Cuticulata

Anemones with fine cuticles
Published 26 July 2023

For all their familiarity to anyone who has spent much time observing life on the seashore, sea anemones can be a surprisingly difficult group of animals to define. Only a single feature distinctive to the order has been identified to date: the presence of a triad of apical flaps at the opening of the nematocyst capsule (Rodríguez et al. 2014). Otherwise, anemones are recognized more by the absence of features characteristic of other cnidarian lineages. Higher-level taxa within the anemones have proven similarly challenging with systematists forced to rely on a limited range of characters, many of them vulnerable to loss and convergence. Nevertheless, the advent of molecular data has allowed many anemone clades to be placed on a firmer footing than was once possible.

Nemanthus nitidus, copyright Nick Hobgood.

One such clade that has been resolved in recent years is the Cuticulata. The members of this clade are often large, deep-sea anemones with thick columns bearing tubercles and/or a chitinous cuticle (hence the clade name; Rodríguez et al. 2014). However, these characteristics are not universal. Reinforcement by the cuticle may make up for the fact that the column contains few perfect mesenteries (that is, sheets of tissue that connect the body wall with the pharynx). The cuticulates form part of the broader clade Metridioidea, a distinctive feature of which is the presence of acontia (Rodríguez et al. 2012). These are nematocyst-dense, thread-like extensions of the mesenteries that may be stretched out of the mouth or through pores on the column. They may provide a defensive function or assist in the processing of food. However, many cuticulates also appear to have secondarily lost acontia.

Burrowing anemone Halcampe decemtentaculata, copyright Siena McKim.

Among those cuticulates that have lost acontia are various deep-sea lineages. The Graspina are one such group in which the basal disc has become flexible, allowing the anemone to grasp onto narrow substrates such as the tubes or skeletons of other invertebrates. On the sludgy base of the deep sea, covered in radiolarian ooze, rocky outcrops are few and far between. The Chemosynthina are specialists in chemosynthetic environments such as hydrothermal vents; members of this group lack acontia but still retain the pores on the column through which they might otherwise be extruded. The Halcampidae are acontia-less anemones that live in burrows, where filamentous protrusions from the column would presumably just get in the way.

Calliactis parasitica on a hermit crab, copyright Julien Renoult.

Perhaps the group of cuticulate anemones to incite the most interest has been the hermit-crab associates of the Hormathiidae. You may be aware that many hermit crabs will pluck an anemone and place it on their shell to provide extra protection. You may not have considered (as I certainly hadn’t until researching this post) that not just any anemone will do. It needs to be one that will actually grasp onto the shell when put in place and remain attached. Two hormathiid genera, Calliactis and Paracalliactis, are among the genera that exhibit this behaviour. By taking up residence on a hermit crab, the anemones gain increased mobility and improved access to food sources. As well as providing protection to the crab with their stinging tentacles, the anemones may also cover cracks and other weak points in the home shell. And in the case of the cuticulate hormathiids, the cuticle secreted by the anemone’s pedal disc forms a structure called a carcinoecium that may coat and even replace the original shell. As the crab grows, the anemone can add to the carcinoecium, thus obviating the need for the crab to regularly abandon its shell in search of a larger one. Interestingly, phylogenetic analysis suggests that the hermit crab association has evolved among the hormathiids twice, with the two genera each more closely related to non-symbiotic hormathiids (Gusmão et al. 2020). Some anemones do not get selected by hermit crabs but do climb onto gastropods in search of a better life. Perhaps this was the first step to becoming truly crabby.

References

Gusmão, L. C., V. Van Deusen, M. Daly & E. Rodríguez. 2020. Origin and evolution of the symbiosis between sea anemones (Cnidaria, Anthozoa, Actiniaria) and hermit crabs, with additional notes on anemone-gastropod associations. Molecular Phylogenetics and Evolution 148: 106805.

Rodríguez, E., M. Barbeitos, M. Daly, L. C. Gusmão & V. Häussermann. 2012. Toward a natural classification: phylogeny of acontiate sea anemones (Cnidaria, Anthozoa, Actiniaria). Cladistics 28 (4): 375–392.

Rodríguez, E., M. S. Barbeitos, M. R. Brugler, L. M. Crowley, A. Grajales, L. Gusmão, V. Häussermann, A. Reft & M. Daly. 2014. Hidden among sea anemones: the first comprehensive phylogenetic reconstruction of the order Actiniaria (Cnidaria, Anthozoa, Hexacorallia) reveals a novel group of hexacorals. PLoS One 9 (5): e96998.

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