The Caprellidea (Crustacea) have undergone an interesting morphological evolution from their ancestral gammarid-like form. Although most caprellid families have markedly reduced third and fourth pereopods (the walking thoracic limbs) and pleons (the posterior body parts), one family, Caprogammaridae, has developed pleon with swimming appendages (pleopods), whereas another family, Phtisicidae, possesses well-developed functional third and fourth pereopods. The unique character status of these families implies that there has been reacquisition or multiple losses of both pereopods and the pleon within the Caprellidea lineages. Although the Caprellidea are fascinating animals for the study of morphological evolution, the phylogenetic relationships among the Caprellidea are poorly understood. One obstacle to studying the evolution of the Caprellidea is the difficulty of collecting samples of caprogammarid species. In this study, we obtained live samples of a Caprogammaridae species and confirmed that its pleon and pleopods could perform similar locomotive functions and swimming movements as observed in gammarids. From the phylogenetic analyses on 18S ribosomal RNA gene sequences, we identified three distinct clades of Caprellidea. The ancestral state reconstruction based on the obtained phylogeny suggested that once lost, the third and fourth pereopods were regained in the Phtisicidae, while the pleon was regained in the Caprogammaridae, while we could not exclude the possibility of independent losses. In either case, the caprellid lineage underwent a quite complicated morphological evolution, and possibly the Caprellidea may be an exception to Dollo’s law.
Caprellid Molecular phylogeny 18S rRNA Pereopod Pleon Dollo’s law
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We express our special thanks to the staff of the Akkeshi Marine Station (Hokkaido University) for their technical support with the fieldwork to collect the C. gurjanovae samples and the researchers and crew of the JARPA II and the Institution of Cetacean Research for collecting and providing us with the Cyamus balaenopterae samples. A research grant from the Research Institute of Marine Invertebrates funded this work.
Supplementary Figure 1Maximum likelihood tree based on the 18S rRNA gene sequences of the Caprellidea, Corophioidea (a gammarid superfamily that is closely related to the Caprellidea [4–6]) and two gammarid outgroup species. According to Fig. 3, the branches supporting each caprellid clade (Clades 1, 2, and 3) are colored. In the maximum likelihood analysis with PhyML 3.0 , the GTR + I + G model selected by the Akaike information criterion (AIC) in Modeltgenerator version 0.85  was used. The estimated parameter values were as follows: nucleotide frequencies, pi (A) = 0.25716, pi (C) = 0.24198, pi (G) = 0.27938 and pi (T) = 0.22148; GTR rate parameters, RA→C = 2.88, RA→G = 5.31, RA→T = 3.73, RC→G = 0.67, RC→T = 13.10 and RG→T = 1.00; proportion of invariable sites = 0.54; and gamma distribution parameter alpha = 0.41. The analysis was performed using SPR branch swapping, and 1000 bootstrap pseudoreplicates were conducted. The numbers under the branches in each tree represent the bootstrap values; values less than 70% are not shown. See Table 1 and our previous study  for the accession number of each taxon’s sequence. (PDF 321 kb)
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