Dive Dry with Dr. Bill

#547: Photo Taxes... or Phototaxis?

Here I go again, tossing out those big words when all my readers want to know is the good stuff... you know, munching and mating. What the heck does "phototaxis" mean and why should I care, you say? First off, it has nothing to do with the government placing new taxes on photographs... not even the ones you hope your significant other doesn't see (you know, the old boyfriend or girlfriend). "Photo" refers to light and "taxis" refers to movement. The simple definition of this biological term is movement towards or away from a source of light. See, I made it easy to understand... right?

On a recent night dive I discovered yet another local example of this phenomenon when I encountered an unidentified sea cucumber in the shallows of the dive park. At first I thought my eyes were deceiving me as something quickly withdrew back into the rocky bottom when my video lights hit nearby. Then I moved more cautiously, keeping my light pointed away until I spotted another sea cucumber extending out of the substrate. As soon as I directed my video lights on it and began to film, it disappeared. This happened two more times, leaving me with a new mystery. I was aware that sea stars (known as starfish by the non-PC crowd) and some brittle stars and sea urchins were able to sense light, but not sea cucumbers. Apparently some can as I've found out.

Now we have several sea cucumbers in our waters. The two most commonly seen are the warty (Parastichopus parvimensis) and the California (Parastichopus californicus) sea cucumbers, and we also get crevice cucumbers and are at the northernmost range of Holothuria zacae and another southern species I have yet to identify. I've filmed each dozens if not a hundred times and I've never seen any reaction to light. They just keep on munching away on the sediment, extracting organic nutrients from it and pooping out "clean" sand. Recently I found out that the California sea cucumber uses more than just its mouth to harvest "food." Apparently it also takes in calories through its anus! Amazingly enough, the anus also serves its usual purpose (excretion) plus the exchange of oxygen and carbon dioxide with the water (respiration) in addition to some ingestion of food.

While this unidentified sea cucumber remains a mystery to me and will be the subject of further research, there are many other examples of phototaxis in our local waters. These are often amply illustrated when one submerges after the sun goes down and the moon comes out, something I love to do during summer months when the golf cart ride back to my home doesn't make me shiver and shake. Night diving is also a favorite this time of year because all of that nasty, dense invasive Asian Sargassum seaweed has died back so I can actually find things. Oops, I guess I kind of digressed... although there is some connection: when it gets dark, I move down to the dive park so I guess I move away from the light?

Another critter that responds to light in a negative way is the sea cucumber's relative, the sea urchin. Normally you won't see them out of their hiding places in the rocky reefs here during the day because they'd get munched by hungry sheephead. The sheephead hide in the reef at night to avoid sea lions, sharks and other predators so the larger black sea urchins are free to wander out and feed on drift kelp or even living holdfasts. When I point my video lights in their direction, they quickly head towards shelter. However, our red and purple sea urchins don't seem to do the same thing... perhaps because hungry lobsters head out to feed at night. Yes, lobsters love sea urchin sushi!

We do see a few abalone in the park despite their near regional extinction several decades ago. During the day they tend to remain well within their crevices, but at night they may wander slightly (but not too far). Once they sense my video beams striking their soft tissue, abalone also begin to retreat from it. I have actually watched a few of them do a 180 degree turn when they sense the light and head back into their nook or cranny. Apparently they don't move well in reverse! Neither does my 25 year old Toyota Tercel on the mainland.

So far the examples I've mentioned above represent what is known as negative phototaxis... the movement away from a light source. One could easily expect species active at night to exhibit this tendency so they are not easily detected by predators. However there are also examples of critters that exhibit positive phototaxis... movement towards a light source. I've observed two species which are drawn to light, and by doing so they risk losing life and limb. Why would any critter do such a thing... why, for "love" of course!

On my recent night dives, as in previous years and also while in the tropics, I've noticed several species of segmented polychaete worms that crawl out of their holes in the reef and hightail it towards my video lights. I'm guessing that astronomy was not a subject they were taught as kids, since they apparently believe they are heading towards moonlight near the ocean's surface. At least I taught my students in astronomy class at the Toyon school how to differentiate between the two.

These worms are so "excited" (hopped up on hormones as we'd say) and animated as they paddle and wriggle their way towards the surface (or my lights) that predators couldn't miss them. In fact I've seen kelp rockfish snatch up hapless worms on their way to erotic ecstasy. The poor worm may literally explode as it is engulfed by the fish's mouth. They often do the same when they strike my video lights or camera housing. The worms are heading towards the surface to spawn and cast off their sperm and eggs in the upper water column so they will disperse the kids farther away from home. It truly is amazing to watch the gamete trails in my video lights. They look like the tails of comets in the night time sky.

© 2013 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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Light sensitive sea cucumber, black sea urchin; and green abalone and spawning worm exploding in front of my camera.

This document maintained by Dr. Bill Bushing.
Material and images © 2013 Star Thrower Educational Multimedia