Stramonita biserialis On Catalina Island: An El Nio Anomaly*

By Phil Liff-Grieff

Stramonita biserialis (de Blainville, 1832)    Summer camp is a place for kids to have fun, relaxation, camaraderie and a wonderful opportunity to get away from the rest of the family for a number of weeks. It is a place where kids can learn new things and try on new roles. And, of course, Parents' Day is an important part of the experience as it allows the campers to replenish their supply of candy, cookies and other essential snack foods.

    In July of 2000, my wife and I took part in the annual Parents' Day ritual as we visited our son at the Catalina Island Marine Institute (CIMI), an outstanding marine sciences-focused camp based at Toyon Bay on Catalina Island. Loaded down with our snorkel gear and bags of candy, we were prepared for a day of camp fun.

    I greatly enjoyed touring the underwater photography lab, the shark and ray tank, the algae lab (doesn't every summer camp have an algae lab?) and the invertebrate lab. I was curious, however, about the hermit crab in the touch tank that had taken up residence in a Thaid shell I had never seen before in Southern California; Stramonita biserialis (de Blainville, 1832). As this species lives south of Ensendada, Mexico, it clearly didn't belong in Toyon Bay. Did someone accidentally drop a shell into the bay? Were exotic shells brought in to give the kids something to find on their marine excursions? No one knew anything about the shell or how it got there.

Stramonita biserialis (de Blainville, 1832)    The day culminated with time to snorkel with the kids in the bay. To my-surprise, I immediately encountered two mature Stramonita feeding on barnacles exposed at low tide. The species was indeed living in Toyon Bay! Plans were quickly made to return to perform a more detailed survey.

    In August, the CIMI management invited us back to make a detailed survey of Toyon Bay and its surroundings. In all, we encountered 46 mature specimens of Stramonita biserialis living in two distinct colonies - one at each point enclosing the bay. No Stramonita were found beyond those two colonies.

    I examined the evidence in front of us in an effort to determine if we were looking at a range extension for the species or simply a one-time anomaly. My biggest clue was the simple fact that all of the specimens observed in the bay were fully mature and no egg masses or juveniles could be found. Clearly, all of the specimens arrived at around the same time and were not adapting well enough to breed.

    The two colonies of this gastropod found in this unlikely location were the result of warm currents spawned by an El Nio condition. They must have settled on the two points where they were found and survived the increasingly cooler conditions that prevailed over the ensuing years. Survived, but not thrived as they were not able to reproduce and should perish within one generation.

    It was very easy to test this hypothesis. The following summer (2001), I found myself at another annual Parents’ Day and I checked the location of the first colony – no thaids were to be found. This past July, I checked the second site and found no specimens had survived there as well. The lifespan of this pioneering colony of gastropods had ended.

    As we look ahead to the next El Nio cycle, I can’t help but wonder what species will find its way into our waters.

*Originally published in the Pacific Shell Club Newsletter “Las Conchas,” Vol. 34, Issue 1, October, 2002.