Pleopod loss and gain
Paired first pleopods in females of most lithodid genera caused Boas (1924) to change his view of the pagurid ancestor from Pagurus to a genus like Nematopagurus or Pylopagurus. Neither of the latter genera were included in Cunningham et al.’s (1992) study, nor are they included here for lack of larval, megalopal, and early juvenile data. McLaughlin & Lemaitre (1997) reported the presence of paired first pleopods in adult females of all Pylochelidae, the diogenid genera Paguristes Dana, 1851 and Paguropsis Henderson, 1888, and several genera of the Paguridae. No paired pleopods are present on the first pleomere in the zoeal or megalopal stages of any paguroid, although Richter & Scholtz (1994) suggested they might be present in embryonic stages. When paired first pleopods do develop in the adult they appear to represent new structures, not modifications of existing megalopal pleopods, and they do not arise until sometime after crab stage 10 (Provenzano & Rice 1966; Sandberg & McLaughlin 1998; McLaughlin & Paul 2002).
Buds, representing developing pleopods, appear, with few exceptions, in the penultimate or ultimate zoeal stages on pleomeres two to five of paguroids. These appendages, fully developed, paired, and usually biramous are characteristic of the megalopa. They are lost entirely in lithodids either with the molt to crab stage 1 or shortly thereafter, and are replaced by newly developing pleopods on the left side only in females at approximately crab stage 5. In contrast, pleopods are reduced in the first and/or second crab stages, and ultimately lost on one side of the pleon, in subsequent juvenile stages in the majority of pagurids. It is unclear what mechanism(s) influence pleopod loss, but loss is not uniform throughout the superfamily. For example, Provenzano & Rice (1966) reported the reduction and ultimate loss of both second pleopods and third through fifth right pleopods in Paguristes sericeus A. Milne-Edwards, 1880 at crab stage 2. The left second pleopod remained absent through crab stage 10. Females developed an egg-bearing second pleopod on the left side of the pleon gradually in later stages, whereas males did not. In contrast, Brossi-Garcia (1987b, 1988) described pleopod loss beginning at crab stage 2 in two species of another diogenid genus, Clibanarius Dana, 1852; but this loss involved only the pleopods of the right side. Variations were also seen in the five species of Pagurus in the present investigation. In studies of P. venturensis Coffin, 1957 by Crain & McLaughlin (1993) and P. kennerlyi (Stimpson, 1864) by McLaughlin et al. (1989), marked reduction was found in the second pleopods on both sides at crab stage 1 and complete loss at stage 2. Similar reduction was observed for third through fifth pleopods on the right side at crab stage 1, with complete loss in crab stage 2. However, a tiny second pleopod bud was apparent on the left side of a presumed female of P. venturensis in crab stage 3, suggesting a return similar to, but more rapid than, that observed in Paguristes sericeus. Comparable second pleopod reduction in Pagurus bernhardus (Linnaeus, 1758) was reported by Carvacho (1988) and in P. ochotensis Brandt, 1851 by McLaughlin et al (1992) but without complete loss in crab stage 2. Clearly, pleopod absence and loss, like asymmetry, lacks a single straightforward explanation.