Megalopa/early juvenile characters and character states.
We have followed the recommendation of Wiens (2001) to explicitly document our character choices. Table 1 provides a summary of the characters and character states for the “postlarval” (megalopa/juvenile) semaphoront. Since data for crab stage 2 are more restricted, we have conducted our analysis at two levels, i.e., those taxa for which complete data are available for the three postlarval stages, and those taxa where data are available only for the first two postlarval stages. Table 2 furnishes the list of paguroid taxa examined and the respective character states for megalopa, crab stage 1 and crab stage 2.
Although much emphasis has been put on the transition to (or from) a crab-like pleon, the carapace of a shell dwelling hermit crab and the carapace of a well calcified, crab-like lithodid similarly represent a transformation from one condition to another. Additionally, the rostrum, common to the majority of decapod zoeae, persists at least through the megalopal stage (Williamson 1982), but undergoes major reduction in juvenile stages of many paguroids. We consider three characters in the category, carapace.
1a. – Calcification. Scholtz & Richter (1995) reported a strongly calcified exoskeleton for most adult reptant decapods, except for “asymmetrical hermit crabs” and some thalassinids, although they considered a soft cuticle as exhibited by “natant” decapods the “original” condition. While a calcified megalopal carapace is present in some lithodids, that is not true for all, nor is it true for most pagurids. In the subsequent juvenile stages, some degree of calcification may be gained, but if a calcified integument is initially present it is not lost, it is simply strengthened. Of itself, this restricted evidence would suggest that a chitinous cephalothoracic integument is the plesiomorphic condition. With any decapod, the postmolt exoskeleton is initially uncalcified (Greenaway 1985). However, this cannot be viewed as indicating an evolutionary precursory condition. The onset of calcification of the carapace varies, but clearly calcification is not an orderly and progressive transformation. We have interpreted an initially calcified carapace as primitive because of the comparable corresponding states of pleonal calcification (see rationale for characters 5-9).
1b. – Carapace delimitation. With relatively few exceptions, the pagurid megalopal carapace consists of a well-defined, chitinous shield and membranous posterior portion. In contrast, among lithodids there seems to be variation in the extent to which the anterior carapace is delimited, but a specific shield is not similarly identifiable in lithodids. Applying Nelson’s Rule, the more general condition would be lack of specific delineation.
1c. – Rostrum. Rostral processes are generally well, or at least better, developed in the megalopa in all families, but show gradual or substantial reduction in subsequent stages. We consider the more general, thus primitive condition, to be a well-developed rostrum.
2. Ocular acicles
The interpretation of what constitutes an ocular acicle varies among authors, although all concur that its presence is apomorphic. Jackson’s (1913: 40) definition as a spearhead shaped “squama”, or Makarov’s (1938: 126, 1962: 120) “as a small appendage ... which is usually oval with a pointed tip” has been interpreted to mean the entire calcified plate of the penultimate segment (cf. Powar, 1969) of the peduncle, as implied by McLaughlin (1974, 1983) and Martin & Abele (1986), and has been specifically defined as such by Sandberg & McLaughlin (1998) and Forest et al. (2000). In contrast, Richter & Scholtz (1994: Figs. 3A, B) and Boyko & Harvey (1999: 383, fig. 2A) have restricted the ocular acicle to the “spinose or platelike anterodorsal extensions.” The calcified plate often present on the second (penultimate peduncular segment), with or without projections, is understood here as representing the ocular acicle.
3. Thoracic appendages
Only two pairs of thoracic appendages are considered in our analysis, as these are the only ones that appear to be correlated with carcinization.
3a. Development of the fourth pereiopod is of primary importance. The more general condition, seen in lithodids, i.e., development of the fourth pereiopod as a walking leg, is considered primitive, rather than an atavism as claimed by Boas (1924), or gene replication of the third pereiopod as suggested by Richter & Scholtz (1994).
3b. The reduction of the fifth pereiopod, although apomorphic when evaluated among all decapods, is a character shared by all Paguroidea. As indicated by Pohle (1989), Richter & Scholtz (1994), and Scholtz & Richter (1995) this appendage in the adult is specially adapted for different functions among members of the superfamily. The more general usage of this appendage is as a gill cleaner, and as such is provided only with setae. We consider setae only as the primitive condition.
4. Pleon carriage.
The megalopal pleon is carried, at least initially, in a straight and fully extended position in all paguroids studied. While it may remain straight or become twisted in subsequent crab stages, it may also be flexed under the cephalothorax and held closely against the cephalothorax. We consider the extended, straight pleon more general, and therefore to represent the plesiomorphic condition.
5 -9. Pleonal tergites.
Integumental calcification of the tergites, like calcification of the carapace, is not easily polarized, although Goffinet & Jeuniaux (1994) considered decalcification of the pleon a secondary loss in paguroids. Fully calcified megalopal tergites are present in some, but not all lithodids, nor are they present in most pagurids. Following Nelson’s Rule, lack of calcification should, being more general or absent, be adjudged the plesiomorphic state. However, character state adjacency demonstrates that calcification, if initially present in megalopal tergites, may be lost, whereas if calcification is not present initially in those tergites, but is subsequently gained, it is not then again lost. Consequently we consider initial calcification of the megalopal tergites the primitive state. Nevertheless, fusion and calcification in each tergite, as well as tergal subdivisions must be considered as characters distinct for each of the first five tergites. Based on the limited data available, the megalopal/juvenile sixth tergite does not provide any significant ontogenetic information.
5. – Tergite 1. Only two characters are applicable to this tergite, calcification and tergal identity. Presence of a calcified tergite is considered plesiomorphic, as is the retention of tergal identity .
6. – Tergite 2. Three characters are needed to evaluate the changes that occur in this tergite: 1) calcification, 2) division, and 3) loss of identity. Character states for tergal division are undivided, marginal plates delineated, lateral plates delineated, and marginal and lateral plates delineated; for tergal identity: distinct, partially lost or obscured, and identity completely lost or nearly so. The primitive states are calcified (for the reason stated above), undivided, and distinct, the latter two being the universal megalopal conditions.
7-9. – Tergites 3-5. In addition to the characters of the second tergite, one additional character is needed, i.e., development of accessory marginal nodules. Lack of development of such nodules is considered the plesiomorphic condition.
Differences in telson shape and armature are observable in early juveniles. The typical, roundly subrectangular or subquadrate, and unarmed telsons of the megalopae are judged to be plesiomorphic.
After reviewing these losses in early juvenile stages among diogenids, pagurids, parapagurids and lithodids, it has become clear that the majority of species undergo a more or less gradual loss. We have, therefore, coded the complete loss and various degrees of gradual pleopod loss numerically using Wilkinson’s (1992, 1995) ranking methods, but these representations are not meant to suggest an ordered transition.
With the molt to megalopa, uropods, if initially present in lithodids, are quickly lost, while uropods in other paguroids are dramatically altered. Here again we have used Wilkinson’s (1992, 1995) method to code the character states, but not to suggest an ordered transformation.
We will consider only cheliped asymmetry here, as it is the only element not incorporated into other characters. Megalopal symmetry is adjudged primitive, however coding of right and left handedness is not an indication of presumed polarity.
14. Zoeal stages .
We have included the number of zoeal stages passed through by each taxon, whether these might be considered to have preceded the megalopa or been inserted following it because there undoubtedly is informational value in the extent of larval influence.