Note on the Development of Cryptoconchus porosus (Burrow)
[Received by the Editor, October 31, 1957.]
In Otago waters specimens of Cryptoconchus porosus spawn at regular intervals from the middle of June until the end of August or occasionally until the middle of September (Brewin, 1942).
The sperms are extremely motile with a body length of about 0.6μ, a flagellar length of 4μ (Fig. Ia).
The eggs are produced in long gelatinous strings which become much extended under gentle wave action, strong wave action being necessary for rupture. The eggs are 20–23μ in diameter, olive green in colour, mediumly-heavily yolked, and bear a white corona of follicle cells about 8μ deep (Fig. Ib). The shape and configuration of the follicle cells would rather indicate a drifting planktonic existence during the early stages of embryological division. Nevertheless very few eggs or early embryos were taken in plankton nets set regularly near coastal rocks where adult Cryptroconchus occur and in the aquaria it was noticed that unless aeration was particularly vigorous eggs and early embryos tended to remain at the bottom.
Note: About 20 hours after liberation of the eggs small ciliates appeared around the follicular coronas (Fig. Id), but whereas ciliate invasion of the cells of the corona (and subsequently of the cells of the embryo proper) occurred rapidly in embryos in an unhealthy state, it was definitely impeded in embryos in a healthy state Thus it would appear that one function of the follicular corona is that it forms a barrier against ciliate invasion.
In the laboratory observations were made over a period of three years. At successive spawnings the newly-laid eggs were transferred to clean, well-aerated aquaria together with a small amount of sperm. A plunger jar apparatus was erected to create something like wave action and the sea water was changed every day Despite these precautions no animals were reared past the late trochosphere stage, cell disintegration taking place between the seventh and ninth days.
Little difference was noted in the timing of the developmental stages in the various batches of embryos, and the following summary of the development of a batch of eggs liberated at 1.30 p.m. on July 2, 1939, may be taken as typical:
|Stage of Development of the Embryo.||Approximate Time After Fertilization.|
|1st cleavage furrow||3 hours|
|2nd cleavage furrow||3½ hours|
|32-celled stage (Fig. Ic)||16 hours|
|64-celled stage||20 hours|
|Gastrulation with invagination (Fig. Id)||22–28 hours|
|Appearance of prototroche (Fig. Ie)||33–48 hours|
|Rupture of follicles and freeing of embryo (Fig. If)||60–72 hours|
|Embryos positively heliotropic and swimming in the upper layers (Fig. Ig)||3–4 days|
|Embryos bottom dwelling (Fig. Ih)||4 or 5? days|
After becoming bottom dwellers, some embryos survived for four days, but no typical shell formation was noted before disintegration, and dense ciliate infection took place.
Fig. 1.—Embryological development of C. porosus a, sperm; b, egg in corona of follicles, optical section; c, 32-celled stage, lateral view; d, early gastrula, lateral view; e, trochosphere with prototroche; f., trochosphere breaking through follicles; g., free-swimming trochosphere; h., early bottom-dwelling stage showing elongation of the posterior end.
These observations correspond fairly well with those of Hammarsten and Runnstrom (1926) on Acanthochiton discrepans in which the escape of the larvae from the corona of follicle cells takes place about the third day of development, and the bottom-dwelling phase is achieved some 12–24 hours later. These workers having available a good supply of Nitzchia managed to rear the larvae through to young chitons, maintaining them in a healthy condition for 1½ months.
Brewin, B. I., 1942. “The Breeding Habits of Cryptoconchus porosus (Burrow)”. Trans. Roy. Soc. N.Z., 72: 186–190.
Hammarsten, O. D. and Runnstrom, J., 1926. “Zur Embryologie von Acanthochiton discrepans (Broun)”. Zool. Jb., XLVII: 261.
Dr. Beryl I. Brewin,
Department of Zoology,
University of Otago.