San Carlos Scuba Diving

It is unlikely that a scuba diver in the Sea of Cortez will encounter a more alien and ornate decapod than the Cortez Barrel Shrimp, Gnathophyllum panamense. Also known as the Spotted Bumblebee Shrimp, this organism is difficult to confuse with any other Gulf of California shrimp species.

A downright beefy and glossy carapace is punctuated with large red or orange spots and smaller white polka dots. Accentuated by purple walking legs and white or yellow chelipeds and tail, the Cortez Barrel Shrimp appears to be an amalgam of highly disjointed parts.

Despite a wide range throughout the Sea of Cortez, from the tide pools of Puerto Penasco to the dive sites of Cabo San Lucas, scuba divers rarely sight the Cortez Barrel Shrimp. The ornate coloration of this shrimp is an excellent example of disruptive coloration and the bright splotches of color actually make the shrimp difficult for predators to visualize.

During the day, the Cortez Barrel Shrimp is reclusive and hangs on the undersides of rocky caves and caverns, or more commonly, underneath small rocks in rubble fields. Scuba divers most often catch glimpses of this beautiful creature during night dives when it emerges to feed. Little is known about the natural history of this shrimp. It has been reported to be a generalized omnivore, although it has also been reported to clean parasites off of fishes, as well as to consume the tube feet of echinoderms. While it is possible that this shrimp is more generalized than other members of the family Gnathophyllidae, it seems unlikely that this shrimp would be substantially different in diet than its kin, which are all starfish specialists.

Fall diving in San Carlos has been nothing short of spectacular this year. One of the most interesting of the sightings this year has been of one of the most secretive denizens – the Pacific seahorse, Hippocampus ingens. This is the only seahorse found in the western Pacific Ocean and Gulf of California. It is also one of the world’s largest seahorses reaching a length of 30cm (1 foot!)
Pairs of these beauties have been found at several dive sites throughout the summer. One particular pair of seahorses has inhabited the same spot at San Antonio Point all summer long and scuba divers have witnessed dramatic courtship displays and even frequent pregnancies of the male.
Although the Pacific seahorse is primarily a nocturnal species, scuba divers who patiently observe from a distance can also see and even hear the seahorses “snicking” at the passing clouds of mysid or possum shrimp that form the basis of their diet.

A pair of Hippocampus ingens at San Antonio Point, San Carlos, Sonora, Mexic0

Even when you know right where they are, seahorses can be hard to spot while scuba diving. Seahorses are masters of camouflage.

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.