzoa, sponges, small Crustacea, foraminifera, etc. A good deal of sand is taken in with it.

9. Haliotis is certainly a browser and uses the radula as a licking tongue. Experiments were. made to ascertain whether it can extract plankton from the water as part of its food as Crepidula does. The results were interesting (see p. 487), but not conclusive, pointing to inability to use plankton.

10. There is no evidence to show that Haliotis is a homing mollusc like Patella, what there is pointing the other way. It is an exceedingly active species.

11. Haliotis demands the highest possible conditions of aeration in its water. Given these it is hardy, and will even live for months after removal of its shell, secreting one or more new ones (Boutan).

12. In the attempt to find out why Haliotis is getting scarcer in Guernsey and the neighbouring islands (leaving Jersey out of account), it was concluded that this could probably be brought about by overfishing, and by a too ruthless disturbance of the beaches, leading to a certain diminution of the food supply over the area within which Haliotis is available for fishing. If this is not the main cause, some kind of natural fluctuation may account for i t; contamination, natural enemies, etc., not appearing to be main factors. The fishing of Haliotis in Guernsey has consequently been totally suspended by the States for a period of two years. At the end of that time more permanent regulations will be devised.