Murderous Cones

The cone shells of the family Conidae have long been the subject of extreme interest from collectors. Their architectural form, polished surface and intricate patterning make it hard to argue that they are things of beauty, indeed. Not surprisingly, this long-standing aesthetic interest has also made them the subject of much taxonomic interest—some for the better, some arguably for the worse. For today's post, I've selected a particular subgroup of the cone shells: the species of the subgenus Textilia.

Bubble cone Conus bullatus, copyright H. Zell.

To describe the generic taxonomy of cone shells as 'messy' is something of an understatement. Part of the problem is that cone shells are another one of those groups in which a high level of species diversity contrasts with a low level of morphological disparity. Though species are readily distinguishable on the basis of superficial features such as colour patterning, they generally hew pretty closely to a particular overall morphotype. This can make it difficult to associate particular species into evolutionary groups. For many authors, the problem has been solved (or at least satisfyingly swept under the rug) by treating all cone shells as belonging to a single genus Conus. But with over 800 known species of conid, many showing intriguing variations in biology and natural history, many have yearned for a more informative system. Those who would divide, however, have disagreed significantly on how many divisions there should be. At the most disassociative end on the scale, one recent system divided the cone shells between no less than 113 genera, separated into five families. A more conservative approach was taken by Puillandre et al. (2014) who recognised four genera of cone shells (in a single family) with the larger genera encompassing multiple subgenera. Textilia was treated by PUillandre et al. as a subgenus within Conus, which remains the largest genus in the family by a considerable margin.

Pallisade cone Conus cervus, copyright James St. John.

Ten species of cone shell were included in Textilia by Puillandre et al. (2014). The species are found in the Indo-west Pacific, between south-east Africa and Hawaii. They are medium- to large-sized cone shells with the largest species, the pallisade cone Conus cervus reaching close to 12 cm in length. The smallest, the Timor cone C. timorensis, is at least 13 mm long. Textilia species have smooth, inflated shells and flared lips on the aperture (Old 1973). Only one species of Textilia, the bubble cone C. bullatus, can be considered well known. Not only is it found over almost the subgenus' entire range (other species are more localised), it is the only species found in shallower waters, being most common from slightly subtidally to 50 m (Hu et al. 2011). All other Textilia species are restricted to deeper waters. Just to confuse matters slightly, the textile cone C. textile is not a member of subgenus Textilia but another subgenus Cylinder.

Video of cone shells capturing fish, from here. The first individual is a striated cone Conus striatus (subgenus Pionoconus), the second is a bubble cone Conus bullatus.

Textilia forms part of a clade of cone shells with a diet composed primarily of fish. A slow-moving gastropod is obviously ill-suited to taking down a fast-moving fish by brute strength alone so cone shells make use of a quite different tactic: lethal poisons. The venom of a cone shell can be exceedingly powerful, enough so that multiple species have been known to cause severe injury or fatality to humans unwise enough to handle them live (cone shells may use their venom for defense as well as for attack). The teeth of the cone shell's radula have been modified into elongate, hollow needles. While most of the teeth are retained in a sac at the rear of the buccal cavity, only a single tooth is in use at any one time. When a suitable prey animal comes within reach, the snail's proboscis is stealthily extended towards it. The active tooth is then fired along the proboscis into the target, injecting a complete payload of toxins. Among Textilia, Conus bullatus is the only species whose toxic characteristics and capabilities have been studied as yet, but it is probably representative of the subgenus as a whole. As with other fish-hunting cone shells, the injected venom carries a mixture of toxic peptides that can be divided between two functional groups (Hu et al. 2011). These have been referred to as the "lightning-strike cabal" and the "motor cabal". The peptides of the lightning-strike cabal are the first to take effect, causing a rapid (almost instantaneous) tetanic immobilisation of the prey. After this, the motor cabal of peptides act to block neuromuscular transmission, preventing the prey from recovering from its freeze. And all this in a matter of milliseconds: as of 2011, at least, C. bullatus had the fastest immobilisation capacities of any fish-hunting cone shell. As beautiful as they are, cone shells are a force to be feared.

REFERENCES

Hu, H., P. K. Bandyopadhyay, B. M. Olivera & M. Yandell. 2011. Characterization of the Conus bullatus genome and its venom-duct transcriptome. BMC Genomics 12: 60.

Old, W. E., Jr. 1973. A new species of Conus from Indonesian waters. Veliger 16 (1): 58–60.

Puillandre, N., T. F. Duda, C. Meyer, B. M. Olivera & P. Bouchet. 2014. One, four or 100 genera? A new classification of the cone snails. Journal of Molluscan Studies 81: 1–23.