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Dive Dry With Dr. Bill: Gooseberries

About six weeks ago I did my last SCUBA dive after logging #100 over the last four months of the year. I didn't stop because the water got too cold. A film crew I've worked with before came into town for six weeks to film the California market squid (Doryteuthis opalescens). I went out on the Scrambler most nights but didn't do a single dive with them. Given my cancer, I didn't think it was a good idea for me to descend to the limits of recreational diving (130 ft) in the dark essentially solo.


The film crew departed the day before Super Bowl so after saying goodbye at the boat, I drove over to the dive park to see if any of my friends were there. Conditions were reported as excellent with 50 ft visibility and no current. Tempting! Then I saw local instructors Larry and Ruth Harris come up the stairs. I walked over to find out what they had seen and they said a nice, big turtle! They showed me pictures of it. Of course every time a turtle has been sighted in the park and I head down to see it, it is nowhere to be found.


Then Ruth showed me several images of a jelly she had taken out by the old "swim platform" (actually a pier float). I was pretty certain it was a comb jelly known as the sea gooseberry. Of course we scientists refer to it as Pleurobrachia but I couldn't remember the species name so I had to look it up in my Beneath Pacific Tides when I got home. Apparently it was P. bachei, the most common member of that family in our waters.


So what is a comb jelly? They are gelatinous invertebrates in their own phylum, Ctenophora. They get their common and phylum names from the eight rows of "combs" or ctenes along the side which are really sets of fused cilia used for propulsion. The ctenes in motion often appear to be bioluminescent, but the rainbow colors observed are due to the diffraction (scattering) of light passing over the fine cilia. However, some comb jellies can also bioluminesce but this species does not.


Although some confuse ctenophores with sea jellies (formerly known as jellyfish), you have nothing to fear from them. They do not possess the nematocysts or stinging cells of the sea jellies. The jelly's stinging cells are used not only for defense, but for capturing prey. Comb jellies use a different approach. They have sticky cells known as colloblasts to snare their munchies. These include planktonic larvae and crustaceans. If you are one of those, a comb jelly might be considered as a voracious predator.


Pleurobrachia bachei is found from SE Alaska all the way down to Acapulco south of the border, down Mexico way. Its body is nearly spherical. There are two long tentacles hanging down from it with numerous side branches. It uses the tentacles to capture prey by swimming up through the water column in a spiral fashion. In addition to the normal comb jelly diet, it will feed on eggs and small fish.


I'm sure all my readers are curious... about the sex life of a comb jelly. Why, not? Munching and mating are two critical functions of all life (well, only the first for me apparently). Ctenophores are hermaphroditic, containing both male and female gonads at once. I can't say much for their mating strategy though as they simply release the gametes into the surrounding water where fertilization may... or may not... occur. Not very thrilling.


Evolutionarily, comb jellies are a bit more complex than sponges (well, except SpongeBob Squarepants) but much simpler than most other life forms. I think they believe in the KISS strategy... Keep It Simple Stupid.



© 2020 Dr. Bill Bushing. For the entire archived set of nearly 850 "Dive Dry" columns, visit http://www.starthrower.org

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