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Dive Dry with Dr. Bill

#590: Driving Me Batty!

Late spring and early summer often signals the beginning of an underwater "season" that I absolutely detest! I'm referring to what I call "bat ray season." Around this time of year these abhorrent creatures make their appearance in the dive park and adjacent waters to feed on disgusting morsels such as worms. They do so by stirring up the sediment, hammering it with their heads to dig holes to unearth tube worms such as Chaetopterus variopedatus to slurp up. Sometimes the visibility drops so low it looks like a hundred open water students are out there doing their training dives.

A few years ago I was diving in Descanso Bay as part of the monitoring program required for the Sea Trek installation there. It was my responsibility to take water samples at several points and then dive to clean the light meters that monitored the clarity of the water. On several of my monitoring days the visibility was near zero thanks to these winged cousins of the sharks. I couldn't even find one of the meters because of it. I kept doing an underwater search pattern about one foot off the bottom looking for it, and several times had bat rays scoot out from under me at the last moment. I'm just glad I wasn't Steve Irwinized by the experience.

Now many divers enjoy encountering these chondrichthians (cartilaginous fish including sharks and rays) on their dives. I, too, have experienced some good dives with them. Several times I've filmed them as individuals bashing the bottom with their "brain cavities" and munching on the worms, clams and other goodies they dug up. When there are just a few of them in the area, you can actually see and film their behavior. I've even filmed them mating on a few occasions. One time at Isthmus Reef I was "hanging" on the wall there when I sensed something behind me, and turned to see a pair of them "flying united" (and I don't mean the airline). That produced some pretty spectacular footage.

The bat ray (Myliobatis californica) is a member of the eagle ray family. The genus name means "grinder" in Greek and that describes how these fish "chew" their food. They are reported to reach lengths of up to six feet and the dainty ladies may weigh as much as 240 pounds while the boys are much smaller. Their blunt, high heads are quite distinctive compared to most other rays in our region. Coloration may be tan to light brown to dark brown on the upper surface and much lighter below. And, yes, these rays do have stingers near the base of their tail, but I've never had a problem with one of them.

Bat rays are found from Oregon down into the Sea of Cortez but are most common from northern California to the Pacific coast off central Baja. Although they have been reported from depths in excess of 300 feet, most are found in relatively shallow water. Although they primarily feed over soft bottoms (who wants to bang their head against a rock all day?), they frequent a wide variety of different habitats. I often see them swimming mid-water through the kelp forests.

Dr. Milton Love states that in many areas these fish make seasonal movements between inshore and offshore waters. You can even see large swarms of them, numbering in the hundreds, swimming through the water. The seasonal migrations usually bring them inshore when water temperatures warm up there and take them offshore in the fall before winter storms hit. They may also move between shallow and deeper water at night.

As mentioned above, bat rays are like junior high school humans... the girls are bigger than the boys. Unfortunately the boys of this species never catch up. In fact the oldest male bat ray was a mere six years while females may live to be over 23 years! Males may become sexually mature when they are a mere 18" long with all of them reaching this stage at about 25" and 2-3 years of age. In contrast, females may begin to mature at two years but are not all mature until about nine years. Of course that is a big difference between them and humans!

A single female may have as many as a dozen youngsters, usually in summer and fall. I have never observed a birth event, but one diver did see a young one emerge right in the dive park several years ago. They mate soon after they give birth, with the "gestation" period lasting 9-12 months per Dr. Love. The developing embryos obtain nourishment from substances created by the female's uterus. Since they are not mammals they don't nurse. You did know that already, right?

Although I most often see them feeding over sand or mud bottoms, marine biologist and diving pioneer Conrad Limbaugh reported seeing them take abalone while feeding in the rocks. Although I focused on the Chaetopterus worms as menu items earlier, because their empty tubes are often scattered around the burrows bat rays dig when feeding, they do relish other tasty critters. These include fish, squid, molluscs such as clams (and their siphons) and scallops, crustaceans including crabs and shrimp, and even echinoderms such as sea cucumbers and brittle stars. Their feeding pits may be as wide as 3-6 feet and the soft bottom may look like the cratered surface of the moon after intensive feeding... at least until the currents erase them.

Fossils related to our current species are known from at least 37 million years ago in California. I guess that means they probably aren't going away any time soon, and I'll just have to get used to them during late spring and summer ... or dive mostly at night when they head into deeper water. But today is my birthday and after a winter and spring of hard work (or was it hardly working) I deserved a birthday treat. As the paper comes out, I'm sitting on a plane heading for the Dutch Antilles where I'll spend some time diving Aruba and Bonaire, and bring back new stories to tell about that adventure.

© 2014 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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Bay ray beginning to feed on bottom and stirring up a real dust cloud with only the tail showing;
flying through the air (er, water) with the greatest of ease.

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