Macrodactyla, Haddon, 1898

Yap, Nicholas Wei Liang, Mitchell, Michela Lee, Quek, Zheng Bin Randolph, Tan, Ria, Tan, Koh Siang & Huang, Danwei, 2023, Tripedalia maipoensis Sun & Tsui & Wong & Cheung & Ng & Or & Qiu 2023, sp. nov., Zoological Studies 62 (29), pp. 1-25 : 20-21

Phylogeny of Macrodactyla and D. armata

In this study, we tested the relationship between M. aspera and D. armata , due to their strong resemblance to each other in having tentacles densely covered with visible nematocyst batteries. Our phylogeny reconstruction did not recover M. aspera and D. armata as a clade. Instead, both formed two distinct groups: M. aspera and the new species, M. fautinae sp. nov., were recovered as a clade, while D. armata was closely related to the actinodendrids (i.e., A. arboreum and Ac. haeckeli ; Fig. 1 ).

Morphological and ecological evidence supported our findings that Macrodactyla species and D. armata belong to two distinct groups. Anatomically, D. armata lacks adhesive verrucae on its column, while these are present on M. aspera and M. fautinae sp. nov. In both Macrodactyla species, the sphincter muscle is circumscribed in form, while this musculature is diffused in D. armata ( Carlgren 1949) . Evidence supporting the close phylogenetic relationship of both macrodactylid species (i.e., M. aspera and M. fautinae sp. nov.) have been discussed earlier in this study (see genus entry of Macrodactyla , above).

Together with our molecular findings and some evidence present in published taxonomic accounts of the Actinodendridae species targeted here, these strongly suggest that D. armata may be actually more closely related to actinodendrids than to Actiniidae , the family in which Dofleinia is currently placed. Our proposition here agrees with Rowlett (2020: 292), who also suggested the close relatedness of Dofleinia to actinodendrids, based on similarities in the ecology and morphology of these animals.

Ecologically, like D. armata , actinodendrids are also well-known to be able to inflict painful stings upon contact; they are popularly referred to as ‘Hell’s Fire Anemones’ ( Ardelean and Fautin 2004; Erhardt and Knop 2005; Fautin et al. 2009). Both D. armata and actinodendrids are hosts to anemone shrimps ( Bruce 1969 2005; Fransen 1989). Additionally, Rowlett (2020: 292) notes that these anemones exhibit, “burrowing behaviour.”

Broad morphological similarities are shared between D. armata and these actinodendrids, particularly: i) in having a smooth column without any visible structures (e.g., adhesive verrucae, etc.) ( Carlgren 1949), and ii) in bearing regions of dense nematocyst clusters along their tentacles (for a brief list of other morphological similarities, see Rowlett 2020: 292). Pertaining to i), while assuming M. aspera to be a morphological variant of D. armata, Rowlett (2020: 292) posited that the presence of verrucae is a diagnostic trait of Dofleinia , despite conceding that this trait was not found on the holotype of its type species (“ Dofleinia … having… adhesive verrucae… strangely absent from many specimens (including the type)”). In light of our molecular findings, and the possibility of Dofleinia being related to actinodendrid anemones, we propose that the presence of adhesive verrucae is not a diagnostic trait of Dofleinia . Regarding ii), we note that this feature does differ in appearance between D. armata and the actinodendrids (for an example of acrospheres of an actinodendrid, see Ardelean and Fautin 2004: 192, Fig. 3 ). Rowlett (2020: 292) suggested that this feature may be homologous between these anemones. At present, the taxonomy of Actiniidae is in chaos and taxa within this family have been found to be polyphyletic ( Daly et al. 2017). To determine if D. armata is truly closely related or belongs to the family Actinodendridae will require further thorough taxonomic and genetic work that is beyond the scope of this present study.

Figures

Fig. 1. Phylogeny reconstruction of Macrodactyla aspera and Macrodactyla fautinae sp. nov. among actiniid sea anemones. Tree topology a result of maximum likelihood (ML) analysis, using a concatenated dataset (i.e., cox3, 12S, 16S, 28S). At the branches, bootstrap ML resampling values, and Bayesian inference (BI) posterior probability values, are presented as ML/BI. Only bootstrap values> 50, and posterior probability> 0.8 are indicated here; those less than 50 and 0.8 are denoted by (-). Taxa indicated in bold denotes the presence of new sequences obtained for this study. Indicated by an asterisk (*), we retain the name Macrodactyla doreensis in this figure to aid with the discussion within the main-text. For sequences used and the full tree, refer to table S1 and figure S3, respectively.

Fig. 3. Living individuals of Macrodactyla aspera, external morphology. A, B, expanded individuals extending from the substratum, in situ, top view. Note splotchy brown patterns on oral disc and tentacles in both A and B, and partially everted actinopharynx resembling thick lips in B. Photographs by KS Loh. C, a live collected specimen (ZRC.CNI.1099), side view. Note reddish-orange column and adhesive, conspicuous verrucae, with grains of substratum attached to it. D, a partially retracted specimen (ZRC.CNI.1099), top view. Note that individual does not completely retract its tentacles; note also the presence of marginal projections.Abbreviations: mp, marginal projections; v, verrucae. Scale bars = 10 mm.