Turridae

Shell of Turris crispa crispa, copyright H. Zell.

At this point, I've made numerous references on this site to the gastropod family Turridae, discussing its members and non-members and alluding to its sordid history. So maybe I should set out the basics of the story properly.

The Conoidea are a diverse group of marine predatory gastropods with over 4000 known living species. They are best known for the production by many species of venom used to paralyse their prey, in some species being potent enought to threaten humans. In the majority of conoideans, this venom is delivered via a tooth that becomes detached from the radula and is held at the end of the retractable proboscis. Until relatively recently, Conoidea were commonly divided between three families. Two of these families, the Conidae (cone shells) and Terebridae (awl shells) were well defined and constrained. The third family was the Turridae, including by far the greater number of species but not really defined within Conoidea beyond 'the rest'. Many of 'the rest' were small, many were restricted to deep water, many were poorly known. Different systems were proposed over the years in an attempt to break the turrid mass into more manageable units but each system differed significantly from the next and no one system became universally accepted. Some authors would focus on the protoconch as their guide to classification, others would focus on the radula, others might call out features of the operculum. One author commented in 1922 that turrids were "considered by those who meddle with them to be more perplexing than any other molluscan family", and this complaint was still being upheld by Kilburn (1983) over sixty years later.

Though it had long been accepted that the 'turrids' probably did not represent an evolutionarily coherent group, it wasn't really until the advent of molecular phylogenies that things started falling into place. Puillandre et al. (2011) identified two main lineages within the Conoidea, leading to the dissolution of the original Turridae into no less than 13 families in order to maintain the already-established Conidae and Terebridae. Turridae in the strict sense was restricted to a much smaller clade of a bit over a dozen genera, sister to the Terebridae (Bouchet et al. 2011).

In contrast to the bewilderment of the original turrid array, Turridae sensu Bouchet et al. is a morphologically quite coherent group. They are more or less fusiform (spindle-shaped) shells, often with a narrow, high spire and relatively weak sculpture. Indeed, but for the fact that most tend to have a long siphonal canal at the base of the shell, they often bear a distinct resemblance to their sister group, the terebrids. The majority of turrids have a multispiral protoconch, indicating an extended, planktonic-feeding larval stage in development, but there are some species with a paucispiral protoconch indicative of direct development.

Radula of Xenuroturris legitima, from Kantor & Puillandre (2012); ct = central tooth.

The radula of turrids usually comprises three apparent teeth in each row. The central tooth is actually formed from three teeth (the original pointed central tooth and two plate-like lateral teeth) fused together; in some species the division between these teeth remains visible whereas in others the central tooth disappears entirely. The main business part of the radula is the single pair of marginal teeth which, as in other conoideans, are enlarged and modified for venom delivery. They have a distinctive 'duplex' form; in older publications, this was referred to as a 'wishbone' form because the tooth appears under light microscopy to be divided between two branches. After the advent of electron microscopy, it was discovered that these two 'branches' in fact represent the thickened margins of an undivided tooth. The larger of the two margins is mostly attached to the radular membrane with only the tip of the tooth being free; the smaller margin is held free of the radula. The thinner part of the tooth between the two margins forms a gutter along which venom can flow. However, the radula is placed in such a position that it cannot be protruded through the mouth in the manner of grazing gastropods. As with other conoideans, prey (in this case probably worms) is despatched through the use of a detached marginal tooth transferred to the end of the proboscis. However, whereas other conoideans such as cone shells may have the tooth functioning like a hypodermic syringe for delivering prey, turrids use their tooth to slash at the prey like a switchblade, with venom passively entering through the resulting cuts. The proboscis is then used to draw the prey back into the mouth, where the radula is used to grasp and swallow it, sucking the unlucky worm down the gullet like spaghetti.

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.

Kantor, Y. I., & N. Puillandre. 2012. Evolution of the radular apparatus in Conoidea (Gastropoda: Neogastropoda) as inferred from a molecular phylogeny. Malacologia 55 (1): 55–90.

Kilburn, R. N. 1983. Turridae (Mollusca: Gastropoda) of southern Africa and Mozambique. Part 1. Subfamily Turrinae. Annals of the Natal Museum 25 (2): 549–585.

Puillandre, N., Y. I. Kantor, A. Sysoev, A. Couloux, C. Meyer, T. Rawlings, J. A. Todd & P. Bouchet. 2011. The dragon tamed? A molecular phylogeny of the Conoidea (Gastropoda). Journal of Molluscan Studies 77: 259–272.