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Dive Dry with Dr. Bill

#490: In the Jungle, The Mighty Jungle

It seems most people have an inordinate fear of predators, even if they live in an urban environment where the only "predators" are loan sharks and corporate bankers. I see viewers grimace when they watch a cheetah take down a gazelle and start eating it alive. A pride of lions can instill irrational fear in someone living in a high rise jungle. And when people think about what lurks beneath the water's surface, it is sharks, sharks, sharks. Sure, under the "right" (or "wrong" if you prefer) conditions, the lion or shark could bite down on one of us. But most of us are never in serious threat when they are doing their "munching."

Now when I think of predators, my mind turns to species like the Navanax. It is unfair to refer to any predator as vicious when all they are doing is obtaining their evening meal. Imagine how beef cattle would envision us if they could perceive you grilling their ground up flesh (or juicy steak) on the backyard barbecue. However, Navanax does come close in my book. These beautiful snails sneak up on their relatives such as the colorful nudibranchs and consume them like vacuum cleaners!

The most common species of these snails in our waters is Navanax inermis, although it occurs in at least two different color forms or "morphs." I've been encountering them both recently in the dive park. They prefer shallow water, being most abundant above 25 ft, but have been observed down to 100 ft. I've been doing a lot of shallow dives lately to extend my bottom time and therefore the time to acquire video footage. They prefer calmer shorelines like Catalina's leeward coast and are often abundant in sheltered regions with sandy or muddy bottoms such as Avalon Bay and the Hen Rock area in the lee of Long Point. Their geographic range extends from Monterey Bay to the Sea of Cortez.

One could describe them as looking like psychedelic cigars due to the bright colors often seen on them, and the shape of the body. Most are 4-5" long, but giants can reach up to eight inches. Not exactly your idea of a vicious predator, eh? Try seeing them from a nudibranch's eyespot. Their base coloration is generally a dark brown overlain with stripes and other markings of yellow, orange and blue. The mouth has a series of palps extending from it.

These carnivores love to feed on their nudibranch kin, but also on shelled snails. To locate their prey, they simply follow the "slime trail" left by a mollusc moving across the bottom. I've watched them chase after purple olive snails on sandy bottoms and they certainly catch up quickly. They may even be cannibalistic, feeding on their little "brothers" and "sisters." Now that's a truly cruel form of sibling rivalry. When they munch on shelled species, the hard parts often pass through the digestive tract intact and are expelled. I think they could even ingest and digest some of my cooking! I wish I could. Burp.

As my regular readers and other perceptive people already know, mating in many species of snails gets rather interesting. Those like Navanax and its nudibranch prey are usually simultaneous hermaphrodites. This means they function both as girls... and as boys (or women and men since they are sexually mature). Talk about gender identity issues. It still takes two to tango (or is it tangle?). They line up on the proper side (the right) and double their pleasure, double their fun by each mating with the other. The great thing is that any Navanax they encounter is an appropriate mate. Imagine how easy that would make Saturday nights at the Chi Chi or Marlin Club! Recent studies have shown that isolated individuals may experience reduction in the number of sperm, and when they finally encounter another Navanax they may favor a more lady-like role as a sperm receiver rather than donor. They lay interesting looking egg masses that look like a lot of yellowish threads joined together into a basket-like shape. Parental care is non-existent... remember, I said they even eat their young.

I mentioned that Navanax inermis has two color morphs. One has much smaller spots often arranged in linear fashion while the other has bolder, often more colorful ones. To add to your confusion, there is a second species known as the white-spotted Navanax, Navanax polyalphos. I see it much less frequently in our waters. The body color is black and it is covered with small white or yellow spots. This species is found from Santa Cruza Island to the north, down to the Sea of Cortez and on to Costa Rica.

© 2012 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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Two color phases of Navanax inermis; two Navanax doing "the wild thing" and the egg mass from such a coupling.

This document maintained by Dr. Bill Bushing.
Material and images © 2011 Star Thrower Educational Multimedia