Anchor Sponges

Sponges are, by their very nature, a challenging group taxonomically. At the macroscopic level, they are often amorphous and indeterminate in appearance. As one taxonomist complained in 1842 (as quoted in Hooper & Van Soest 2002): "there is so much that is in common to them, and each adapts itself so readily to circumstances and assumes a new mask, that it requires a tact, to be gained only by some experience, to recognize them under their guises; while we labour, perhaps in vain, to devise phrases which shall aptly portray to others the characteristics of objects that have no fixed shape, and whose distinctive peculiarities almost cheat the eye". Reliable identification typically requires the close examination of microscopic details, in particular the conformation and arrangement of the mineralised spicules that make up the skeleton of many sponges.

Myxilla incrustans, copyright B. E. Picton.

The Myxillidae are a family of marine sponges that, so far as we currently know, are most diverse in temperate and frigid waters. Like other members of the class Demospongiae, the most diverse of the recognised sponge classes, they have a skeleton of spicules constructed from silica. Different arrangements of spicules allow the body of the sponge to be divided into two layers. In the outer ectosoma, which can be thought of as the 'skin' of the sponge, elongate spicules are vertically radiating or placed in 'bouquet' arrangements with a palisade of vertical spicules surmounted by radiating clusters. These spicules generally have each end similar and may be smooth or spiky. In the inner choanosoma, within which are placed the feeding chambers of the sponge, elongate spicules are placed in a reticulate arrangement. These spicules generally have one end pointed and the other blunt.

Skeletal arrangements and individual spicules from various Myxillidae, from Hooper & Van Soest (2002).

Mixed in amongst these larger megasclere spicules are smaller microscleres that do not form part of the main structural skeleton, though presumably they do help hold the sponge body together. In myxillids, the microscleres generally take the form of anchorate chelae, small curved structures with incurved rounded prongs at each end. Members of the boreal genus Melonanchora have a mixture of chelae and a different type of microsclere shaped like a ribbed rugby ball (Santín et al. 2021). In the Indo-West Pacific genus Psammochela, growing sponges will also incorporate sand from the surrounding environment to supplement the microscleres (de Voogd 2012).

Growth habit of Myxillidae can vary from encrusting to massive to branching. The species Stelodoryx procera, found around the Azores, has a distinctive growth habit with a flattened main body at the end of an elongate stalk. On the whole, though individual species may be distinguished by growth habit, species within a single genus may differ greatly in form. For determining genera, examination of spicules is really the only way to go.

REFERENCES

Hooper, J. N. A., & R. W. M. Van Soest. 2002. Systema Porifera: A guide to the classification of sponges vol. 1. Kluwer Academic/Plenum Publishers.

Santín, A., M.-J. Uriz, J. Cristobo, J. R. Xavier & P. Ríos. 2021. Unique spicules may confound species differentiation: taxonomy and biogeography of Melonanchora Carter, 1874 and two new related genera (Myxillidae: Poecilosclerida) from the Okhotsk Sea. PeerJ 9: e12515.

Voogd, N. J. de. 2012. On sand-bearing myxillid sponges, with a description of Psammochela tutiae sp. nov. (Poecilosclerida, Myxillina) from the northern Moluccas, Indonesia. Zootaxa 3155: 21–28.