I rarely dove deeper than 100 feet during my first 40 years of on-again, off-again diving. In the 60's it was because the freshwater sites I dove didn't get that deep. After I moved to Catalina in 1969, I still was a fairly "shallow" guy since most of the kelp forests I study are above 100 feet. I didn't see any reason to engage in deep diving, especially given the equipment of those early days when we dove with J-valves, and no SPG's (submersible pressure gauges), octopuses (other than the mollusc kind) or BCD's (buoyancy compensator devices).
I often wondered why I rarely encountered one of my favorite starfish, the bat star, on these dives. Occasionally I would see small ones under rocks in shallow water, but it was an uncommon event. Later I dove the colder northern Channel Islands from Anacapa out to San Miguel Island, and found bat stars in an array of colors well above 100 feet. Hmmm... my scientific mind had to create a hypothesis that I could test with a little bit of observation, giving me a good reason to dive more (as if I need one).
In my preliminary research, I learned that bat stars are found from Sitka, Alaska, south to Cedros Island off Baja California. Now my finely trained scientific mind realized that this might be a largely cold water species. What would any self-respecting cold water critter do as it extended its geographic range into warmer waters to the south like Catalina? Well, conventional wisdom suggests they would go deeper to find cooler water. A similar but opposite migration occurs when SoCal "warm water wussy" divers go south as winter, and colder water, approaches.
When I started my deep diving exploits about five years ago, I began seeing bat stars at depths below 100 feet (and occasionally above that). In fact, they are known to frequent depths from the intertidal to nearly 1,000 feet. Glad I didn't have to dive that deep to find them. The bat stars at depth were mature adults, reaching diameters up to about eight inches. While the young ones I found in the shallows were almost always orange, those at depth had a wide array of colors and formed assemblages that were quite beautiful... at least under a video light. Although most had the usual starfish (er, sea star) compliment of five arms, individuals may have four to as many as nine! They get their name from the bat-like webbing between the arms.
I frequently turn sea stars over to film their undersides. Most of the bat stars I looked at had their stomachs turned inside-out... although they quickly drew them back through the mouth into the protection of their hard calcium carbonate exoskeleton. Bat stars feed on a variety of plant and animal matter, and although some of it is live, much of it is dead... and therefore unable to flee. They use sensors at the tips of each arm to locate their meal. The bat star has plenty of time to evert their stomachs and envelope the food to digest it. They secrete strong digestive juices that liquefy the meal so they can then "slurp" it up. As scavengers, they perform a vital function by cleaning up organic debris that has fallen to the bottom.
Also on the underside, living in the grooves on each arm where the tube feet are found, are small, dark segmented worms. These worms are not parasitic on the bat star and do it no real harm. Instead they munch on the leftover bits and pieces of food. This is a good example of a commensal relationship between two species.
The dorsal surface of the bat star shows its hardened ossicles, or plates of calcium carbonate that form the exoskeleton and protect the internal organs. Most starfish have structures known as pedicellariae on top that grab larvae that attempt to attach to the starfish and crush them. Bat stars don't have them, but their dorsal surfaces are quite free from encrusting marine life. The fuzzy appearance of a bat star's surface comes from small structures that serve as a kind of gill to aid in capturing oxygen from the water.
A while back I was diving Blue Car Wreck between the Edison plant and the East End Quarry. I dropped down to about 150 feet to film a few adult bat stars, but also found several on the steep rocky reef at about 60 feet where I could spend some real time with them. When I turned these bat stars over, their stomachs were all tucked nicely into their bodies. There was nothing on the rocks they were feeding on. However, they are vulnerable in this upside-down position and quickly began performing some acro-bat star-ics, slowly using their tube feet and arms to turn themselves over. This made for some compelling video which the accompanying still images can not do justice to.
© 2007 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 49, 10:00 AM and 5:00 PM weekdays and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. Please help me climb out of self-imposed poverty... buy my "Munching and Mating in the Macrocystis," "Great White Sharks of Guadalupe," "Calimari Concupiscence: Mating Squid, " "Playful Pinnipeds: California Sea Lions," "Belize It or Not: Western Caribbean Invertebrates, Fish and Turtles," "Gentle Giants: Giant Sea Bass" or "Common Fish and Invertebrates of the Sea of Cortez" DVD's. Yes, take Dr. Bill home with you... we'll both be glad you did!
Colorful adult bat stars at depth and a sequence showing one turning itself over in a display of acro-bat star-ics.
This document maintained by
Dr. Bill Bushing.
Material and images © 2007 Star Thrower Educational Multimedia