There are over 700 extant species within the genus Conus, also known as the cone snails. These include some of the most beautiful shells in the seas, but the snails that secrete these shells have also developed an amazing and sometimes deadly hunting strategy. The radula in most snails is a scraping tongue, used to harvest algae or other organic matter. In cone snails however, the radula is folded and rolled into a harpoon, which is connected to a venom gland. The venoms produced by cone snails (conotoxins) can be among the most potent venoms produced in the animal kingdom. It has been estimated that a single sting from the Australian Geography Cone, Conus geographus, can deliver sufficient toxin to kill 70 adult humans.
So why would a lowly snail need such potent toxins? Well, in most cases they don’t, but it depends on their diet. Cone snails can be broadly classified by their prey preferences. There are cones that eat fish (piscivorous cones), worms (vermivorous cones), and there are cone snails that eat other molluscs (molluscivorous cones). Among these, the vermivorous and molluscivorous cones are not particularly harmful to humans (with a few exceptions to be discussed below). Their toxins are highly specialized only to the worm or snail anatomy, and do not necessitate the incredibly fast incapacitation required for subduing vertebrates.
The piscivorous cones are a different lot. Imagine a slow-moving snail trying to capture a fish. Without being hit by some substantial weaponry, a fish would quickly dart away after being stung. Even if it only made it a few yards before succumbing to toxin, this is still an incredible distance for a snail to track its dinner. To counter this, these cones have developed an amazing array of peptides, delivered in a toxic cocktail designed to rapidly paralyze and kill the fish. A single snail may have as many as 100 different toxins delivered in a single sting, each targeted to a specific part of the vertebrate physiology. Some toxins paralyze peripheral muscles, while others shut down the central nervous system or stop the transmission of pain signals. The end result is a sting that will have a nearly immediate paralytic effect on vertebrates.
Conus dalli, is an interesting case. It is a molluscivorous cone, yet it may in fact be one of the most deadly members of the genus Conus. It is in the same group (the tented cones, so named for the attractive triangular markings on their shells) as Conus textile. Conus textile has been implicated in a number of human deaths. A near look-alike for this deadly cone from the Pacific, Conus dalli is only found in the Panamic region, from Panama through the Sea of Cortez. Dall’s cones been found to share a large number of toxin motifs in common with Conus textile, but also possess 3 conotoxins that had not been previously identified.
What then does the presence of these potentially deadly animals have on scuba diving in the Sea of Cortez? If you aren’t the type to pick up everything you see, there is no threat from these at all. They certainly aren’t about to stalk a scuba diver in order to get the feast of a lifetime. No, all of the reported stings have been to humans handling these snails, most often putting the attractive shells in their pockets. Eventually, the snails decide that they have had enough, and well, that’s that.
Mainly nocturnal, Dall’s cones will emerge from under rocks to hunt and are occasionally observed by scuba divers out for a night dive. They can be differentiated from all other snails in the Sea of Cortez by their glossy cone-shaped shell covered with black or brown “tents” on a white or tan background, and by the bright red tip on their siphon. They are found more often at offshore islands, and in the San Carlos and Guaymas areas, nearly all encounters by scuba divers occur at Isla San Pedro Nolasco. So if you are one of the lucky few to see these deadly beauties on the prowl, enjoy knowing that you may observe these remarkable creatures without fear, so long as you respect their wishes to be left alone.