Looking for Sea Monsters

By Phil Colarusso

Tunicates covering native eel grass. Photo by Dann Blackwood.

As a diver for EPA, I am often asked, “what’s the scariest thing you’ve ever seen underwater?”  Most people expect the response to be sharks or stingrays.  They are generally disappointed and perplexed when I say sea squirts.

Let me explain.  Sea squirts, also known as “tunicates,” are seemingly benign-looking gelatinous, filter-feeding animals.  They grow in large colonies or as individuals.  They don’t bite, sting or even generally move, once they have attached themselves to a solid object, like a rock or pier.

So why the concern?  Well, here in the northeast, like many other places in the U.S., we’ve been experiencing an invasion of non-native tunicates.  Mooring lines, piers, boat bottoms, rocks, shellfish, and even eelgrass have been overgrown by various invasive sea squirts.  Anything that does not move quickly is at risk of being engulfed by the rapidly reproducing and growing colonial species of sea squirts.

Sea squirts are prolific filter feeders, processing more water per unit body weight than oysters, blue mussels or quahogs.  Very few animals feed directly on them because they are not a native species.

Our dive unit has been studying the impact of these invasive species on the ecology of coastal salt ponds on Martha’s Vineyard.  We initially became interested in this topic while doing eelgrass restoration work, when we noticed large numbers of eelgrass shoots covered with exotic sea squirts.

Tunicates cover a boat hull. Photo by Dann Blackwood.

As part of our study, we quantified the abundance of invasive sea squirts in a number of coastal ponds on Martha’s Vineyard.  We quantified the short term impact of the presence of sea squirts to eelgrass by measuring plant growth, size and morphology and sugar concentrations (end product of photosynthesis).  Finally, we measured light reduction by sea squirts.

We found that sea squirts reduce the amount of ambient light that reaches the plants by between 70-80 percent.  Plants covered with sea squirts grew at a much slower rate and had fewer leaves per plant, so the presence of sea squirts was having a measurable negative effect on eelgrass.

We recently began a new study designed to look at the impact that sea squirt feeding may have on the normal food web dynamics in these coastal salt ponds. We will be using stable isotopes to construct food webs in a test pond with an abundance of sea squirts and compare that to a comparable pond without sea squirts.

The sea squirts are likely competing with native commercial shellfish species (scallops, mussels, quahogs, oysters) for food.  Our concern is that an explosion in the abundance of the exotic sea squirts could result in upsetting normal food web dynamics, potentially reducing populations of commercially important shellfish.

The scariest things in the ocean don’t always look that way!

About the author: Phil Colarusso is a marine biologist in the EPA Region 1 Coastal and Ocean Protection Unit and a member of the  EPA New England Dive Unit. He’s been with EPA since 1989.

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