Dive Dry with Dr. Bill

#230: Munching NOT Mating- Shark Attack!

A friend of mine recently said she thought I was focusing too much on mating in my columns. I responded that it was spring, and during that season a young man's (even a mature man's) fancy turns towards things other than a juicy steak or a tasty blueberry pie. I think it is genetically programmed in us red-blooded American males, and I fall into that category! However, in fairness, I'm going to focus on that other "M" word... munching... in the next two columns. Funny, you'd think there'd be more concern about this "M" word since it entails violence and death, while the other "M" word involves pleasure and love (among the invertebrates?). Our society has some strange values IMHO (in my humble opinion).

A while back, we visited Torqua Springs as our third dive site on SCUBA Luv's King Neptune dive boat. Now, I've never been a big fan of this dive site because it has a lot of silt on the reef which often gets stirred up by ocean currents, or munching bat rays, thus limiting my visibility and ability to film. This dive was no exception. Captain Tony asked me to check the depth where we were anchored, and although it was only 15-20 feet deep, I could barely see the hull of the ship! Such conditions usually bode poorly for my underwater videography, and my ability to make exciting finds to report to my readers or viewers.

Conditions were so bad that I decided I'd abort the dive after just 10-15 minutes, and started heading back towards the boat. However, I had to stay under at least 20 minutes to log this as an official dive. Shortly after I turned around and moved into shallower waters, I saw something of interest in "the distance" (well, at least at the limits of my visibility). It was a large sheep crab. Starting about February each year, these fairly large members of the spider crab family enter shallower waters reportedly to mate (oops, I used that "M" word again). I do encounter them engaged in that delightful activity on occasion, but I'm not going there in this column!

I moved closer to the sheep crab and realized it was feeding on something, and that something was of fairly good size... perhaps 3-4 feet long. What I saw in its grasp was the "skeletal" remains of a large fish with plenty of meat left on the "bone." The crab was munching away, tearing pieces of flesh off the backbone in the region of the pelvic fins (and you prefer that to the other M?). All that remained above that region was the spinal cord with little meat left on it. My camera started rolling to capture this violent and horrific feeding behavior since Munching is a favorite subject for my underwater videos (second only to that other M). Despite the poor conditions, when I edited the video that night I was surprised that it was pretty clear.

I took a few breaks while filming to investigate the meal itself. I quickly realized that the back "bone" of this fish was made of cartilage rather than calcium carbonate. This tasty meal was a shark! When it comes to encounters between sharks and invertebrates (as well as other prey), the munching is usually done by the shark, but in this case it was the munchee rather than muncher. From what was left of the tail section and pelvic region, I could quickly see it was a male shark since the claspers were untouched. Although much of the shark was in someone's stomach, I was 99% positive it was a swell shark. I think the sheep crab thought it was "swell" too. So this shark attack is an attack on the shark, not by the shark. Of course people tend to follow the relatively few (only five fatal last year around the globe I believe) attacks by some sharks on them, rather than the millions or tens of millions we humans kill... often cutting off their fins and letting them fall helpless to the ocean bottom to bleed to death.

Now sheep crabs can be active predators, but it is highly unlikely one could take down even a docile shark of this species. I've seen them feed on defenseless dead bat rays, gorgonian soft corals and sea cucumbers. I've watched some pretty amazing video of the large octopuses in the Pacific northwest taking down sharks even larger than this one, and the sheep crab does have ten legs instead of just eight arms like the octopus. However, I doubted it captured this swell shark even if the shark had been taking its daytime nap in the rocks. Another clue that the crab was not the real cause of this shark's demise was the fact that so much of the fish's "meat" was missing from the forward part of the body... far too much for the crab to have consumed and remained the size that it was. I've set records at all-you-can-eat restaurants (13 pounds of Prime rib at Marmacs, beating out one of the 1970's Lakers who held the previous record... right Chuck?) , and I doubt I could have eaten that much. Based on that, it also seemed unlikely that the crab encountered this shark after it died.

Now what might have killed the swell shark, and left enough of a meal for the sheep crab to finish off, or at least try? I can only speculate, but I'm good at that (except when it comes to investments, other than my house). It has been well documented that California sea lions will take leopard sharks and eat them like corn-on-the-cob (without butter). They have been observed doing this several times in the waters of Little Fisherman Cove where the USC Wrigley Marine Science Center is located. Now swell sharks tend to be smaller in size than fully grown leopard sharks, so this species could easily be taken by a hungry sea lion.

But why did the sea lion leave enough of a meal for something else when it could easily eat the entire fish? Now I'm really going to go out on a limb. Perhaps a great white shark came onto the scene and either munched the sea lion, or at least chased him away. Remember, portions of the original "Jaws" were filmed near Torqua Springs (although using the mechanical shark if I remember correctly). Now that would be a food chain: great white shark feeds on sea lion that feeds on swell shark, leaving enough for the sheep crab to gorge on. Sounds like payback time to me. But then, that's just an educated guess... right? Just call me a "Deep Sea Detective."

© 2007 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. Please help me climb out of self-imposed poverty... buy my "Munching and Mating in the Macrocystis," "Great White Sharks of Guadalupe," "Calimari Concupiscence: Mating Squid, " "Playful Pinnipeds: California Sea Lions," "Belize It or Not: Western Caribbean Invertebrates, Fish and Turtles," "Gentle Giants: Giant Sea Bass" or "Common Fish and Invertebrates of the Sea of Cortez" DVD's. Yes, take Dr. Bill home with you... we'll both be glad you did!

Sheep crab munching away on the swell shark; curious Gary Garibaldi and remnants of the shark munchee.

This document maintained by Dr. Bill Bushing.
Material and images © 2006 Star Thrower Educational Multimedia