Munida, Leach, 1820

Cabezas, Patricia, Macpherson, Enrique & Machordom, Annie, 2009, Morphological and molecular description of new species of squat lobster (Crustacea: Decapoda: Galatheidae) from the Solomon and Fiji Islands (South-West Pacific), Zoological Journal of the Linnean Society 156 (3), pp. 465-493 : 489-490

MUNIDA

The genus Munida is by far the most diverse taxon of the family Galatheidae , and is widely represented in the waters of the South-West Pacific. In an earlier study, we suggested the monophyly of the group in the area ( Machordom & Macpherson, 2004), excluding the species Munida callista Macpherson, 1994 , which was recently ascribed to a new genus ( Cabezas et al., 2008). The complexity of the group has previously been described in terms of extreme morphological convergence and heterogeneity in divergence values within species ( Baba, 2005; Macpherson & Machordom, 2005).

The study of the combined data set and the separate 16S rRNA data set supported two differentiated clusters. The first cluster occupied a basal position, and included the type species M. rugosa from the North-East Atlantic and M. delicata from the Solomon Islands, whereas the species M. oblongata sp. nov. from the Solomon Islands and M. devestiva from New Caledonia were also included when we only considered the 16S rRNA data. Mean divergence values among the species were quite high (10.96% for 16S rRNA and 15.8% for the COI gene), but were within the range cited for other decapod taxa ( Ptacek et al., 2001; Harrison, 2004). For the COI gene, the value could be an underestimate because of saturation traces found in the third codon positions. For both genes, these values were higher than the mean divergence reported for the genus ( Machordom & Macpherson, 2004), and similar divergence values have been found in species such as Enriquea leviantennata ( Baba, 1988) and Babamunida callista ( Macpherson, 1994) , which were excluded from the genus Munida on the basis of morphological and molecular information.

Additionally, the new species M. oblongata sp. nov. is morphologically very close to the species M. parca from New Caledonia, although genetically they show a deep divergence. Although this pattern seems to be common in decapods ( Knowlton, 1986), additional explanations are also possible. The undersampling of more closely related taxa might lead to the same pattern. A future increase of the sampling effort is essential to evaluate the overall diversity of galatheids in the area. And extinction events in the group could be an alternative explanation, although with our data it is not possible to confirm this hypothesis.

The second cluster included the rest of the species of the genus, all from the South-West Pacific. Three new species of the genus Munida appeared in this cluster. Although most relationships at the internal nodes were unresolved, M. lailai sp. nov. was highly supported as a sister group of the species M. parca , and these two species were supported as a sister lineage to M. caeli sp. nov. Divergence values between these two species were very low, although they were similar to divergences reported for other species complexes such as Munida militaris Henderson, 1885 + Munida rosula Macpherson, 1994 + Munida compressa Baba, 1988 ( Machordom & Macpherson, 2004). Morphologically, the two new species resemble M. parca , and although morphological differences are subtle, these were constant in all of the specimens examined. The phylogenetic position of the species M. mendagnai sp. nov. was not resolved, but it appeared in the second cluster with M. caeli sp. nov. and M. lailai sp. nov. in all the analyses. All three new species included in this cluster showed a low genetic divergence, with a very similar morphology.

This marked difference between the first and the second cluster was emphasized in a previous study ( Cabezas et al., 2008), in which M. rugosa was not included in the ingroup of the genus Munida , and showed a clear genetic difference with respect to the rest of species of the genus. Morphologically, there are no apparent characters to differentiate species from both clusters, although differences in epistome shape and armature, as have been observed between Munida and Babamunida ( Cabezas et al., 2008) , as well as antenna and antennula insertions, should be considered. On the other hand, the heterogeneity observed in the divergence values could indicate that different radiation events have shaped the diversification of the genus.

All of this information emphasizes the need for further more detailed analyses of more species from the Indo-Pacific and Atlantic-Mediterranean oceans, as well as the study of more conserved genes, to clarify the taxonomy of this group of species.

EVOLUTIONARY CONSIDERATIONS

The taxonomic status of the new species was supported by both morphological and molecular data, although intrageneric relationships could not be deduced with confidence for all of the species. This low resolution seems to be the general pattern for the family Galatheidae , and has already been noted in other studies in which relationships at the genus level were also unresolved ( Machordom & Macpherson, 2004). This conspicuous feature might be related to a scenario resulting from a rapid speciation process, rather than from a lack of resolution or paucity of the molecular markers used. The number of informative positions and the low levels of saturation indicate that both genes are appropriate for the study of this group. In addition, previous works have described the mitochondrial genes 16S rRNA and COI as powerful tools to elucidate phylogenetic relationships in crustacean decapods ( Porter, Pérez-Losada & Crandall, 2005), and in other marine groups (e.g. sponges; Blanquer & Uriz, 2007). Our phylogenies displayed a large number of short internal branches relative to long terminal branches. However, some groups of species also showed short terminal branches, which suggest recent speciation events.

Although the data did not show a clear geographic structure, New Caledonia seems to be the ancestral area for the different genera, although a deeper phylogeographic study is necessary to test this fact.

These results suggest an evolutionary history shaped by rapid diversification and radiation events, probably related to the marine and geological complexity of the area ( Holloway & Hall, 1998).

Events of rapid diversification are common within Crustacea, including decapods ( Schram, Feldmann & Copeland, 1978), making it difficult to resolve relationships at ancient nodes, and to thus estimate divergence times. The Eocene ( Schram, 1986) has been proposed as the scene for extensive decapod radiation, and Machordom & Macpherson (2004) located the diversification of the genus Munida in the Late Miocene, using two mitochondrial genes. An older estimation has been suggested by Porter et al. (2005), dating the diversification of Anomuran crabs (including the Galatheidae family) in the Permian– Triassic, using a multi-locus estimation procedure based on nuclear and mitochondrial genes. It is clear that the use of mitochondrial markers alone is insufficient for resolving such ancient speciation processes, as the substitution rate leads to a high probability of homoplasy. Thus, further investigations, including additional morphological and molecular characters, are still required to understand the evolutionary history of the group.

Previous studies have demonstrated the usefulness of nuclear genes to resolve phylogenetic relationships at high taxonomic levels ( Tudge & Cunningham, 2002; Ahyong et al., 2007). Morphologically, the extreme convergence observed in this family makes it very difficult to find new morphological characters of phylogenetic value, and the most common morphological characters used in the traditional taxonomy of the family show a high degree of homoplasy. Nevertheless, characters not previously considered, such as those describing the epistome region, can provide new synapomorphies ( Cabezas et al., 2008). If combined with additional genes with lower substitution rates, such as conserved genes, these characters could improve the resolution obtained at the basal nodes of the phylogenies.