The larval development ofArgulus coregoniThorell (Crustacea: Branchiura)

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Abstract
The larval development of Argulus coregoni from the first to the ninth stage was studied, with special reference to the morphological changes of the appendages and some other parts of the body. A newly hatched larva is a copepodid-form measuring 0·6–0·7 mm long and approaches the shape of the adult after moulting into the second stage (0·7–0·9 mm). Throughout the larval stages, the first maxilla shows the most remarkable modification. The larvae of the first to the fifth stage are equipped with two strong, curved claws, which act as a clasping organ, at the terminal segment of the first maxilla. The basal segment expands considerably at the third stage (0·9–1·1 mm) prior to forming a sucker at the sixth stage (1·7–2·2 mm). The claws start to degenerate at the fifth stage (1·4–1·8 mm), but are still recognizable as a rudiment after the seventh stage (2·2–2·6 mm). The other distinct morphological changes that occurred are: (1) bifurcation of the anterior part of the dorsal ridges on the carapace, (2) development of the male accessory copulatory organ at the basal segments of the second to the fourth legs, (3) number of posteriorly directed minute spines on the ventral surface of the carapace, (4) number of elements in the supporting ribs of the suckers, (5) number of spines found at the first segment of the second maxilla, and (6) number of setae on the four pairs of thoracic legs. The larval forms of A. coregoni younger than the fourth stage (1·1–1·4 mm) are indistinguishable from those of A. japonicus and A. foliaceus.

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