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Dive Dry with Dr. Bill

#554: Love or Live?

I'm starting to feel a slight chill in the air when I head down to the dive park for my night dives. Fortunately the water is still pretty warm, and I won't have to head to the tropics until at least November at this rate. An unusual sighting on a recent dive warmed the cockles of my heart and I spent 95 minutes underwater filming to bring it all back to you! Aren't I special? I didn't think so.

I hadn't descended more than five feet when I saw something unusual... several large Ophioderma panamense (formerly O. panamensis) right on top of the seaweed on the rocky reef. Why is that unusual, you rightly ask? Normally these relatives of the sea stars, sea urchins and sea cucumbers are found hiding deep in the reef or under rocks to avoid predators like hungry sheephead. Why were they out in the open and up high where a nocturnal muncher could easily take them?

First, I'd better tell you that I'm referring to a species of brittle star (a group is known scientifically as ophiuroids... no relation to hemorrhoids). Now we know a star fish isn't a fish, and a sea horse isn't a horse. Well, you may have figured out that the brittle star is not a star, at least not one like the Sun, Sirius or Betelgeuse. Their bodies possess a central disk with five long, slender arms radiating out from the center. However, brittle IS an appropriate name since these arms may easily autotomize, or break off, in the mouth of a hungry predator leaving the rest of the critter to slither to safety. I did see a number of them with one or two arms missing from such encounters.

So why, pray tell, would a munchable brittle star risk life and limb by climbing up to the highest point on the reef, far from safety? Certainly it wasn't an opportunity to climb Lookout Mountain for a chance to see the scenery. After all, they don't have focusing eyes anyway. No, they do it for the same reason many other species put themselves in harm's way... for love! Yes, these brittle stars exerted a lot of energy to reach Lover's Lane high atop the rocky reefs of the dive park.

I had seen a few of these brittle stars out on a previous night dive and given my scientific analytical mind, had guessed at their intent. However, on this night I'd get proof of that hypothesis. I literally saw over 100 of these brittle stars in a relatively small area on the reef. One has to wonder what environmental cues caused them all to emerge at the same time... or was it a form of chemical communication released when they are "in the mood?"

I was filming one when I noticed a cloud of tiny brownish spheres emanating from slits in the brittle star's central disk. Again, my finely honed cerebrum deduced that it was a cloud of eggs released by a female in hopes they would encounter the gametes from a nearby male and get fertilized. A little later I watched (and filmed) as one hunched up high on the tips of all five arms and released another cloud of eggs. Obviously it was attempting to gain about six inches of extra altitude so the currents would carry its offspring to distant shores!

This was the first time I've ever seen brittle stars spawn. On Santa Catalina Island in the Sea of Cortez on a Christmas Day, I watched as dozens of sea urchins released their milky white clouds of gametes into the water. My first thought was "Where is a fire extinguisher" until my brain kicked in and I realized what I was seeing wasn't smoke. I've also watched video of their relatives, the starfish and sea urchins, spawning.

All these invertebrates are playing the lottery... casting off lots of "bets" in hopes a few will fertilize and eventually grow into an adult. It's a real game of "chance" (kind of like the local dive instructor of that name). Personally, I don't like the odds. I'm glad that mammals like us use internal fertilization instead to increase the odds. Even so, it is not often easy to conceive that way either.

Ophioderma panamense is found from the northern Channel Islands south to the Sea of Cortez and on down to the Galapagos and Peru. It generally frequents habitats such as under rocks or in kelp holdfasts from the low intertidal to a depth of about 130 feet.

The central disk may be up to nearly two inches across and is reasonably smooth with tiny granules on the surface. The color is gray-brown to olive green with white bands evident on the long slender arms which may reach lengths of 3-4 times the diameter of the disk. Gravid females bearing eggs often show lumps around the edge of the disk which are the eggs packed in the gonads. When they spawn, the eggs are released through a number of slits along the margin of the underside of the disk.

These brittle stars will come out of hiding at night to feed. However they generally stay on the bottom. A smart ophiuroid doesn't risk being munched while looking for its munchies. They crawl about using a "rowing" motion of the arms in search of their food which may include detritus, dead critters or even small living prey. These ophiuroids are real gluttons... they may eat so much that their central disk bloats up and looks like the dome where the Spruce Goose used to be. My Mom used to get so embarrassed when I'd do that at the all-you-can eat buffets, she'd go out to the car... but Dad and Gramps would egg me on! Burp.

© 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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Ophioderma brittle stars out on the reef and releasing clouds of eggs into the water.

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