Followup on Bean Clams

September 23,  2018

Question: I saw your Jan. 25 column regarding the bean clam population explosion on Newport Beach shores, and have a follow up question. My husband and I have been witnessing the masses of bean clams on the beach for over a year now, and today, as were walking upon the low-tide shoreline, we noticed that all of a sudden, many of them were opened up. This happened overnight. What made them die so suddenly? (Jeannine)

Answer: It’s impossible to be sure from a photo alone, but it’s likely that either the bean clams (Donax gouldii) recently spawned or an infectious agent was to blame.

Microscopic examination of the tissues could perhaps further detect whether the gonad shows signs of recent release of gametes for spawning or whether there are active infections from bacteria, viruses or parasites, such as trematodes that use this species of clam as an intermediate host. Unfortunately, this kind of testing (known as histology) can’t be performed on necrotic (dead) tissue, so samples would have had to have been collected and tested immediately after the event.

Mass mortalities are natural occurrences among this species, and are known to occur in similar species found worldwide. Our native bean clam populations have been observed to decline following a spawning event during the warmer summer months (June through September). Bean clams become reproductively viable around 1-1.5 years old and have been observed to die shortly after spawning. Their spawning season occurs June through November, although some clams can continue to spawn up to their third summer.

Almost 70 years ago, biologists determined that an infectious organism was the cause of a mass mortality of bean clams they were monitoring on the shores of La Jolla, when that population that had numbered in the millions fell to less than a dozen individuals several months later.

These naturally occurring phenomena shed light on how these species seem to disappear from our shore until the next large recruitment event.