On the effects of an Epicaridan Parasite, Gyge branchialis, on Upogebia littoralis
Authors: Tucker B. W. 
Published: April 2021

ABSTRACT
The effects of Epicaridan Isopods on their hosts have not previously been studied in detail. Gyge branchialis at Naples is particularly suitable for this purpose on account of its commonness. In 1924 21-5 per cent, of Upogebia littoralis at Naples were found to be parasitized. Some difference in the sex-ratio in old and young, parasitized and non-parasitized Upogebia are discussed. Amongst normal animals adult females outnumbered adult males by 3 : 2. Amongst young animals of about 3-6 mm. carapace length and parasitized material of all ages there was a marked excess of males. It is thought probable that there is in fact a real preponderance of males in the whole population, but that ovigerous females probably tend to remain nearer the mouths of their burrows and thus render themselves more liable to capture. Fixation of the parasite normally takes place when the host is under 17 mm. long (carapace length : 6-6-5 mm.). The available evidence suggests that the length of life of Upogebia is about (or at any rate not less than) three years. The life of the parasite is apparently normally coextensive with that of the host. The effect of the parasite on the general vitality and viability of the host is negligible. Moulting and growth are not materially interfered with. Parasitized males have chelae agreeing in size and appearance with those of the female and develop the appendages of the first abdominal segment, which are normally present in the female only. Parasitized females are unaltered externally, except that the chelae tend to average very slightly smaller than in normal females. The genital pores are never obliterated in either sex. The growth of the chelae in normal and parasitized animals, both absolutely and relatively to the body, are studied by means of graphs based on a large series of measurements. The parasitized male growth curves are closely approximated to the female type, while those of the female are not significantly altered. (For a fuller summary of the results on chela growth, see pp. 32-3.) On account of the early age at which parasitization takes place all individuals are modified, in contrast to crabs parasitized by Rhizocephala, in which a considerable percentage of animals are unmodified, having evidently been parasitized too late in life. Scattered oocytes occur amongst the male germ-cells in the testes of many normal males. These cells are never very numerous, but are seldom absent. They accompany the normal cells in their development and are eventually detached into the lumen of the testis, where they degenerate. They can sometimes be recognized amongst the older spermatogonia, but have not been traced with certainty farther back than this, so that it is uncertain whether they arise by metamorphosis of spermatogonia or are distinct from these from the outset. No evidence is found in Upogebia littoralis of any seasonal variability in the numbers of these cells. Comparison is made with Upogebia major, which has paired abdominal extensions of the testis which appear to be constantly ovarian in character (Ishikawa), and Upogebia stellata, in which Runnstrom and the present writer have found similar posterior extensions of the testis containing oocytes at the hind end. Upogebia stellata may also develop rudimentary oviducts and female genital pores in the male. The condition of the testis in parasitized males shows a wide range of variability from one in which it is only slightly reduced and spermatogenesis is proceeding vigorously to one of complete atrophy. The tendency to develop oocytes is much accentuated. They tend to become conspicuously more numerous and develop farther than in normal males, occasionally even forming yolk. In a few cases a whole tract of testis appears to have been completely converted into ovary. Spermatogenesis may occur even in the oldest parasitized males and may continue locally even in greatly reduced testes ; on the other hand, it may cease before reduction of the organ has proceeded very far. Sperm production and a marked development of oocytes may sometimes be found in different regions of the same testis. Rarely the lining of the vas deferens undergoes a more or less marked chitinization. In the large majority of parasitized females the gonad is completely absent. When it persists in a reduced form it is always found to be an ovary in a state of diminished activity, but not otherwise abnormal. The early age at which parasitization takes place results in the gonad pursuing its development from an early stage under the parasite’s influence. The development of the testes in normal and parasitized males and females is outlined. Even normal animals show considerable variability in the degree of development of the testis at given body sizes. Males appear to be all sexually mature from about 6 mm. carapace length onwards, though they probably do not breed until later. Occasional cases of much earlier sexual maturity occur. Two males of carapace length 4 mm. were found to be producing ripe sperm. Females of 8-9 mm. carapace length may be found bearing eggs. In parasitized males differentiation of the testis is markedly retarded and in some cases evidently completely checked, the ducts appearing to be in general rather less sensitive than the gonad. At a time not much antecedent to 8 mm. carapace length some of the less drastically affected individuals may begin to form sperm. In many, after a period of decreasing sperm production, spermatogenesis ceases entirely (cf. however, section 11). The correlation between the condition of the gonad and the degree of modification of the secondary sexual characters and even between the different changes in the gonad itself is exceedingly loose. Males with testes entirely absent may be the earliest parasitized, but the variability in the modifications must be attributed largely to constitutional differences in the animals. The modifications evidently persist, at least for a considerable time, if the parasite is removed ; but whether, in these circumstances, they are permanent is not established. Two exceptional conditions are described : (1) An unparasitized male with female appendages. The con dition of the gonad suggests that this is a case of natural recovery from parasitization. (2) Two (or three) parasitized males without female appen dages. These might be supposed to be exceptional cases of very late parasitization, but this interpretation is impossible in one case and doubtful in the other. In the first the gonad is completely aborted ; in the second it is practically normal, but the chelae are markedly reduced. They are thought to be more probably constitutionally abnormal cases, where the organism has failed to respond to parasitization in the normal manner. Previous work on parasitic castration is reviewed and discussed. Notwithstanding suggestions to the contrary, the modifications of sex characters in parasitically castrated Crustacea are exclusively in the direction of feminization, as is also the case in The lia parasitized by Aphelopus (Kornhauser). The feminization is positive and unequivocal and cannot be interpreted as a return to or retention of primitive, undifferentiated, or juvenile features. The solitary instance in which parasitization is accompanied by a definite exchange of characters between male and female is that of stylopization in Hymenoptera. It is concluded that the appearance in stylopized females of certain characters normally confined to the male must be due to some peculiar conditions controlling the development of these characters, and that their appearance is in some way dependent on the general nutritional disturbance and not on any directly masculinizing influence of the parasite. Geoffrey Smith’s theory that the parasite, by withdrawal of nutriment, reacts on the host in a similar manner to an adult ovary is considered to be the only one which makes any approach to a real explanation. Objections to this theory on the ground of the experimental and other evidence against any effect of the ovary on the soma in Arthropods are considered. All that the castration and gonad-grafting experiments and the evidence from abnormal individuals with male soma and female gonads prove is that the somatic characters are uninfluenced by the gonad in normal development, and that gonad and soma are so adjusted to one another that in the above cases the demands coming from the ovary are at no stage strong enough to interfere with the constitutional (i. e. ? chromosomally determined) predisposition of the somatic cells to metabolize in the male way. But a more insistent drain on the same food materials might overrule this predisposition and compel an alteration of metabolism, and it is suggested that the parasite exerts its effect by bringing to bear a demand of the same kind as, and not less drastic than, that of an adult ovary on an immature organism.