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Dive Dry with Dr. Bill

#650: A Sticky Subject: Sea Urchins

Last summer's unusually warm water temperatures caused many ecological disruptions in our island waters. I have written about a number of them over the past year. One I have only touched on is the impact on our local sea urchin populations. Those of who who took biology at some point probably remember that sea urchins are members of the spiny skinned phylum Echinodermata along with the sea stars (starfish for most of us), sea cucumbers and brittlestars.

Starfish populations along much of the West Coast were hammered the last few years by a pathogen that literally caused them to waste away. Today there are very few starfish in our waters, especially the knobby starfish (Pisaster giganteus), ochre starfish (Pisaster ochraceus) and fragile rainbow star (Astrometis sertulifera). The spread of this pathogen, known as a densovirus, may have been aided by the unusually warm water. Die-offs of seastars have been observed in previous decades as well, but never as wide spread or involving as many species as this event.

Sea urchins are also subject to diseases that are enhanced by increased impact and spread during warm water episodes like El Niños. One of the most disastrous die-offs occurred during the 1982-84 El Niño but I have also observed lesser events during the late 1970s, 1990s and about 2006. Last summer I began noticing dead sea urchin tests (exoskeletons) on the bottom. Most of the tests I observed did not show significant indications of bacterial infection, suggesting it may have been the elevated temperatures alone that killed them.

At shallow depths in Catalina waters we have several different species of sea urchin. The primary ones are the red urchin (Strongylocentrotus franciscanus), purple urchin (Strongylocentrotus purpuratus) and black or Coronado urchin (Centrostephanus coronatus) but an observant diver may also spot the heart urchin (Lovenia cordiformis) or the slate pencil urchin (Eucidaris thouarsii). And, of course, there is always the sand dollar (Dendraster excentricus) which is a highly flattened form of sea urchin.

The red and purple urchins are more northerly species known from Alaskan waters down into Baja California while the black urchin's distribution extends from southern California down to the Galapagos. Thus the affinities of the first two species are to the north. They tend to be the species most affected by warm water episodes and, indeed, the many dead urchins I observed starting last summer were mostly these two types. The black urchin is accustomed to warmer water and thus generally survives warm water episodes.

Another sea urchin that may be encountered by divers is the white urchin (Lytechinus anamesus). I have often seen large numbers of these smaller urchins feeding on drift kelp and other algae at deeper depths down to 200 ft. Because this urchin usually lives in these cooler waters, it is less affected by warm water in the upper water column.

On my last few dives I've been seeing white sea urchins at much shallower depths, in the 40-50 ft range. Most of them are not feeding on drift seaweed on the bottom, but are on our slowly recovering giant kelp as well as deeper water kelp species. Why I ask? I'm always looking for answers to the puzzles I encounter while submerged because that's what scientists do... even retired ones. Why are the white urchins up in shallower water and munching down on our attached kelp?

Of course the "answers" I come up with are speculative and hypothetical. They are in need of scientific testing, but here's my initial take. With most of our giant kelp absent over the past 10 months, there has been little drift kelp to sink down to deeper depths and provide food for the white urchins. Many of the deeper water kelp like oarweed and Agarum were also impacted by unusually warm water even at depths of 100 ft. The Asian exotic Sargassum horneri dominated the landscape, only recently dying out since it is an annual rather than perennial. It could have provided a lot of drift material on the bottom at deeper depths, but it also contains chemical defenses known as polyphenols that make it taste "bitter" to herbivores including the white urchins.

So with little decaying drift material on the bottom at deeper depths, I'm guessing the sea urchin larvae settled on what they sensed as good food for growth... the recovering giant kelp up in shallower water. Many larvae of marine critters drift and develop in the plankton as they disperse to new places with the currents. When it comes time to settle out and grow, many are capable of sensing the right conditions in the environment. That may include the right type of substrate, the right type of food or the presence of other members of their species. So that's my story and I'm sticking with it until someone more ambitious than I conducts a real scientific study.

In the interim, I'm just going to relive an early experience with sea urchins when I watched "Thunderball" decades ago and saw Sean Connery, er James Bond, "tastefully" remove sea urchin spines and suck the toxin from the foot of the lovely Domino Derval, played by Claudine Auger. Sigh. Remember, ladies... if you're ever in need of similar medical treatment, yours truly is a "doctor" (although I don't have a foot fetish)!

© 2015 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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Black or Coronado sea urchin and purple urchin; white urchins on blades of giant kelp.

This document maintained by Dr. Bill Bushing.
Material and images © 2015 Star Thrower Educational Multimedia