Despite the ecological and economic importance of lobsters to Maine, there is still much we don’t know about how they interact with their environment during their earliest life stages.
Researchers at the University of Maine and Bigelow Laboratory for Ocean Sciences are trying to figure out what baby lobsters eat in order to make sure they are better prepared for climate change as many of their main food sources start to decline.
In recent years, the number of juvenile lobsters compared with adult lobsters has declined, suggesting that lobsters are dying off in the early stages of life. Richard Wahle, director of the Lobster Institute at the University of Maine and one of the lead scientists on the research project, explained that lobsters go through different stages where they “metamorphose sort of like a butterfly.”
Somewhere between the first stage and the final stage, monitoring has shown a significant number of those little lobsters are dying.
At the same time, researchers have noticed major changes in the zooplankton population in the Gulf of Maine due to its warming waters. That includes the decline in some species like calanus finmarchicus, which Wahle said are like “little butterballs” that feed everything from herring to whales.
In their larval stage, lobsters spend several weeks in the water column feeding on zooplankton to fuel their growing and changing bodies. Scientists in Maine are working to figure out whether larval lobsters aren’t making it to adulthood because there isn’t enough for them to eat.
“These zooplankton are the main food supply for the baby lobsters as they’re growing,” said Evelyn Layland, a master’s candidate in marine biology at the University of Maine School of Marine Sciences. “One of the possibilities is that they may be lacking for food. What we’re looking at [is whether] there’s a link between the declining numbers in particular species of copepods and zooplankton in the Gulf of Maine and the decline in the lobsters.”
Despite their importance to Maine’s economy and ecosystem, scientists don’t know much about what lobsters eat during these key early stages of development. Lobster larvae grow between the size of a peppercorn and a fingernail and are notoriously difficult to study, so scientists have to get creative with their methodology.
For her experiments, Layland uses an aquarium equipped with a video camera to record how lobster larvae at different stages capture and handle prey. She observes free-swimming lobsters in the lab tank and those that are tethered to tiny sticks, and watches how they handle a number of different prey that vary in size and swimming abilities.
“I can use what I learn from lobsters feeding on larger, faster or slower [prey] and extrapolate onto the known zooplankton community and try to figure out which species are out there make for good food sources for the lobsters,” Layland said. “That’s the main question that we’re hoping to answer.”
In addition to what Layland is doing in the lab, the researchers also want to see how larval lobsters dine in their natural habitats.
Alex Ascher, graduate research assistant at the Darling Marine Center, does the work of carefully dissecting itty bitty lobster larvae, whose organs have the “consistency of very wet tissue paper,” to see what exactly is in their stomachs using a new methodology called environmental DNA, or eDNA.
So far, Ascher said he has found lobster larvae to be generalist eaters, meaning they’ll chow down on anything they can get their claws on, from copepods to crab larvae. One thing he hasn’t seen a lot of in lobster stomachs, though, is zooplankton like calanus finmarchicus.
“That doesn’t mean that calanus isn’t important,” Ascher said. “It could potentially just mean that the small percentage of lobster larvae that do capture calanus are the ones that survive and go on to be adult lobsters.”
It is possible, though, that there is not a cause and effect relationship between the zooplankton and lobster populations. The same warming waters that are affecting zooplankton could be causing lobster larvae to die off at the sensitive stages, or predators that have increased in numbers could be dining voraciously on both populations.
However, the early results are leaning toward the food limitation hypothesis.
“Let’s say you have columns for each of these hypotheses accumulating evidence,” Wahle said. “The evidence is accumulating more in favor of the food limitation hypothesis than the other ones. I’m not saying that we’re ruling out any of them. It could be a mixture of influences, too.”
Wahle said that researchers should eventually have an idea in the next year or two about what lobster larvae eat and what that means for Maine as waters continue to warm.
“Baby lobsters become adult lobsters, adult lobsters fuel a good portion of Maine’s economy,” Ascher said. “If we understand the factors that affect larval survival it gives us a seven-year head start on what the adult population might look like and what the fishery might look like. It helps us plan better and more sustainably for the future.”