The Effects of Parasitism

THE EFFECTS OF PARASITISM Giard (1887) and later Smith (1906) have described in detail the changes that occur in crabs parasitised by Sacculina and other parasitic crustacea. Sacculina, an internal parasite, is a cirripede crustacean, and part of its body projects to the exterior under the abdomen of its host, while root-like processes ramify to all parts of the crab's body, avoiding the vital organs and absorbing nourishment chiefly from the blood. It attacks males and females and in both causes atrophy of the sex-glands and consequent sterility. The only effect of this in females is an acceleration in assumption of adult sex-characters. Parasitised males, however, gradually take on more and more of the female characters, their great claws become relatively smaller, the abdomen broader, the swimmerets enlarge and become fringed with the hairs to which, in females, the eggs are attached. Most of the affected crabs die, but in a few the parasite disappears and the repro ductive organs are regenerated. In a female a normal ovary develops, in a slightly feminised male, a normal testis, but in some males a sex-gland is regenerated, in which both ova and sperm are found ; real male hermaphroditism is produced.

Geoffrey Smith (1906) who investigated this problem found that the blood of the normal female crab differs in chemical constitution from that of the male. It contains fatty substances which are used in the production of the yolk of the egg. These fatty substances form an important part of the food of the parasite Sacculina. That which Sacculina absorbs cannot be used in yolk formation, and as the eggs cannot develop, the ovary degenerates. In the male these fatty substances are present in but small quantities. The parasite demands more and the whole physiology of the male crab is altered to meet this demand; the male thus assumes the female type of metabolism, and conse quently female characters.

The interest of this case is that it permitted Smith to ques tion the validity of the conception that in all forms the gonads functioned as organs of internal secretion, contributing a pecu liar product to the blood streams. Smith argued that the gonad, far from adding anything to the blood stream, removed some thing from it.

Sex-reversal.

It is somewhat difficult, when one first ap proaches this subject of the transformation of sex, to grant that reversal is possible in the higher forms in which the sexual differ ences, morphological, physiological, and mental, are so sharply emphasized. But when one remembers that what is rare and exceptional in one form can be facultative in others, and that all forms have much in common, the difficulty vanishes. It is readily conceded that the oyster, for example, may regularly change its sex. The native oyster begins its life as a male and then, when one or two years old, may and indeed commonly does, become a female. But that is not all, for Orton (1921) has shown that such an oyster, after becoming "white sick," i.e., after shed ding its ova into the mantle cavity, and whilst still carrying its own embryos, can, within the space of a month, become equipped as a male once more. That which is usual in the oyster may be, under certain conditions, not rare in the more highly organised forms. Similar instances of facultative sex-transformation are those furnished by Crepidula Plana, parasitized Cymotheids and Epicarids, the starfish, Asterina gibbosa, and the slug Limax maximus. In all these cases the direction of the transformation is male —4 hermaphrodite female.