Zemacies: A Toothless Wonder

The Recent species Zemacies queenslandica, photographed by Jan Delsing.

What makes an organism a 'living fossil'? The phrase is one that has been thrown about a bit over the years but whose actual definition can be ambiguous. Many people use it to refer to a species that is supposedly little changed from its distant ancestors. An alternative interpretation, however, and I suspect the phrase's original inspiration, would be something that was first discovered as a fossil, only to be found alive at a later date. The coelacanth, of course, would be the textbook example of such a case (without necessarily being a good example). But you might be surprised at some of the animals that could be called a 'living fossil' under this definition. The white-tailed deer would be one, as would the bush dog of South America. And so would the subject of today's post.

Zemacies is a genus of conoid gastropods known from the south-west Pacific. It is relatively large as conoids go, with some growing over three inches in length. Shells are a slender, fusiform shape, often with prominent nodules on the whorls. The first known species, Z. elatior, was described from the Miocene of New Zealand. Over time, additional species were described from New Zealand and Australia, extending the age range of the genus from the Palaeocene to the Pliocene (Powell 1969). It wasn't until 2001, however, that living species of Zemacies were recognised in the deep sea around New Caledonia and Queensland.

Figure from Fedosov & Kantor (2008) showing appearance of pyriform organ in foregut of Zemacies excelsa (left), with cross-section of organ to show internal structure (right). Abbreviations: bl, bulb-like structure; gt, glandular tissue; ms, muscles; sgp, semicircular glandular pad; tf, tall folds underlain by glandular tissue; tn, tentacles.

The discovery of living specimens (or, at least, living until the time of their collection) led to the revelation that Zemacies was a very intriguing genus, in a way that would have probably never been guessed from fossil material alone. As has been described previously, one of the great innovations of conoids was the modification of the radula into a system for the injection of paralysing toxins. When researchers investigated the soft anatomy of Zemacies excelsa, however, they discovered that it turned away from this trend. Zemacies has lost both the radula and its associated poison glands, as well as the associated proboscis. In their place, one side of the foregut has grown a pear-shaped outgrowth, referred to as the pyriform organ, that is covered with glandular tentacles (Fedosov & Kantor 2008). This structure is so unusual that its initial discovery lead to the proposal of a new subfamily to separate Zemacies from all other conoids (it has since been placed by Bouchet et al., 2011, as a distinctive member of the family Borsoniidae).

Unfortunately, the bathyal habitat of living Zemacies means that we as yet have no idea of its preferred prey and consequently little idea of how the pyriform organ is used when feeding. The internal cavity of the pyriform organ contains an array of longitudinal muscles, suggesting that it can be extended out the front of the animal in place of the usual proboscis. The tentacles may function to grasp or adhere to the prey, and/or the glandular tissue underlying them may secrete toxins or digestive enzymes, while the action of the pyriform organ against the foregut wall during withdrawal of the prey may serve to crush it (I wonder what the efficacy of this arrangement may be against something with a strong but not calcified cuticle, such as a deep-water crustacean). A similar foregut structure (also associated with loss of the radula and proboscis) has been identified in another conoid genus, Horaiclavus, though phylogenetic analysis of the Conoidea indicates that the two genera almost certainly evolved these structures independently. Horaiclavus also differs from Zemacies in that the muscular foregut outgrowth lacks any associated glandular tissue and is presumably entirely mechanical in its action (Fedosov & Kantor 2008). Perhaps one day someone will finally observe one of these deep-sea genera in their native habitat and provide us with a solution to the mystery of their life styles.

REFERENCES

Bouchet, P., Y. I. Kantor, A. Sysoev & N. Puillandre. 2011. A new operational classification of the Conoidea (Gastropoda). Journal of Molluscan Studies 77: 273–308.

Fedosov, A., &. Y. Kantor. 2008. Toxoglossan gastropods of the subfamily Crassispirinae (Turridae) lacking a radula, and discussion of the status of the subfamily Zemaciinae. Journal of Molluscan Studies 74: 27–35.

Powell, A. W. B. 1969. The family Turridae in the Indo-Pacific. Part 2. The subfamily Turriculinae. Indo-Pacific Mollusca 2 (10): 215–415.