Hermit crabs are the commonest intertidal anomuran crustaceans. Diogenes miles (Herbst) is an inhabitant of surf-beaten sandy shores along coasts of peninsular India. This species is outstanding among other diogenid hermit crabs in having, unlike others, an extremely flat and broad body well adapted for its occurrence in long, narrow-mouthed gastropod shells mostly of Oliva and to lesser extent Gypraea and Conus. During the present study it was observed that, contrary to what was expected, D. miles docs not breed throughout the year but appears to have its breeding season restricted to the summer months. This was indicated by the collection of ovigerous females of the species only during the summer months near Bombay, Ratnagiri and Karwar along the Indian west coast. Eggs are minute, bright crimson to scarlet when immature and are easily shed, even with slight disturbance. Larval development consists of 3 zoeal and a megalopa stages, described in the present paper, after specimens reared in the laboratory. Some of the salient features of the zoea larvae are: a long, pointed rostrum, smooth carapace, usual triangular telson with process formula 7 + 7 in the first zoea but complete reduction of the 4th process in the third zoea. The megalopa has the typical unequal chelipeds, the left being larger, only 3 pairs of pleopods, and the telson is rectangularly rounded. Though the deep curve of the carpus develops right from the megalopa stage, the inward right-angled bend of the cheliped, so characteristic of the adult, is not fully developed even in the 5 subsequent instars.
The porcellanid crabs in their larval phase show typical anomuran features but develop a crab-like form from megalopa through the juvenile instars. However, the adult species characters are not yet noticeable in the juvenile phase thus leading to confusion in identity both at specific or even at generic level. With attainment of the crab-like form, the reproductive appendages, like male pleopods, show a brachyuran pattern. In absence of external sexual dimorphism, sex determination is difficult in this group. Very little information is available as to how and when the sexual appendages start developing in Porccllanidae. With the purpose of tracing the development of the adult taxonomic characters and that of the reproductive appendages (and sex differentiation), the present paper describes the juvenile laboratory development of two porcellanid crabs, viz. Petrolisthes rufescens (Heller) up to instar III and Pisidia gordoni (Johnson) up to instar IV. Taxonomic features: In both the species the definitive adult features like those of the carapace front and its armature, the chelipeds and other pereiopods, the sternite of the third maxillipeds, etc., develop in the third instar. Sexual appendages: (a) In Petrolisthes rufescens the primary pleopods get totally reduced at least up to instar III and, therefore, the sexual appendages appear to be developing as a secondary series of pleopods, particularly in the female. (b) In Pisidia gordoni, at least in the male, differentiation starts from instar II onwards with the first pleopod modifying into a male appendage, while the remaining pleopods gradually become reduced.
The aesthetascs of the prawns are chemo- and mechano-receptors which exhibit diversity in number and shape depending on habitat and environment. In marine forms (both adults and larvae) they arc more numerous, longer, slender and uniformely thin wallcd. In adults of freshwater prawns they are fewer, shorter, divided into a thick walled stalk and a thin distal cob-like portion. However, they arc structurally different and positioned differently in the atyid genus Caridina and palaemonid genus Macrobrachium. Further, Caridina exhibits sexual dimorphism with males possessing a greater number of aesthctascs-bearing segments than females, unlike Macrobrachium which does not show such dimorphism. As far as larvae are concerned, in the 'inland species' of both genera, which complete their metamorphosis purely in freshwater, the 'freshwater- type aesthetases' appear in the first and second zoeal stages in totally or partially abbreviated type of development respectively. On the other hand, the early zoeal stages of 'coastal species' possess 'marine-type aesthetascs' indicating salinity dependence for their successful metamorphosis. These aesthetascs get shortened in late larval stages and subsequently get transformed into typically 'freshwater type' from postlarva onwards. A unique kind of 'branched larval aesthetascs' has been observed in two species of Macrobrachium. Thus, despite their common sensory function, the aesthetascs exhibit structural variations depending on environment and also play an important role as salinity indicators in the culture of freshwater prawns.
The inland prawn genus Macrobrachium Bate, 1868, is supposed to have originated from marine ancestors sometime in the beginning of the Pleistocene and since then its members have invaded freshwaters to different degrees. As such, these prawns are found to occupy almost all types of aquatic bodies right from purely marine through estuarine, riverine to impounded waters. Consequently, they have evolved various types of developmental patterns. Based on information available on more than 40 species, three basic types of larval developmental patterns can abe recognized in this genus viz., Prolonged or Normal Type (with 8 to 20 stages), Partially Abbreviated Type (with 2 or 3 stages) and Completely Abbreviated Type (with only 1 stage). However, there are several species showing transitional developmental patterns. Thus, the process of freshwaterization seems to be still continuing in the genus. Incidentally, the above knowledge can be applied in prawn culture as a basis for selection of a suitable candidate for forage species.