Dive Dry with Dr. Bill

#593: Dr. Bill and the Argonauts

A few weeks ago, my occasional dive buddy Catherine came out with instructor Casey Connell and a group from his dive shop. Although I mostly dive solo, I always enjoy the company of a lovely mermaid (unless she's a total air hog... or air head, which Cat is not). I joined her for a planned dive out to the "swim platform" (actually an old float from the pier according to Lorraine Sadler who helped Jon Hardy tow it out there and sink it). It offers structure and critters and the possibility of a giant sea bass sighting. Unfortunately some in the group didn't hear my directions to head out to 55 feet and turn left following that bottom contour so we ended up out in Descanso Bay instead.

On our way back in, I encountered the shell of an invertebrate only occasionally seen in our nearshore waters. It is a rather beautiful and very fragile shell, so I handled it gently as I brought it over to Cat to see. Most of my readers know that invertebrates generally use shells or exoskeletons for protection from predators. Therefore, if you are as brilliant as most of my readers are, you are probably wondering why this shell was so fragile. The simple answer is that it isn't to protect the critter inside from the maws and jaws of hungry carnivores. Instead it functions as a BCD, or buoyancy compensating device, for the animal inside.

Yes, I know you are curious and, like most, you would like me to offer you a name so you have a label you can deal with regarding this critter. If I told you it was an Argonaut, would that help? Probably not for many of you. Instead, you might think of "Jason and the Argonauts," a 1963 film Chuck Liddell and I screened at our 1979 Catalina Island Film Festival. While not many came to see this flick, it did inspire one young man to become an aspiring film maker according to his biographer. His name... Tim Burton and "Argonauts" was apparently one of the first films he ever saw! He was inspired by the incredible scene of pirate skeletons fighting with cutlasses.

But the argonaut I refer to was never the star of a movie I'm aware of except for Jacques Cousteau specials. The one I refer to is also known as the paper nautilus. Until I researched this column, I thought there was just a single species known scientifically as Argonauta argo. Instead, I discovered that there were at least seven or eight species recognized by researchers. My limited expertise on this group leads me to believe the one I found is most likely Argonauta argo or possibly A. nodosa although that species usually has a more elongated shell. But what is the "paper nautilus?" Actually, it is not a nautilus at all if you're thinking about the much better known chambered nautilus (Nautilus pompilius, although there are five other species). It is a pelagic or open water octopus!

Argonauta argo is known from most tropical and subtropical waters around the globe. They hang near the surface and munch on crustaceans, molluscs, jellyfish and salps. They feed mostly during the day using their large eyes to locate prey. They use their tentacles to capture and draw it towards the mouth. Then they bite it using their beak and inject their saliva which contains a toxin. If their munchie has an external shell, they drill through it and inject their saliva just as many of our octos do with clams. As mentioned, their shell is not for defense. Instead they utilize color change to camouflage themselves as well as ink which confuses the chemical senses of their predators (tuna, billfish and dolphin).

I should note that the shell of the "paper" nautilus is most assuredly not made from the fibrous pulp of plants. If so, it would quickly become quite soggy and sink taking our octopus down to the bottom of the sea. They much prefer life near the surface! It is actually created from calcite and serves both for buoyancy and to protect the female's eggs while she drifts around. The critter herself may reach a length of about four inches but the shell may be as long as 16 inches. The ones I find are always much smaller. The males are about one quarter the size of the ladies, and weren't even discovered until the late 19th century. Sounds almost like junior high school! Although I do encounter shells occasionally, it is pretty rare to find a living animal.

Like other octopus, the paper nautilus has eight arms. Two of these are web-like and specialized to capture food. In the male one of the arms (the third) is modified for reproduction and is known as the hectocotylus. I have mentioned this structure with respect to our common two-spot octopus, and how its arm has an erectile tip known as the ligula. The male argonaut prefers a different method of protection and hides his in a pouch until it is "needed." Biologists once thought the detached reproductive arms of the males were parasites when found inside the female! Sadly, the poor males are reported to die shortly after they fertilize the female. Maybe Nancy Reagan was right when she commented "Just say no." Celibacy may have its advantages... at least if you are a paper nautilus! My mileage may differ. The lucky ladies get to mate many times during their lifetime!

© 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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The chambered nautilus and a paper nautilus in Cat's hands; and views of the paper nautilus shell.

This document maintained by Dr. Bill Bushing.
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