Physiology of Finfish and Shellfish (2+1)

3.1.33. CLASS CEPHALOPODA - (ceph-a-lo-poda)

Cephalopods usually are usually dioecious (two sexes), and fertilization is internal. Courtship, which in some species can be quite elaborate, is often a precursor to copulation. Gonads are located in the posterior of the body.

In some species, males can be distinguished by the modified sucker discs found at the tips of his longer two tentacles. The male uses these long arms to remove a sperm packet from his mantle cavity and to then insert this packet into the female’s mantle cavity. This arm is autonomous and sometimes can get broken off while mating and is retained within the female’s cavity. If this happens, the male simply grows a new one: no problem!!

Within two months of mating, the female octopus will attach long strands of clustered eggs (resembling a cluster of grapes) to the ceiling of her lair. While the eggs are incubating, the female will gently caress them to keep them clean and free of bacteria. She also keeps a steady flow of fresh, oxygenated water flowing over her precious eggs. When they are ready to hatch, her caresses become more rigorous, this helps the young to escape from their egg sacs.

The pelagic Cephalopod family Argonautidae, commonly knows as the paper nautilus, have a remarkable adaptation for egg deposition. The two dorsal arms of the female are greatly expanded at their tip to form a membrane. The expanded portion of each arm secretes one half of a beautiful calcareous bivalved shell. She then deposits her eggs directly into this case. The shell acts as both a brood chamber and a retreat for the female. The posterior of the female usually remains in this shell. The male, which is much smaller than the female, does not have such a shell and is often found in cohabitation within the same shell, more or less as a freeloader.

Environment and maturation of fish eggs

The gonads, ovaries and testes, of all vertebrates are similar in structure and function. The gonads are usually paired (although they may be fused or single in some groups). The testes in all vertebrates produce millions of sperm at a time. As we shall see, the major differences in male reproductive systems involve the mechanisms for transferring those millions of sperm to the ova.

Ovaries produce ova surrounded by fluid filled sacs called follicles. These follicles burst releasing ova into the coelom and then the oviduct. The number of ova produced at any one time varies considerably depending upon the habitat of the animal and the amount of parental care the young will receive. Thus fish, like the cod, which release unprotected eggs into the ocean and do not care for their young may produce and release more than a million eggs at once. Other fish and amphibians which do not broadcast their ova as far afield or which give some measure of parental protection to the eggs may produce thousands of eggs at once. Birds and mammals, which invest considerable parental energy in protecting their offspring both before and after birth, produce only a few eggs (ova) at one time.

Once ova are released from the ovary, they are transported through the oviduct toward the exterior. In the case of fish and amphibians, which release shell-less eggs, the oviduct is essentially an unspecialized tube leading indirectly to the exterior. Birds and reptiles produce shelled eggs; the shell is critical in protecting the egg from desiccation in the terrestrial environment. The oviducts in these animals are modified, being in close association with a shell-forming gland and capable of considerable distension to accommodate the enormous, inflexible eggs that result.