Cirripeds are of either male, female, and hermaphrodite sex, Male is dwarf (or complimental) and usually attaches to female or hermaphrodite. Four sex combinations are found among cirripeds : (1) female and dwarf male ; (2) female, dwarf male and hermaphrodite ; (3) dwarf (complimental) male and hermaphrodite ; and (4) only hermaphrodite.
BROCH (1922) pointed out that of cirripeds separate sex is a characteristic they share with most of the other Crustacea. Hermaphrodite sex in cirripeds has evolved from separate sex, which is more primitive than hermaphrodite.
Sex in cirripeds does not intimately correspond to their evolutionary stage. It is, however, closely related to ecological conditions they live in.
Sex combinations with a male, such as (1), (2) and (3) are found in taxa (or groups) having low population density or living as parasites on coelenterates, echinoderms, or on other arthropods. The existence of a dwarf male guarantees efficient reproduction under these ecological conditions. Sex combination (4) of only hermaphrodite is characteristic in taxa (or groups) with high population density. Their gregariousness guarantees hermaphrodites efficient reproduction by mutual fertilization.
Gregariousness was observed already in the oldest cirriped fossil Priscansemarinus from the Middle Cambrian Burgess Shale and
Cyprilepas from the Silurian of Estonia. NEWMAN (NEWMAN et al. 1969) concluded that two different size classes found in
Cyprilepas indicate either sexual dimorphism or different stage of growth. Evidence of gregariousness found in
Cyprilepas indicates that they are hermaphroditic, and that the size difference is not sexual dimorphism, but is different stage of growth.
Sexual polymorphism in these cirripeds resulted from sexual selection for aquisition of the most efficient reproduction strategy under the ecological conditions mentioned above, in addition to sessile mode of life and copulative mode of reproduction. Sexual polymorphism originated as a result of change from free living to sessile mode of life, perhaps in the early Cambrian or latest Precambrian.
Existence of dwarf males is known in some aschelminthes, molluscs, annelids, arthropods, and vertebrates. The same ecological factors produced sexual polymorphism in these groups.
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