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Dive Dry with Dr. Bill

#558: A Thorny Subject

As I write this week's column, our local waters are still warm and cozy and let me do my long night dives in comfort. Of course by the time you actually read it, things may have changed and I may be off in the tropics somewhere! But I'm getting ahead of myself... like I usually do. I'm not even sure where I'll go this winter... perhaps the Egyptian Red Sea, Zanzibar or South America. Only one trip is pretty well set... Jamaica next May for my niece's wedding. Right now I'd better get back to my local night diving!

I descended on this particular night planning to film more of those light sensitive sea cucumbers or mating southern kelp crabs or possibly even the great white seen around the harbor a while ago. What I found was quite different from what I expected... a thornback (Platyrhinoidis triseriata), the first time I've ever seen one in the dive park. The only other one I filmed was at 150 ft off Sea Fan Grotto back in 2007 and it caused me to go into deep deco. Ruth Harris says she's seen a few here so maybe I just need to make an appointment with my optometrist.

Two years ago I was spending the night up at Howland's Landing with Jean-Michel Cousteau, Dr. Dick Murphy and other old friends from the early Cousteau programs here on Catalina in the 1970s. We were standing out on the pier in the dark looking into the shallows. I was totally shocked to see perhaps a dozen thornbacks feeding along with a number of bat rays, round stingrays and shovelnose guitarfish. Given that, I wondered why I don't see them more frequently on my night dives.

So what is a thornback, Dr. Bill? Well, I've always called them a ray but apparently I've been wrong. Others have referred to them as a type of guitarfish. Apparently they are wrong too. Thornbacks are relatives of the sharks and rays, but they are in their own tiny little family consisting of three species. They are more closely related to the rays than the guitarfish. Their genus name Platyrhinoidis means broad snout in Greek and the Latin species name triseriata comes from the three rows of spines (the "thorns") on their back.

Thornbacks may reach a length of about three feet. They are flat like round stingrays, but a fairly uniform grayish or brown on the dorsal surface rather than mottled like the stingray, and white underneath. The best diagnostic feature is the three rows of spines on the back and tail. Dr. Milton Love states some anglers think these are poisonous, but they are not. Thornbacks do not sting, unlike the round stingrays I see in the dive park at night.

This species' maximum range extends from Tomales Bay north of San Francisco south to the Sea of Cortez and possibly down to Ecuador. Their reported depths range from the surf zone (where I saw them with Jean-Michel) to about 450 ft where I doubt I'll ever see them! Some reports suggest they may migrate seasonally, perhaps into the deeper water. They frequent soft bottoms and are often buried in sand or mud during the day (does that make them "sedimental?"), but may also be seen swimming about as I did back in 2007 at a depth of about 150 ft.

I was surprised to see a second thornback in the dive park about a week later. It is possible that these thornbacks might be in the dive park because market squid are running in our area. In addition thornbacks may feed on crustaceans such as mysid shrimp, crabs or amphipods; worms, sea pens and fish including sardines, anchovies, gobies and surfperch. They detect at least some of their prey by sensing electrical currents using the ampullae of Lorenzini just like their shark relatives. In turn they are eaten by elephant seals, which we don't have many of in our waters, and sharks. Native American tribes also used them as a food source. Too bad the Pimugnans didn't leave us a Native American Cookbook. My occasional dive buddy Paula Marrama caught one off the Venice pier about a week ago. Fortunately she released it.

The girls are sexually mature at about 19 inches while the precocious boys reach that state at 15 inches. The young develop inside the mother, surviving on the yolk of the egg since there is no placenta. Live young of about three inches are delivered generally during summer. Anywhere from one to fifteen embryos may develop each year. OctoMom has nothing on this species!

© 2013 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 29, 10:00 AM weekdays and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. You can also watch these episodes in iPod format on YouTube through my channel there (drbillbushing). 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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Four views of the thornback swimming at night in our dive park.

This document maintained by Dr. Bill Bushing.
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