Towards a phylogenetic classification of Leptothecata (Cnidaria, Hydrozoa)

Abstract

Leptothecata are hydrozoans whose hydranths are covered by perisarc and gonophores and whose medusae bear gonads on their radial canals. They develop complex polypoid colonies and exhibit considerable morphological variation among species with respect to growth, defensive structures and mode of development. For instance, several lineages within this order have lost the medusa stage. Depending on the author, traditional taxonomy in hydrozoans may be either polyp- or medusa-oriented. Therefore, the absence of the latter stage in some lineages may lead to very different classification schemes. Molecular data have proved useful in elucidating this taxonomic challenge. We analyzed a super matrix of new and published rRNA gene sequences (16S, 18S and 28S), employing newly proposed methods to measure branch support and improve phylogenetic signal. Our analysis recovered new clades not recognized by traditional taxonomy and corroborated some recently proposed taxa. We offer a thorough taxonomic revision of the Leptothecata, erecting new orders, suborders, infraorders and families. We also discuss the origination and diversification dynamics of the group from a macroevolutionary perspective.

Figure 1. Cladogram of the phylogenetic analysis of Leptothecata with minimum filtering (matrix 16S18S28S_N).

Support values are integer numbers or decimals (no leading zeros). Parametric support above the branch (aBAYES/aLRT), non-parametic support below the branch (BS/SH-aLRT); values in plain text indicate non-significant support (aBAYES < 0.95; aLRT < 0.9; BS < 75%; SH-aLRT < 0.85); significant support values are in bold face (aBAYES ≥ 0.95; aLRT ≥ 0.9; BS ≥ 75%; SH-aLRT ≥ 0.85). Colors were used when support is significant for at least 3 out of 4 methods and fine black lines for those with low support (two or fewer significant values). The bars above branches indicate clades found in the topology obtained from the Leptothecata 16S18S28S matrix; bars beneath the branches indicate the same for 16S18S28S_Nrw4. To the right of the description of major clades are details on the newly proposed groups, emphasizing suborders and orders (black box). Purple dots indicate unstable terminals (rogue taxa) and yellow dots were likely misidentified. Rogue taxa were not considered in the evaluation of the clades in other analyses, and together with the ones marked by yellow dots, are considered incertae sedis in the proposed taxonomy. Basic taxonomy (species and genera) from the WoRMS database.

Figure 2. Cladogram of the Leptothecata obtained from the unfiltered (matrix 16S18S28S).

Branch colors, support values and notation for unstable taxa as described in Fig. 1. Names within quotation marks (“ ”) are non-monophyletic groups (traditional taxonomy from WoRMS).

Figure 3. Cladogram of the Leptothecata obtained from the intensively filtered (matrix 16S18S28S_Nrw4).

Branch colors, support values and notation for unstable taxa and taxonomy as described in Fig. 1.

Figure 4. Cladogram (A): Classification of the Leptothecata following Bouillon.

Colors indicate representative clades. Codes at the base of the branches from Table 3. Asterisk indicates non-monophyletic, original proposal. Cladogram (B): comparison of the clades following Bouillon (left, defining the families) with the principal clades and lineages we propose herein (right, see Fig. 1); the symbol ^o indicates clades already defined in the literature. Colors indicate representative clades and the numbers at the bases of the branches indicate a summary of the relative position according to Bouillon and this study. Clades defined by more than one family in Bouillon are indicated by a combination of colors and number of the family in each case. Rogue taxa were not included and double asterisks indicate families absent from Bouillon.