Megalopal and early juvenile phasenext section
As previously indicated, while the megalopal stage in porcellanids has been included in the majority of descriptions of larval development, very limited data are available on the first and later crab stages. In the most complete juvenile study to date, Petrolisthes armatus (Gibbes, 1850) was followed by Brossi-Garcia & Guimarães Moreira (1996) through 12 juvenile stages. From their study, one would conclude that sexual maturity is reached more rapidly in porcellanids than in lithodids, as Brossi-Garcia & Guimarães Moreira noted the appearance of gonopores at the second juvenile stage. In contrast, incipient gonopores were occasionally observed only as early as crab stage 5 in Lithodes aequispinus by McLaughlin & Paul (2002). In P. armatus, Brossi-Garcia & Guimarães Moreira (1996) reported and illustrated generally symmetrical pleopod reduction beginning in crab stage 1. Although these authors indicated that sexual dimorphism, not only in gonopore delineation but in pleopod development, was observable in crab stage 2, they illustrated complete absence of juvenile pleopods in stage 3 males while development of the male second pleopods as gonopods was well underway. Rudimentary or vestigial pleopods in females were described and illustrated for stage 3, with further reduction by crab stage 6. It would appear that the authors did not differentiate whether these rudiments were remnants of early juvenile pleonal appendages or actually buds of newly developing female pleopods. However, the illustrated absence of the fourth pleopods in stage 3 and their reappearance at stage 6 would suggest that females at crab stage 3 were still undergoing megalopal pleopod loss, but by stage 6 adult pleopods were beginning to develop, as adult females of Petrolisthes are provided with paired pleopods on pleomeres 2-5. Despite the precocious appearance of gonopores, the division of the telson into the seven-plate condition of the adult appears not to have yet begun, as Brossi-Garcia & Guimarães Moreira gave only setal counts for the telson in juvenile stages 1-11.
The first four crab stages were described by Paul et al. (1993) for Pisidia gordoni (Johnson, 1970) with pleopods of the second through fifth pleomeres reduced in the first crab stage. By the second stage, the posterior three pairs were absent or vestigial whereas, the second pair had been transformed into developing male gonopods in one specimen. However, in two other specimens, adjudged by the authors possibly to be females, complete pleopod loss by crab stage 3 was observed. At the fourth crab stage, the telson began to show indications of the seven-plated adult condition. Paul et al. (1993) also described the first three juvenile stages of Petrolisthes rufescens (Heller, 1861). In this species pleopod loss began with the molt to the first crab and no pleopods were present by crab stage 3. Paul et al. (1993) considered it more probable that their few specimens were incipient females rather than that a delay in the development of male gonopods occurred in this species. Division of the telson in P. rufescens, however, was already observable in the first crab stage and was nearly complete by the third crab stage.
Only the first crab stage was described for Allopetrolisthes angulosus (Guérin Méneville, 1835) by Wehrtmann et al. (1996). These authors reported pleopod reductions on pleomeres 2-6 (sic), but with the pleopod pair of pleomere 2 approximately three times larger than the remainder. A more significant difference was reported for the telson, where the full complement of five adult plates was already well defined.
In the development of male gonopods, Paul et al. (1993) noted that considerable similarity existed between brachyurans and porcellanids, but expressed the belief that this was another example of convergence. Our data are too meager to formulate any real comparisons with the transformations taking place among lithodids and porcellanids, but it is quite obvious that considerable heterochronic development occurs in both groups.
Knowledge of zoeal development in the Porcellanidae is appreciably more complete. Despite their distinctive, very elongate rostral and posterolateral carapace spines, porcellanid zoeas were considered by early carcinologists essentially to be very similar to those of the Brachyura (Müller, 1863). However, only two characters might conceivably link the porcellanids to the Brachyura, i.e., the reduced, somewhat rod-shaped antennal exopod and the lack of a functional exopod on the third maxilliped (Williamson 1974). Only in Petrocheles spinosus Miers, 1876, does the number of porcellanid zoeal stages exceed two, and only in this species do uropods develop before the megalopal stage. Wear (1965) described five zoeal stages for P. spinosus, with uropods developing in stage three, but with no pleopods developed in either the zoeal or megalopal stages. In all other porcellanids for which larval data are available, developing pleopods are present in ZII and become functional in the megalopal stage, only to be reduced or lost in subsequent juveniles stages. While paired, egg-bearing pleopods reappear in females, pleopods are completely absent in adult males unless the second pair develop as gonopods.