Offshore seaweed is both affected by winter’s heavy seas and summer’s relatively toasty waters. The former generates strong groundswells that rip kelp and other seaweed from their rock attachments, and the latter promotes seaweed decay and sloughing due to high water temperatures. Currents then push these heaps of seaweed onto beaches such as La Jolla Shores.
Left to its natural devices, stranded seaweed (called beach wrack) is soon visited by seaweed flies. Unlike most insects, these flies are committed to beach living. Seaweed flies look similar to houseflies but have heavier and flatter bodies to deflect constant coastal breezes. They can fly but don’t really want to. When disturbed, they will take to the air but stay low to the sand.
Lanna Cheng, marine entomologist at Scripps Institution of Oceanography (SIO), said, “In front of SIO, we have about ten different seaweed fly species. We don’t really know much about them.”
We do know what the flies are doing in the wrack. They are mating and depositing eggs. When the larvae hatch, they feed on the weed or the bacteria coating the weed. Chewing holes in the kelp inflicts damage, which more bacteria attack. In this way, the larvae may be farming their food source, and as they do, they simultaneously break down the kelp so it becomes watery and literally dissolves. Without seaweed flies, wrack would not decompose or would do so very slowly.
Interestingly, in the 1970s, the U.S. Navy funded a project directed at using kelp to generate natural gas. Navy researchers dumped a load of kelp into a tank but it refused to rot. With no understanding of kelp ecology, they did not realize seaweed flies were part of the equation.
The seaweed fly reproductive cycle is partly determined by temperature within a mound of wrack, which mimics a greenhouse ” warm and moist. Larvae may experience incubation temperatures more than 86 degrees, well above ambient. The warmer it is, the faster the larvae become flies, which may be a brief 10 days. The cycle is also dependent on, and therefore synchronized with, the tides. Reproduction begins right after a month’s highest tide and must be completed before the next highest tide (30 days) or the larvae will drown.
Adults have more survival options. When the tide rolls in, the flies seem to vanish into thin air, and in a way they do. They head to shady, protected places like the underside of Scripps Pier, where they congregate until the tide recedes.
Left to unnatural devices, beach wrack is manually removed by raking the sand using heavy equipment, so-called beach grooming. The goal is to reduce unpleasant smells and seaweed flies for beachgoer enjoyment. How this activity impacts coastal ecology has not been seriously studied but known consequences exist. It takes a village of flies to rapidly break down wrack, so reducing fly populations on a groomed beach affects neighboring ungroomed coves, leaving those beaches smelly.
While larval flies recycle seaweed, adult flies prey on such delicacies as bird droppings. The flies are themselves easy prey for some beetles and shore birds since flies flit no farther than about 50 yards from the beach. Despite this short local range, seaweed fly geographic range is breathtaking; populations have settled from at least as far north as Washington and south to Chile.
According to Cheng, no one at UCSD is studying seaweed flies because insect research has not been a focus for local scientists, but that doesn’t mean it shouldn’t be done. Said Cheng, “A large, comprehensive study is needed. We need to know more about fly taxonomy, biology, ecology, populations and genetics. What part do seaweed flies play in the environment they inhabit?”
Since coastal organisms are predicted to be particularly susceptible to the impacts of global climate change, maybe this is a good time to invest some resources into learning about seaweed flies.
” Judith Lea Garfield, biologist and underwater photographer, has authored two natural history books about the underwater park off La Jolla Cove and La Jolla Shores. www.judith.garfield.org. Questions, comments or suggestions? Email [email protected].
