Abstract
The larvae of Chasmagnathus convexus reared in the laboratory were obtained from tow ovigerous crabs collected in Ise Bay in April 1972 and fed Artemia nauplii. This species consists of four zoeal stages and one megalopa stage in the complete development. Twenty eight days were taken until the young crab metamorphosed. 1. The zoea stage. The carapace is 0.6 mm in length. It possesses a rostral and a dorsal spine, and a pair of lateral spines on the carapace. The abdomen is composed of six distinct segments. There are a pair of lateral knobs on either side of the second segment, and the same little paired spines on the third and the fourth segment. The paired larval appendages; antennae, mandible, maxillae and maxillipeds, are observed. Numbers of black chromatophores are seen on the surface of the body, and a few pale yellowish green chromatophores only on the abdomen. The additional changes in number are found through the four zoeal stages in following parts: (1) Swimming hairs of the first and the second maxillipeds, (2) segments of the abdomen. (3) setae on the telson, (4) setae on the coxopodite, the basipodite and the scaphognathite of the maxillae, (5) aesthetes on the top of the first antenna, (6) chromatophores. In the second or the third zoeal stage, the rudiments of five parts of the megalopa stage; (1) endopodite of the second antenna, (2) pereiopod, (3) pleopod, (4) third maxilliped, (5) gill can be observed. They are develop in length in the fourth zoeal stage. 2. The megalopa stage The carapace is 1.7 mm in length, 1.4 mm in width, and is in good shape. The dorsal surface of the carapace is smooth and bald. The front bears four appendixes with round tops. The short rostral spine is projecting downwards. The dactylus of the fourth ambulatory leg bears three long feelers at the distal portion. The abdomen is divided into seven distinct segments. The seventh segment is rectangular and its posterior margin swells out, with one point emarginating on its central portion 3. The first young crab stage The general aspect of the body is similar to that of the adult crab. The carapace is 3.5 mm in length and 3.3 mm in width. Many black chromatophores arid pale yellowish green ones are observed on the surface of the carapace and the ambulatory legs.
