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Dive Dry with Dr. Bill

#586: A Monster Muncher

I've been far too "high and dry" and have amassed less than 20 dives the entire year. No, I'm not suffering from some tropical disease (other than wanting to head there again this coming winter). I had my first physical in decades and my doctor said I was healthy as a seahorse. Not sure what he meant by that. I find my lack of diving to be inexplicable. Perhaps I need to visit my shrink so he can delve into my subconscious. Wonder what strange things he'd find there! The water has been unseasonably warm this winter and spring. Heck, even in my "new" water cooled wetsuit (see images) I felt toasty on two dives this past weekend. Have I finally seen it all? Hardly... I've just begun to scratch the surface in my first 50+ years on SCUBA.

My two dives this weekend followed the "identical" profile... out past the edge of the kelp to look for the giant sea bass that has been sighted there recently, then along the edge of the kelp forest to the wreck of the Suejac at the harbor end of the park, and back through what I call the Kelp Kathedral... the beautiful shallows between the inner edge of the kelp forest and the Casino groyne (breakwater). I was surprised to find the "bottom" temperature (not to be confused with my wetsuit bottom) dropped 3 to 4 degrees overnight depending on which of my two dive computers you believe. However, the water was still quite comfortable for dives of about one hour each day.

On both days I nearly ran head-on into the wreck. Yep, visibility was that bad. Visiting divers often ask how to find the Suejac and I tell them the easiest way is to head "south" at a depth of 70 feet. If you can't see the wreck, you'll at least hit your head on it! I often find interesting critters on this wreck and Saturday was no exception. Frequently the creature of interest is a beautiful nudibranch or shell-less snail as I've filmed several different species roaming the ship's hull. This time the bright coloration that immediately caught my eye was that of a rather large (7-8") relative of the nudibranchs... not to mention a fierce predator on its cousins! Yep, down at about 70 ft I saw the brightly colored sea slug known as Navanax, Navanax inermis to be "specific."

Wikipedia states that this sea slug's common name is the California aglaja, but I don't remember hearing a single diver (or even marine biologist) refer to it by that name! Most simply use the genus name Navanax even though there is more than one species of Navanax in our waters. To be fair there are some biologists who think the slug should be moved into the genus Aglaja, but most of us prefer the older name (at least for now). Sometimes it takes a while for a new name to catch on, especially when the old one has been burned into your brain for decades. This species may be differentiated from the other one occasionally seen in our waters, Navanax polyalphos, because it has brightly colored broken lines along the length of its body and that species has white spots.

As you can see from the images (check my Facebook albums), the coloration of this species is brilliant and varied. Even though I'm partially color blind, I see a rainbow-like mix of colors... blue, yellow, orange, and brown. Some individuals are more brown and yellow but this one had the more varied coloration. The one I filmed was at the upper end of the size range since individuals reach up to about 8 1/2 inches long.

Navanax inermis is a vicious monster muncher (aka "big" predator)... at least if you are a nudibranch or a bubble snail. It employs its chemical senses using organs known as chemoreceptors to locate and track its prey. I've occasionally sat on the sandy bottom in our own harbor as well as White's Landing and Goat Harbor watching these slugs following bubble snails and purple olive snails as they plow through the sand. I don't remember ever being patient enough to film them eat one (things do happen at a "snail's pace" after all), and I'm not one to set up an attack by placing an innocent nudibranch in front of an oncoming Navanax! That would be like chaining an impala to a stake so you could film a lion stalk, kill and eat it! They feed by using a structure known as the buccal bulb (a highfalutin' biologist word for a mouth-like cavity) to suck their prey in.

In researching this column I discovered that Navanax has been observed attacking and eating California sea hares (Aplysia californica) of similar or smaller size, although the larger individuals escape if attacked. The scientists from Chapman University in Orange also noted that sea hares that survive an attack exhibit more frequent withdrawal reflexes that help serve as a defense against future attacks. In other words, these sea hares have "learned" from their initial attack that an approaching Navanax is bad juju and they try to avoid them.

Although some refer to Navanax as a nudibranch, it is not. They are both members of the informal snail group known as the Opisthobranchs. That word means having the single set of gills behind (and to the right) of the heart. In contrast most of the snails landlubbers encounter and many of the marine and freshwater snails are Prosobranchs which have their gills placed in front of the heart. Opisthobranchs are characterized by two pairs of tentacles and the gill placement. Navanax is in the suborder Cephalaspidea while true nudibranchs are in the suborder Nudibranchia. Don't worry... you won't be tested on that!

Caution: parents, I'm about to get into some gory details here about mating so beware if your young children are reading this. Like other opisthobranchs (including nudibranchs), Navanax is hermaphroditic. If you've been paying attention to my columns, you know that means each individual possesses both sex organs. Makes it far easier to find a date on a Saturday night (although I have no intention of trying that myself!). The male organ is located to the right of the head and the female opening is on the right side of the rear of the animal. I'll leave the full details to your imagination, but will add that like some other opisthobranchs (including our local sea hares) they may form ménage à trois or even a full-on orgy, with the ones in the middle functioning as both male and female at the same time. On my dives this weekend I did find a mating ménage à trois of three California sea hares on my Saturday dive and they were still going at it 24 hours later! Wow... and no sildenafil citrate needed (look that up in your Funk & Wagnalls)! The Navanax spins a lovely egg web that is often found attacked to rocks or resting on the sand after mating.

© 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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My "water-conditioned" wetsuit last weekend and Navanax on the wreck of the Suejac
plus an egg basket from a successful mating attached to seaweed.

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