Japanese Skeleton Shrimp under scrutiny
In 2003, mussel growers in the Carleton area were concerned. An invasive species, the Japanese Skeleton Shrimp, had landed. It swarms onto the collectors and mussel culture lines. Would it put mussel spat at risk or slow its growth? Who knew? To determine whether the species was harmful, a Fisheries and Oceans project studied the "Japanese threat." Very closely.
An adult male is 35 mm in length. Nothing exceptional there, except that their numbers sometimes swelled to 200 000 individuals per square metre in the Gulf of St. Lawrence, Chaleur Bay and the Magdalen Islands. These uninvited guests are rather aggressive; they shamelessly dislodge indigenous Skeleton Shrimp and take over artificial culture structures, a daunting sight for any worker when the lines are raised.
"Although much was known about the biology of the Japanese Skeleton Shrimp, we knew very little about the nature of this crustacean's interactions with mussel spat prior to our study. With the help of Christian Turcotte, a Master's student at the time, we scoured the grey literature before establishing and testing our scientific hypotheses. It was a real intellectual challenge," says Bernard Sainte-Marie, Research Scientist and head of Marine Invertebrates Biology and Conservation at the Fisheries Science and Aquaculture Branch of Maurice-Lamontagne Institute (MLI) in Mont-Joli.
"Understanding how this invasive species works was part of our project. Trying to control this invading population was an equally interesting technical aspect," the researcher added.
The art of destruction
Led by Bernard Sainte-Marie, in collaboration with Marcel Fréchette (MLI) and Réjean Tremblay (Université du Québec à Rimouski), this scientific investigation remains complex, as the crustacean in question can harm mytiliculture in many ways.
"We started by asking ourselves if the Japanese Skeleton Shrimp was feeding on young mussel larvae. Given the size of the invasive species and the considerable deterioration of their stomach contents, we opted for an immunoassay: agglutination tests for a mussel antibody. This indicated the presence or absence of mussel tissue in the Japanese Skeleton Shrimp's stomach. In this case, the tests showed that the Japanese Skeleton Shrimp was not feeding on young mussels," explained Mr. Sainte-Marie.
So, is there such fierce competition for space that the spat has difficulty developing? Not exactly, according to the researcher. "Based on our observations of the culture sites, the Japanese Skeleton Shrimp covers the spat, but does not take its place. Even if there are hundreds of individuals per linear centimetre," he added. In fact, the high density of Japanese Skeleton Shrimp only limits mussel growth and the speed of their market availability. In general, regardless of temperature conditions, it takes 18 to 24 months for a mussel to reach marketable size.
Lastly, the researcher and his team decided to examine one of their more promising hypotheses. Is the Japanese Skeleton Shrimp competing with the spat, not for space, but for the food resources available in the water column?
Why not? The Japanese Skeleton Shrimp, with its filtering ability, has an advantage. In the mussel cultures, it clings to the mussels that surround the propylene collector ropes using hooks in its back legs. With its antennae, it can sweep the water column and catch the zooplankton and phytoplankton before the mussels can.
"True food thieves! A real case of kleptoparasitism. The smallest skeleton shrimp take advantage of the mussel's inhalant current to feed, to the detriment of the neighbouring shellfish. This increases the species's capacity for growth compared to that of the mussels in a rather critical phase in the spat's development. The fewer invaders per square centimetre, the better the mussels will do!" explained Bernard Sainte-Marie.
Conducted by the Maurice Lamontagne Institute in partnership with growers in Chaleur Bay in the Gaspé, this research was the first to bring this kleptoparasitism to the world's attention. This would also explain why the Japanese Skeleton Shrimp is found in such large numbers on artificial culture structures, and rarely on natural structures, where its natural predators (fish), like those of the mussel, are most present.
Invasion under control
Part of the Japanese Skeleton Shrimp population is decimated each year due to extreme winter conditions. Nevertheless, this natural cull is not enough. There are always females that escape winter's icy kiss to breed once more in spring and summer. Young skeleton shrimp can then benefit from the pumping activity of juvenile mussels.
Also, "to better control the invasion, a brine treatment process can be used. This involves raising the collector lines, then dipping them in sequence into a salt-saturated pool continually fed with sea salt crystals. This takes from 30 seconds to 1 minute, but is enough to almost instantly kill the young Japanese Skeleton Shrimp. Like the adults of the species, their external gills are very sensitive to salinity. A process of reverse osmosis is quickly triggered and death is guaranteed," says Bernard Sainte-Marie.
But how do the mussels survive this rather salty cleansing? Once they are submerged in the brine, they hermetically seal themselves. There is little risk. Once the treatment is complete, the lines are returned to the water 2 or 3 metres below the surface.
"Although this technique is very effective, its use at sea is restrictive and the associated costs could discourage growers. It is only used when needed, in cases of heavy infestation," concludes Bernard Sainte-Marie. The Japanese Skeleton Shrimp had better watch out.
To learn more about the Japanese Skeleton Shrimp and to distinguish it from similar species, consult the Aquatic Invasive Species Identification Booklet (online version or PDF).
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