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The Decline of Seagrass Meadows

Zostera! Eelgrass, Zostera marina, is a flowering, marine vascular plant that remains submerged all the time. This is quite a feat for vascular flowering plants, and only a few dozen species world wide are capable of growing completely submerged in a marine environment. Eelgrass creates and extremely important habitat, its upright structures and complex root system create a 3-D living space for many different types of animals. It is (or was) the dominant habitat forming SAV (submerged aquatic vegetation) throughout much of the coastal waters in the northeastern United States. Unfortunately, for various reasons, eelgrass meadows have seen drastic declines, and in many locations eelgrass only exists in a mosaic of small patches. This is extremely bad news as many of the important, and formerly important, commercial and recreational fisheries of the northeast US are dependent on Zostera at some part of their life cycle as a nursery and foraging ground. Some of the species are finfish like tautog, bluefish, fluke, winter flounder, porgies, while others are shellfish such as blue mussels, hard clams, oysters, bay scallops, and blue crabs. Many of the aforementioned species support or once supported vibrant fisheries. Many of those fisheries have collapsed, also for various reasons. However, is it possible there is a link between the crash of the fisheries, the decline of Zostera and the failure for recovery on both ends?

Bay Scallop on Eelgrass

Argopecten on Zostera! Bay Scallops, Argopecten irradians , have developed a very close relationship with eelgrass, Zostera marina. As larvae, they are passively transported, and tend to settle in eelgrass meadows when the current is dampened by the 3D structure of the seagrass. This same 3D structure provides post-set juvenile scallops a spatial refuge from predation. Even as larger juveniles and adults, scallops are capable of, and have been shown to, actively select eelgrass habitats.

Other species also use eelgrass

grass shrimp A number of other species utilize eelgrass as a habitat. Included are grass shrimp, like the Palaemonetes pugio, other decapods such as blue crabs, bivalves such as hard clams, gastropods (snails), and numerous fish species, including winter flounder, tautog and cod.

Invasion of the bottom snatchers….

Some call them sea squirts. Some call them tunicates.  To some it looks like pancake batter.  Didemnum vexillum is a potentially harmful invader along the coast of  New England, specifically in George’s Bank.  Since it was first discovered in 2002, it has spread rapidly.  As a colonial organism, it expands rapidly, forming vast mats of intertwined individuals.  It needs a hard substrate to colonize, and the cobble bottoms off New England and south to Long Island Sound are ideal substrates.  In fact, in Long Island bays, we often find Didemnum on cinder blocks and concrete pilings.  This invader likely came here via important Asian shellfish, but the exact time and point of introduction are not known.    Regardless, this species has received numerous bad press, mostly due to the fact that its preferred substrate is also valued habitat for commercially important sea scallops and groundfish such as cod and haddock.  However, research from Woods Hole lab of NOAA’s Northeast Fisheries Science Center has started to investigate potential positive qualities of this invader, as some species are enhanced by the squirts presence.  Some species of marine worms and Cancer crabs appear to  thrive among Didemnum colonies.  This could have food web implications.  Marine worms are favorite food source of winter flounder, whose stocks are suffering.  The crabs that thrive among the invader are also important food sources to higher trophic level organisms.  In addition, if the crabs are consuming the sea squirt (which is total conjecture on my part, as the article doesn’t say anything about this), than that high amount of production from the squirts can then get transferred up the food chain.  Granted, the invader isn’t good news, and that few species appear to like this new habitat does not in any way mean that this invader is not harmful.  But it does support a research hypothesis that I am working with in local bay waters.  Marine systems are incredibly resilient.  There has been no instance of extinction of a native species due to the introduction of a new species in a marine system.  There might be negative consequences, and in the case of Didemnum, the consequences are many.  That being said, this research indicates that native species are able to adapt to invasive species.  This has certainly been the case of Codium fragile, and invasive macroalgae, and scallops in Long Island waters.  In my research, and my readings, it appears as though those “invaders” which are potentially habitat forming are much more likely to have some positive interactions with native species.  There is still a lot to learn about Didemnum vexillum and its overall ecosystem impacts (some research underway at SoMAS has identified the ability of Didemnum to filter extremely small particles out of the water column, as well as investigated how colonies of Dideumnum might affect water column filtration in small coastal waters).  And however unpopular, it is important that valid research results are always reported, even if they go against the common theme of invaders being strictly negative.  All the information needs to be out there to paint the clearest picture possible.  While it is unlikely that this new research will change many opinions, and while Didemnum is still harmful to some species, it is possible that the George’s Bank marine ecosystem is responding to the invader, adapting to the change, and will continue to support fisheries despite its presence.  Who knows?

3 comments to Invasion of the bottom snatchers….

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