San Carlos Scuba Diving

News and Views

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.

San Carlos saw a very quiet spring break this year with families far outnumbering college students. The weather was beautiful, often in the 80’s, but it seems our winter water is clinging on for a last hurrah. Temperatures this last weekend ranged from 62-65 F. There is 70 degree water attempting to push its way north right now, and is currently about 20 miles south of San Carlos. Now it’s a waiting game, requiring sustained northerly winds to pull the warm equatorial waters into the Gulf and change the water circulation to a summer downwelling pattern.
While a tad chilly, the diving has remained excellent with visibility often exceeding 50 feet. The arrival of large jellyfish in the last few weeks has stirred some apprehension among divers, but have no fear, these are not a variety that tends to sting humans. This particular species combines plankton harvesting with photosynthesis to acquire energy, and this is an ideal time to get in the water and witness the transient bloom of these graceful cnidarians.
When warmer water arrives, they will disappear till next winter, being replaced by Physalia pacifica, the not-so-friendly Pacific Man O’ War. This is a small surface-restricted species that packs a wallop of a sting. Easily identified by its small deep blue bell that extends above the water, they are only a hazard in the top 1-1.5 feet of the water column. Scuba divers can avoid them by simply submerging, but snorkelers should be constantly vigilant if it is jelly season.
Most people crave the arrival of the warmer waters, but for those wishing to see how “the other half “lives, the next few weeks should be exciting. As the Sea of Cortez transitions to summer, the influx of deep-water nutrients slows, and photosynthesis consumes what is left. This creates a massive base for the food chain and during the next month or so, the Gulf of California will explode with organisms intent on either breeding with one another, or eating each other. Either way, it will be quite a show.

Located roughly midway down the Sea of Cortez on the western coast of Sonora, Mexico, the town of San Carlos is a top weekend destination for scuba divers from the Western U.S. A rich and varied ecosystem, the area is subject to tremendous swings in temperature, nutrient levels, and fauna between the winter and summer months.

Divers accustomed to the bright colors and crystal clear waters of the Caribbean may at first be put off by the more subdued bottom topography and variable visibility, but will soon appreciate the diversity of this region, and the sheer density of life that inhabit these waters.

When planning for a trip during the fall or spring, it is advisable to get recent information on water temperatures, as they may change substantially over a matter of a week or two. Changes in water temperatures typically lag behind changes in air temperatures by several months.

Spring, for instance will generally have settled over San Carlos quite solidly by March, with air temperatures reaching 85-95 degrees Farenheit/29.5-35C, but water temperatures will often be 62-68F/16.7-20C. During March and April, as the days grow longer and the water temperature creeps up, visibility can be highly variable, from 40-50 feet some days, to a near “pea soup” the next, as phytoplankton blooms move swell and ebb. April diving is substantially warmer than March, with waters ranging from 72-77F/22-25C, but a good 5-7mil wetsuit is still a must for most people at these temperatures

The end of May is the traditional start to the summer dive season in San Carlos.
Summer diving is outstanding. The warmer water and increased sunlight have consumed much of the dissolved nutrients, leaving crystal clear waters. A substantial base of the food chain is converted during the spring into algal biomass to feed other consumers. By June, temperatures range from 82-89F/27-32C, and will remain warm through September.

The months of October and November are transition months in the Sea of Cortez, Typically they are still relatively mild but as the circulation patterns along the Midriff Islands (near Bahia Kino) change, the arrival of sudden upwelling can bring with it a 10 degree drop in water temperatures and an associated drop in visibility over a 1-2 week period.

The arrival of these winter waters brings with them an explosion in marine life. Many of the larger billfish and tuna will depart, but the plankton draws myriad smaller creatures from the deep such as the brilliant Dall’s goby, nudibranchs, and many other species. For those equipped with a heavy wetsuit or drysuit, winter diving in San Carlos can allow scuba divers to see an entirely new mix of species that are absent from the summer dives.


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Welcome to the San Carlos Scuba Diving, News and Views section of In this section we will be featuring periodic updates about the current water temperatures, visibility, and best of all, the animal sightings. We hope that our readers will find this information helpful and entertaining. We would love to hear from you if you have any comments, announcements, or dive reports you would like to share. You may post your comments in response to any of the earlier posts, or you may email us at and our editor will make sure to share your announcements and reports, so long as they are within the scope of this board. Please email us if you have any suggestions about what you would like to see on, or if there are improvements that you would like to see.
Cheers, SeaCortez (editor)

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