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Dive Dry with Dr. Bill

#630: A Thorny Topic

Today's column was actually written last summer but not published then. After seeing its subject during the Avalon Harbor Underwater Clean-Up, I thought it appropriate to do so now. My regular readers know that during the warmer months Dr. Bill eschews (gesundheit) diving during daylight and sneaks down to the dive park during the dark of night to dive and film. It is just too crowded when the sun is up, in large part "thanks" to the poorly supervised hoards of snorkelers that one local operator dumps on us. Many mainland dive instructors have complained to me about the interference they pose for divers, especially those in training classes. Of course there is a serious downside to diving at night... I almost never see a single bikini!

After having the good fortune to film an as-of-yet unnamed "stinger" (the calycophoran siphonophore featured in a recent column), I returned to the dark two nights later to try to get some more footage. Wouldn't you know it... I didn't see a single one of them! However any dive during a hot spell is well worth it to cool down my core temperature and make sleep more comfortable. The kelp bass were out hunting like mongrel hoards and often prevented me from getting any video of subjects I saw. Several times I found morays out in the open hunting, but the goldarned kelp bass would swarm them ruining my video. I've mentioned before that I frequently see morays shadowed by kelp bass which probably are hoping the moray will flush out a few tasty blacksmith they can capture on the fly. I think they are also jealous of the way the "eel" can writhe through small cracks and crevices in the reef.

On this dive I saw several round stingrays (Urobatis halleri) cruising the reef looking for munchies. These rays reach maximum lengths of nearly two feet with disc diameter of about one foot. Most of the ones I see are brown with spots all over their upper or dorsal surface. Although the ones I see are pretty docile and I've never had one sting me, they are the prime culprit when mainland beachgoers get zapped while wading in the surf off sandy beaches. You should always use the "stingray shuffle" when doing so to scare them away before they try to defend themselves against your big feet. An interesting tidbit from Dr. Milton Love is that each summer these rays grow a new stinger just behind last year's model. The older one is usually shed by fall, but occasionally individuals are observed with two to as many as four spines! Ouch, ouch, ouch, ouch.

As I was approaching the end of my 83 minute dive, I thought I saw another round stingray heading my way. At least it looked like one head-on from a distance. Then it abruptly turned and I realized it was a thornback (Platyrhinoidis triseriata), a spineless distant relative of the round stingray. Thornbacks belong to a small family consisting of just three species, and they are the only member seen in our waters. These cartilaginous fish are known from Tomales Bay down into the Sea of Cortez, with reports of them in Ecuadorian waters.

It is pretty easy to distinguish them from the round stingray when you get a view of the full body. Their heads and discs are round but somewhat pointed compared to the stingray. Coloration is usually gray and solid rather than brown and spotted. The tail is thick with two prominent fins and no stinger whereas the round stingray's is long and slender (and painful if you get struck!). Thornbacks have three clearly defined rows of "thorns" on their dorsal surface.

In some areas of the mainland coast thornbacks seem more prevalent in summer while in other areas they are seen more frequently in winter. They tend to be more active at night so I see them infrequently on my summer and fall night dives. During Cousteau Family Camp up at Howland's Landing, Jean-Michel, Dick Murphy and I stood on the pier and watched many of them feeding in the shallow water along with bat rays and shovelnose guitarfish. Thornbacks feed on small crustaceans like mysid shrimp and amphipods as well as fish, worms and squid.

No "Dive Dry with Dr. Bill" column would be complete without a full expose of a species' sex life. Girl thornbacks are sexually mature at about 19 inches while the boys mature at smaller size, about 15 inches. I will refrain from describing the actual reproductive act itself out of respect for my younger readers and any sexually repressed older ones. Suffice it to say that fertilization is internal (you can let your imagination run wild with that). The embryos develop within mama using the yolk sac for nourishment. Other than that the female provides no additional nutrition. As many as 15 young are born live during the summer months. I'd love to film that event!

According to Dr. Love, the Native Americans along our coast would commonly eat thornbacks. Of course way back then there were no McDonalds in the neighborhood, so Big Macs were out. Heck, the second one in the country appeared in the town next to us and that was way back in the 1950s and in high school a burger, fries and shake were a mere $0.47 (or 23 minutes of work at the minimum wage of $1.25 back then). Today there is no commercial fishery for thornbacks although they are caught with some frequency by anglers from shore or piers. If you catch one don't worry, the "thorns" on thornbacks are not poisonous... just be sure you can tell the difference between them and the round stingray!

© 2015 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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Various angles of the thornback ray and a round stingray for comparison

This document maintained by Dr. Bill Bushing.
Material and images © 2014 Star Thrower Educational Multimedia