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Dive Dry with Dr. Bill

#411: What Did ROV See?

No, that's not a typo... I'm not talking about Karl Rove or his predictions for election day. Shudder. A few weeks ago I wrote about the ROV (remotely operated vehicle) used by Ocean Gate and Scott Cassell in Expedition Catalina before its abrupt and unfortunate end. That article only talked about what we saw on a practice run in Avalon Bay. What the good doctor... and his even "gooder" readers want to know is, what did it see in The Deep around our island. After spending a week processing video shot by the ROV and the submersible Antipodes, I can finally give you a peek at some of its discoveries in our waters.

The first selection of ROV images I'll submit for your viewing pleasure are from depths of about 450 to 550 feet. This is within the range for some advanced technical divers using rebreathers and mixed gasses. Some seem to believe I'm a rather "deep" guy, but this is well below my maximum descent of 200 ft. I've imposed this limit because I only dive the gas mixture that the Creator gave us... air. Since most popular field guides to the critters of the West Coast are based upon images compiled by SCUBA divers, these books rarely include the denizens of the deep unless they can also be found within recreational diving depths. When critters from such depths are sampled, it is usually using equipment that tends to damage them, so it is great to get images of them in their intact, living state.

The image in the upper left is of a sea pen, probably the white sea pen Stylatula elongata. If correct, this would be an apparent extension of this species' depth range since it is listed to a maximum of 200 ft. by Dr. Dan Gotshall in his Guide to Marine Invertebrates: Alaska to Baja California. Not sure how deep Dan has dived, but he's a well known marine biologist. I have observed them in waters from 50 to about 200 ft at a number of Catalina dive sites. These cnidarians (formerly called coelenterates) are colonial relatives of the corals, anemones and jellyfish (or sea jellies if you are of the PC crowd... and I'm not talking about those like me who don't use Macs). Each individual polyp has eight feather-like tentacles that capture food such as plankton drifting down from above or in the currents along the bottom.

In the upper right is another cnidarian that appears to be a soft coral or gorgonian, or possibly a sea pen known as the orange sea pen (Ptilosarcus gurneyi). Soft corals of several species are found at normal diving depths off Catalina, but most are limited to the upper 100 ft of the water column. Like their sea pen relatives, these cnidarians are colonial and use the tentacles on their feeding polyps to capture plankton and other food from the surrounding water. The orange gorgonian is known from depths down to 1,000 feet, but I can't reliably assign this specimen to that species. If it is that sea pen, a species I've never seen, it is not typical of the ones in most field guides. The polyp bearing lateral extensions are more sparse and thicker than in most photos of this species. However, the bulbous base or peduncle that anchors it to the bottom is typical of this critter.

In the lower left you see an assemblage of sea stars (known to the non PC crowd as starfish). Looking at other images, it appears many of them are of one of two closely related species, the California sand star Astropecten armatus, or the spiny sand star, Astropecten verilli. The latter is known from San Pedro to Ecuador and the former from Point Reyes to Baja, so both species are found in our waters. The depth range of A. armatus is reported as down to 180 feet so if they are of this species, this would be an extension of their depth range as well. Some scientists report that A. verilli may be found as deep as 1,600 ft, so it could be this species. These sand stars forage on top of the substrate or by burrowing under it to look for snails since they favor escargot (without the fattening butter). To further complicate matters, there is a third sea star known by the common name "sand star" with the scientific name Luidia foliolata. A few of the individuals in the ROV images looked as if they might belong to this species. It is known to depths past 2,000 feet. Prey for this sand star includes clams, sea urchins, sea cucumbers, brittle stars, worms and small crustaceans.

At the right in the last image are two spherical objects that undoubtedly are white sea urchins, Lytechinus anamesus. Although I see them beginning in depths of about 70 ft, the ones I see above 200 ft are generally small in size. They may reach up to 3" in diameter which appears to be the size of the ones seen by the ROV. This species is known down to 1,000 ft. They are generally believed to feed on algae, which makes them dependent on drift kelp and seaweed that falls to the bottom. However, I have also read accounts that state they will eat critters, including other species of sea urchins, and have seen them scavenging on dead animals in the 100 to 200 ft. range.

Critters living in the cold waters at such depths are often found in shallower water as one moves north along the West Coast of North America. For example, if the second image is of the orange sea pen, that species may be found in waters as shallow as 25 ft off British Columbia and Alaska. However, I'm not going to dive there to film them until I patch the many leaks in my drysuit! This trend of moving to deeper, colder waters in warmer southern seas is known as a submergence. We have a good example in our waters of the reverse. The beautiful scythe butterflyfish is found in shallow water here, generally in the 30 to 60 ft range, where the temperature is cool. In the ocean off Mexico, it is more common in depths of 500 ft to avoid the warmer surface waters there. Now I have no idea why it would want to do that!

Down at these depths there is no sunlight for algae to photosynthesize, and thereby create food for other critters. The food webs for these deep benthic or bottom dwelling critters must fall down like manna from heaven (or the near-surface waters in this case). This is the reason why suspension feeders such as the sea pen and gorgonian can live here. This food source also provides scavengers like the white urchins with "munchies," and they will amass on kelp detritus that falls to the bottom. The sand star and other starfish are most likely active carnivores feeding on critters buried under the substrate, as well as scavenging on debris. In the darkness of these deep waters, the food webs are quite different from those up in the shallows where giant kelp and other seaweeds provide sustenance for many of the critters we see in the kelp forests.

© 2010 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 49, 10:00 AM and 5:00 PM weekdays and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. Please help me climb out of self-imposed poverty... buy my DVD's (see this link). Yes, take Dr. Bill home with you... we'll both be glad you did!

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White sea pen and a soft coral... or the orange sea pen; sea stars and white sea urchins seen by the ROV.

This document maintained by Dr. Bill Bushing.
Material and images © 2010 Star Thrower Educational Multimedia