Contributions to Zoology, 73 (3) (2004)Patsy A. McLaughlin; Rafael Lemaitre; Christopher C. Tudge: Carcinization in the Anomura – fact or fiction? II. Evidence from larval, megalopal and early juvenile morphology
From hermit to king, or king to hermit?

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Semaphoronts

The term semaphoront has been interpreted differently by various authors (Hennig 1966; Brooks & Wiley 1985; de Queiroz 1985; Nelson 1985; O’Grady 1985; Wheeler 1990). Our use of the term, like our application of cladistic analysis is perhaps unconventional. We use semaphoront to indicate an assemblage of individuals of a monophyletic group at an identifiable and comparable period in their life cycles. In the present investigation we have examined two semaphoronts, megalopal/juvenile and larval. The former semaphoront includes all paguroid species for which data on particular aspects of megalopal/juvenile morphology (megalopa, crab stage 1, and crab stage 2) are available.

Marques & Pohle (1998) stressed the need for developmental homology, emphasizing that if a phylogenetic hypothesis is based on non-homological semaphoronts, that hypothesis will likely be wrong. Having found no evidence to the contrary, we have followed Hennig’s (1966) auxiliary principle and assumed [developmental] stage homology in a “postlarval” (megalopa/juvenile) semaphoront. Regrettably, data are not complete for all stages beyond crab stage 1. Maddison (1993) and Hawkins et al. (1997) have reviewed the problems arising from missing data, versus missing characters and pointed out some of the erroneous results that can occur when computerized analyses encounter missing information. Consequently, we have limited our data to taxa experimentally observed, reported in the reliable literature and/or, where character adjacency has permitted, extrapolations. However, given the reversals observed in lateral and marginal plate divisions in Lithodes at crab stage 3, we have not pursued formal cladistic analyses beyond crab stage 2. This stage limitation notwithstanding, we have utilized the data from crab stage 3 and beyond in drawing our ultimate conclusions.

Confirmable reports of direct development in the Paguroidea by Barnard (1950), Dechancé (1963), and Morgan (1987) have described hatchings at the megalopal stage (pre-imago); in all other studies one or more zoeal (larval stages) also are involved. In our analyses, the megalopa is considered the basal point of the “postlarval” semaphoront whether abbreviated development is considered advanced (cf. Rabalais & Gore 1985) or primitive (Wolpert 1990, 1994). Among members of our “postlarval” semaphoront, the occurrence (insertion) of the megalopal stage (pre-imago) is universal.

Our larval semaphoront was to have included those paguroid species for which pertinent zoeal data could be paired with megalopal/juvenile information, at least at the generic level. However, we encountered two major problems. The first dealt with character selections. Phylogenetic brachyuran larval studies most frequently have placed considerable emphasis on setal differences in primary zoeal feeding appendages (Clark & Webber 1991; Marques & Pohle 1995, 1998; Clark 2000; Ng & Clark 2000). Unfortunately in paguroids, these appendages, i.e., paired mandibles, maxillules, and maxillae, may be well developed or rudimentary, depending upon whether or not the larval and/or megalopal stages are lecithotrophic (Van Dover 1982; McLaughlin et al. 2001, 2003). In the zoeal studies considered in our review, all paguroid species had type 1 (cf. Van Dover et al. 1982) maxillary scaphognathites in zoeal stage 1 and followed those authors’ type A sequence through subsequent stages; however, setation was noticeably reduced in species known to be at least facultatively lecithotrophic.

Lecithotrophy has not been seriously investigated in paguroids. However, Anger (1989)and Harvey (1996) have reported the occurrence of ‘secondary lecithotrophy’ in the megalopal stage of several Pagurus species and Harvey has indicated that both zoeal and megalopal stages of Paguristes tortugae Schmitt, 1933 are lecithotrophic. Anger (1996) and McLaughlin et al. (2001, 2003) have recorded similar lecithotrophy in species of Lithodes and Paralomis. To avoid the introduction of seemingly morphological differences that may in fact be differences in developmental patterns (cf. Chai 1974), feeding appendages had to be omitted from our consideration.

The second problem dealt with ontogenetic stage homology, a matter of critical concern in phylogenetic analyses. In the Paguroidea, although the most common number of zoeal stages is four, that number does vary among and within genera, and occasionally even within species. Such variation in some taxa simply reflects environmental influences (Cabrera Jiménez 1966; Lang & Young 1977; Brossi-Garcia & Hebling 1983; Siddiqui et al. 1991), while in others these variation actually are deletions [elimination of a stage ] (Gore 1985; Clark 2000) (or insertions; cf. Wolpert 1994). Marques & Pohle (1998), when confronted with variations in the number of zoeal stages in their study groups, were able to demonstrate that the second stage of their ingroup was homologous with the second stages of the out-groups despite variation in the total number of stages. Clark (2000) however, considered specific morphological attributes as characters across a series of stages with polarity conferred by the timing of their appearances. Specifically, a given character in all taxa presumably was homologous across all stages; its polarity was considered derived if it appeared earlier in the sequence of zoeal stages than the same character in the out-group. This reasoning requires that stages also are ontogenetically homologous among all taxa. Perhaps this is a reasonable expectation in some brachyuran groups; however it is certainly not the case in paguroids.

For example, a review of paguroid larval characters has shown that while some of the variations observed appear to be simply terminal deletions or insertions, e.g., four rather than five zoeal stages or vise versa, some more importantly would seem to be initial deletions (stage 1, or elements of it, eliminated or passed through prior to hatching). Initial deletion means that the state of a character in species hatching in a more ‘advanced’ condition will not be developmentally homologous with that same character in a species without advanced hatching. In one of the more readily recognizable scenarios, the first decapod zoeal stage (ZI) (prezoeas excluded) is defined as having sessile eyes regardless of the number of zoeal stages (Gurney, 1942: 123). However, in Lopholithodes (cf. Crain & McLaughlin 2000a), Lithodes (cf. McLaughlin et al. 2001), Paralomis (cf. McLaughlin et al. 2003), Anapagurus (cf. Ingle 1990), and Pylocheles mortensenii (cf. Saito & Konishi 2002), the eyes at hatching are stalked or at least only partially fused to the orbital wall. In Lithodes only three zoeal stages precede the metamorphic molt, suggesting perhaps that the first stage has been deleted. In Paralomis where the molt to megalopa is preceded by only two zoeal stages, it would appear as though at least one or perhaps even two zoeal stages have been deleted. In contrast, in the diogenid genus Paguristes, where two or three zoeal stages also are reported (Hart 1937; Rice & Provenzano 1965; Provenzano 1978), ZI eyes are sessile, suggesting that perhaps stage deletions in this genus are terminal. Alternatively, Ingle (1990) for the pagurid, Anapagurus chiroacanthus (Lilljeborg, 1855), and Crain & McLaughlin (2000a) for the lithodid, Lopholithodes mandtii, reported stalked or at least only partially fused eyes in ZI; both taxa have four zoeal stages, the number most common for species of Pagurus, which have sessile eyes at hatching. Do differences in ocular development among species of Paguristes and Pagurus, with two or three and four zoeal stages, respectively, but sessile eyes initially, reflect the primitive condition, while species of Anapagurus and Lopholithodes, each also with four zoeal stages but stalked eyes, reflect the derived condition in equivalent stage ZI larvae? If so, is this derived state ontogenetically homologous with the “derived” states seen in “ZI” Lithodes and “ZI” Paralomis with only three and two zoeal stages, respectively? We have found no evidence to allow us to assume that this is true. The condition of the eyes at hatching is just one of a number of stage-variable characters of this complexity that cast critical doubt on developmental stage homologies or equivalent sets of ontogenetic stages among paguroids. Without these, meaningful phylogenetic analysis cannot be expected. Consequently, we have not cladistically evaluated our larval phase data.