Dive Dry with Dr. Bill

#702: It's The Little Things in Life

Some divers power their way past the reefs and sandy bottom without seeing much of anything... unless it is rather Yuuuge! I often wonder why they bother to dive at all. I take pleasure in the little things in life, and one has to move slowly and keep looking for these tiny treasures. It also helps if you have better eyesight than I do. My vision underwater is about as good as a moray's.

I always submit new videos to the annual Scuba Show Film Festival in Long Beach. You can see many of them on my YouTube channel (DrBillBushing). One idea I've not yet produced was to look at the community of critters that live on a blade of our giant kelp (Macrocystis pyrifera). A truly fascinating mini-ecosystem exists for you to see... if you can.

One of the very common encrusters on kelp blades is a worm known as Spirorbis. I first learned of this group back in my Harvard marine biology class more than 50 years ago. When I arrived on Catalina four years later, I was pleased to see my old friend living not only on giant kelp, but also other seaweeds, rocks and the shells of invertebrates. You may have seen them on kelp that drifted up on the beach.

These miniscule worms build coiled calcium-based tubes that attach to the substrate and are a mere 2 to 5mm across. The tubes don't offer much protection from larger critters such as fish which bite holes in the kelp blade. In some species the tubes coil clockwise while others go counter-clockwise. There are anatomical differences between species, but in general the soft body consists of an abdominal segment with a feeding segment known as the thorax.

This anterior portion of the worm contains a series of tentacles, often numbering 10, that serve as the feeding appendages. Using these structures, the worm captures food by filter feeding. One of the tentacles contains a cup-shaped operculum that can seal up the shell if the worm withdraws into it.

So much for munching... time for mating. Actually that is a pretty boring topic too with this group of species. Many are hermaphrodites and have both sex organs in one animal. Some species will fertilize themselves while others prefer two to tango. Some may mate all year long while others may be somewhat seasonal.

Some reproduce by broadcast spawning, casting out the male and female gametes and rely on chance to ensure fertilization in the water column. This requires both the male and female structures to release at the same time. It would be counter-productive if the female organ was in the mood in June while the male preferred December! Others are apparently able to capture and store sperm until the eggs are ready to be fertilized. Some species actually brood the fertilized eggs and the resulting larvae develop either within the shell or in a modified operculum.

Apparently these worms are considered pests by those who keep aquaria because they attach to the walls and hard substances in the tank. Tough... if you're going to imprison my fishy friends, you deserve to deal with the consequences!

There are many species in this genus of worms. They are very difficult for anyone other than a specialist to tell apart, so often they are just referred to by their genus name. Somewhere some taxonomist must be sitting in a lab trying to come up with a useful key to tell them apart. I just don't have that kind of patience.

© 2016 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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Kelp blade encrusted with Spirorbis tube worms and close-ups showing feeding tentacles
and operculum (images on right courtesy of Merry Passage)

This document maintained by Dr. Bill Bushing.
Material and images © 2016 Star Thrower Educational Multimedia