Project Synopsis

The Questions
Why Sphaeromatid Isopods?
Sphaeromatidae: Unresolved Taxonomic Issues

Our goal is to understand the relationships of sphaeromatid genera, identify generic and subfamilial groupings and develop phylogenetic hypotheses that can provide a sound basis for future biogeographic, mating-system evolution, ecological, behavioral, and other studies. Our investigation will be guided by morphological and molecular analyses of evolutionary relationships within this family (Crustacea: Isopoda: Sphaeromatidae). This analysis will necessarily have to consider the relationship of the Sphaeromatidae to the other families of the Flabellifera, and a revision of the Sphaeromatidae is seen as pivotal in developing a better understanding of those family relationships. We will address two questions that combine our research backgrounds in crustacean morphology and molecular systematics.

(1) What subfamilial and generic phylogenetic relationships among the Sphaeromatidae are revealed by morphological and molecular analyses?

(2) Are the same phylogenetic hypotheses inferred from molecular and morphological datasets?

The Questions

The first question (sphaeromatid phylogenetics) is important because sphaeromatid isopods are both speciose and abundant. Sphaeromatids, with rare exceptions, are strongly endemic, and this endemism at generic and species level is potentially highly informative in biogeographic analysis once the phylogeny is understood. About one quarter of all described crustacean genera are isopods, and about 15% of all isopod species are sphaeromatids. This family is one of the three largest families in the suborder Flabellifera (the others being Cirolanidae and Cymothoidae) and includes 99 described genera with over 680 known species. Present knowledge of this family represents less than 50% of the potential diversity based on observed, undescribed taxa (Bruce, pers. comm.). For the Sphaeromatidae as a whole, the enormous diversity in morphology and life history traits obfuscates evolutionary relationships among genera and subfamilies. New data generated by this study will shed light on the evolutionary relationships of the Sphaeromatidae and related isopod taxa.

The second question (compatibility of phylogenies based on molecular and morphological data) is important because convergence and parallelism of body form are rife within this group. Sphaeromatids' impressive exploitation of diverse habitats, in combination with diversity in female life history strategies and elaborate male combat structures, has resulted in extraordinary levels of homoplasy. These features, combined with a historic taxonomic focus on somatic characters to separate genera, a lack of specialists (Hansen provided a revision in 1905, and revisionary studies began again in the 1970s and 1980s) and a focus on regional faunas (i.e., small and artificially restricted datasets), have resulted in the use of plesiomorphic and homoplasious characters to define sphaeromatid genera as well as the subfamilies. Although emerging as important taxonomic characters, mouthpart characters have been little used in familial or generic descriptions (Bruce 1993). Mouthpart characters and other new morphological characters will provide an expanded dataset with which robust phylogenetic inferences can be made. Compatibility or incompatibility of molecular and morphological phylogenies will provide a fresh perspective on character evolution within this family and will provide hypotheses of relationships among the genera of Sphaeromatidae.

Why Sphaeromatid Isopods?

Sphaeromatid isopods are ideal for addressing life history, ecology, biogeography, and phylogenetic hypotheses, because:

However, many (perhaps most) of the large genera are of doubtful monophyly.

The Sphaeromatidae is one of the three largest isopod families, all of which are in the suborder Flabellifera (the others being Cirolanidae and Cymothoidae). Sphaeromatids occur worldwide and are most abundant in marine communities to 1400 m, and are occasionally present in permanent freshwater habitats. They inhabit rocky shores, sandy beaches, coral reefs, coastal rivers, and thermal springs, as well as, algae, sponges, tunicates, and mangroves. They are most diverse in warm-temperate, southern hemisphere shallow (intertidal to 200 m depth) marine habitats. They play an important role as prey for epibenthic fishes and are commensals and scavengers. An understanding of the phylogenetic relationships of the family is pivotal in unravelling the phylogeny of the Flabellifera, and hence the Isopoda.

The importance of and interest in sphaeromatids extends beyond systematics because of their diversity in life histories, reproductive behavior, and sexual polymorphism (e.g., polychromatism: Bocquet et al. 1950, 1951, Tinturier-Hamelin 1962, 1963; biogeography: Carlton 1987; sense organs: Brandt 1988; life history, physiology, and reproductive behavior: Holdich 1968, 1976, Shuster 1981a, b, Jormalainen and Shuster 1997, 1999; male sexual polymorphism: Shuster 1987, Shuster and Wade 1991a, Shuster and Sassaman 1997; sex ratio: Heath and Ratford 1990, Shuster et al., 2001; parental care: Shuster 1992, Thiel 1999).

Variation in brood pouch morphology among females (Harrison 1984) and extreme sexual dimorphism among males (wide variation in uropod shape, size and ornamentation; cephalic shape, size and ornamentation; and variation in body size and growth rate; Hurley and Jansen 1977, Shuster 1987, 1992) make sphaeromatids excellent organisms for studying mating system evolution. However, this research direction (and similar comparative questions) can only be pursued with a robust phylogeny in hand.

Sphaeromatidae: Unresolved Taxonomic Issues

Despite its ubiquity, diversity, and biological importance, the family is in a state of confusion. Few morphological and no molecular studies have been used to resolve sphaeromatid phylogenetic relationships among subfamilies, genera, or species. We do not have a clear understanding of morphological character transformations in the family, and new characters must be identified before genera can be unambiguously defined and a morphological phylogenetic analysis is possible. Based on the preliminary work of Bruce (1994a, b, 1995, 1997, pers. comm.), wide variation exists in mouthpart morphology, a fertile source of new morphological characters.

Presently the family consists of three subfamilies (Sphaeromatinae Latreille, 1825; Dynameninae Bowman, 1981; and Cassidininae Hansen, 1905, though there are several more available names), but their relationships are unresolved, and the monophyly of these subfamilies remains untested (Figure 2). Over the past twenty years sphaeromatids have received considerable systematic attention, but revisions have focused mostly on regional species descriptions (Holdich and Harrison 1980, 1981a, b, Harrison and Holdich 1984, Kensley 1984, Müller 1991, Kussakin and Malyutina 1993, Kensley and Bursey 1996, Kensley et al. 1997). Leading isopod systematists (Wägele 1989, Harrison and Ellis 1991, Brusca and Wilson 1991, Bruce 1994a, 1995, 1997, and Wetzer and Bruce 1999) all agree that a modern analysis of sphaeromatid isopod systematics is long overdue. The historical development to the present state of sphaeromatid systematics is outlined in Table 1.


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National Science Foundation

This project supported by grant DEB-0129317 from the National Science Foundation


Natural History Museum
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