A boulder heavily colonized by Didemnum (photo credit: Dann Blackwood, USGS)

During the 2006 field season, Dr. Robert B. Whitlatch and colleagues from the University of Connecticut's Department of Marine Sciences will be conducting research operations aboard the R/V Connecticut to investigate the distribution and density of the sea squirt Didemnum sp. (right) in the eastern region of the Long Island Sound.

With remotely operated vehicle (ROV) support from UConn's National Undersea Research Center and funding from Connecticut Sea Grant, researchers will conduct video transects along select habitats that have been mapped using sidescan sonar. The sidescan imagery provided by the Connecticut Department of Environmental Protection's Long Island Sound Resource Center and the U.S. Geological Survey will provide a detailed map of the bottom types found in Fisher's Island Sound, particularly the gravel habitat areas that are likely to be colonized by Didemnum.

Sidescan coverage area in Fishers Island Sound (click map for higher resolution image)

The ROV will provide detailed video and still imagery to allow researchers to estimate the percent of the seafloor carpeted by the invader. This will provide a baseline that will be revisited at the end of the summer to guage the growth of the sea squirt invasion. Changes in the abundance of Didemnum will be compared to other sessile invertebrates and will provide data on the ability of these invaders to compete with native animals and to deal with changes in temperature, oxygen, and food availability.

Background Information

The many descriptive terms for aquatic organisms living beyond their natural geographic range include "invasive", "introduced", "non-native", "exotic", "non-indigenous", "alien", and "aquatic nuisance". Although some introductions occur naturally, a variety of human-mediated pathways now transport great numbers of aquatic species long distances in relatively short periods of time. As a result, species from one continent may find their way past natural barriers to other continents far removed from their native lands. Whether deliberately or accidentally introduced through human activity, these introductions occur globally and all affect local biodiversity. In some cases, their presence in new habitats may also have serious socioeconomic or health impacts. There are over 50 introduced species living in LIS. For more information, please see the Connecticut Sea Grant's website (http://www.seagrant.uconn.edu/lisinv.htm). The organism of interest for this mission is the compound sea squirt (Didemnum sp.).

These invasive ascidians were introduced to the New England area from the Pacific and first documented in 1993. The population currently occupies an expanding region from Maine to Connecticut, including Georges Bank off the coast of Massachussetts. Through both sexual and asexual reproduction, microscopic individuals are dispersed via ocean currents and settle to form large colonies (up to 45 cm long) and juveniles are present from July to November. Didemnum colonies can display a coloration from cream to white and a number of morphologies ranging from finger-like projections to laminar sheets. Referred to as fouling organisms, the tunicates attach to rocks, docks, pilings and forms encrusting mats on seafloor, usurping benthic habitat.

Invasive Species Ecology

There is significant interest in why and how successful invasions occur 1) because invasions may directly or indirectly alter local community composition, 2) because the performance of ecosystems may be altered as a result of changing biodiversity which may be associated with invasions, and 3) because there may be significant economic costs associated with these ecosystem-level changes. Although all ecosystems do not appear to be equally invasive, the factors determining the susceptibility of a community to invasion are still often unclear. Invasion theory generally holds that the more species that are present in a community, the more competitive the environment, and the less likely an invasion is to be successful. The most commonly cited mechanism behind this phenomenon is that as species accumulate, competition intensifies and fewer resources remain available for new colonists.

1. What are the characteristics that make a species a successful invader?
   In the field of invasion ecology it has been an exceedingly difficult task to predict which marine species will become "invasive" or "nuisance" species and which will not. It is well recognized that most non- native species do not survive where they are introduced and fewer become pests after they are transported to new habitats/regions. Explanations for this pattern generally focus on lack of tolerable environmental conditions or to the lack of an 'open' niche for the invader in a new habitat/region. Attempts have been made to predict life history traits of species that will be successful invaders and the focus has usually been on the number and size of propagules, age at first reproduction, organism growth rate, etc. Some studies have found correlations between life history traits and invasion success and others have suggested that attributes of the invaded community (i.e., disturbance levels, species diversity) affect its ability to resist exotic species invasion. Our approach has been to undertake comparative laboratory and field experimental studies of recent marine invaders into southern New England, with particular attention to shallow subtidal epifaunal organisms.
2. What effects do invaders have on resident fauna and flora?
   The focus of most biological invasions has been on competitive relationships between natives and exotics. While some of the more publicized invasions do have significant impacts on local biota, many apparently have little or no impact on native biota. At a time when marine invasions are rapidly increasing in frequency due to increased human mobility and trans-oceanic transport, there is insufficient understanding of what causes the differential susceptibility of marine communities to invasion. Our approach has been to use a combination of field and laboratory experiments to begin dissecting biotic and abiotic mechanisms which influence the interaction of invaders and resident fauna.
3. Why are some habitats apparently more vulnerable to invasion than other habitats?

Ecosystems which have reduced biodiversity or that are stressed by environmental degradation and climate change appear to be more vulnerable to invasions. For example, we have experimentally demonstrated that enhanced species diversity directly increases the resistance of subtidal fouling assemblages to invasion and that surveys in a number of coastal habitats in southern New England also revealed an inverse correlation between resident species richness and the number of non-native species in those habitats.

From Stachowicz, J.J., J.R. Terwin, R.B. Whitlatch, and R.W. Osman. 2002

Climate change has its greatest effects on promoting invasive species at the southern and north boundaries of ecosystem types. In addition to greater vulnerability to invasion, increasing temperature will increase the number of warmer water invaders into temperate regions. Temperate coastal regions appear to be the most vulnerable to the effects of climate change on invasion by non-native species. For example, we have correlated a doubling in the abundance of invasive species ascidians in eastern Long Island Sound to a significant increase in seawater temperatures over the past two decades in that region.

For more information on invasive species ecology and other marine benthic population dynamics, please visit the Dr. Whitlatch's Team Benthos website.