Native vs. Invasive:

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Native vs. Invasive

Tunicates in Immunology

Organisms of the Intertidal Zone

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            Throughout the world, marine invasions of non-native species are impacting the dynamics and relationships of established communities. There are cases of “natural” invasions, or invasions without human influence. These are possible when geographic barriers (e.g. glacial recession) or physiological barriers (e.g. climate change) are crossed (Ricciardi). However, long-distance ascidian invasions are primarily a result of anthropogenic activity such as boat traffic.

The zebra mussel has had numerous negative impacts on the Great Lakes, and has spread into the Mississippi and Hudson Rivers.

  In harbors, there is generally minimal species richness and higher pollution levels that result in greater environmental strain (Lopez-Legentil et al., 2006). This allows for opportunistic species to capitalize on the vulnerable conditions.

            It has been generally accepted that Botryllus schlosseri and Botrylloides violaceus are invasive or introduced species to the eastern Atlantic coastline. B. schlosseri, found on all continents except Antarctica, is believed to have originated from the Mediterranean Sea region (Stoner et al., 2002). First identified in England in 1766, the ascidian was most likely introduced to the eastern coast in the 1830s but was first reported in Brooklyn, NY, in 1871 (Ricciardi; Lopez-Legentil et al., 2006). Later observations were made in Australia and New Zealand (1928), Japan (1929), and San Francisco Bay (1940s) (Stoner et al., 2002).
 
B. violaceus is as abundant as B. schlosseri. The native range of B. violaceus is believed to be Japan and Southern Siberia to southern China (Cohen, 2005). Though there was confusion with collections and identification, the first clear records on the west coast of the United States are from the 1970s in San Francisco Bay. It was first inadvertently introduced to New England at approximately the same time by a scientist visiting Woods Hole, Massachusetts (Groshloz, 2001).

B. violaceus
surrounding B. schlosseri; Nahant, MA

Although these organisms usually first settle on man-made structures in harbors or marinas, it is possible for them to spread to open habitats and natural substrates (Lopez-Legentil et al., 2006). This movement was seen in Nahant in the intertidal zone. Between the observations and an intriguing paper (Stachowicz 2002) I studied, I decided to pursue the topic of invasive vs. native ascidians on the northeast coastline of the United States. This is what I learned:

In order to understand discussions relating to native vs. invasive species, I found these definitions to be beneficial. To begin, a native species is one that occurs naturally in a certain location while an invasive species has been introduced to an area. An endemic species is native solely to one place. Cryptogenic species are unable to be identified as native or introduced without further investigation (Carlton, in press). This was exemplified on the west coast when Botrylloides violaceus was indistinguishable from a similar species, Botrylloides diegensis, until 1997 (Cohen, 2005). A resident species is grouped as “native” once it has been present in its respective community for a certain amount of time (Carlton, in press).                                        Color morphs of B. schlosseri and B. violaceus; Nahant

    Stachowicz (2002) compared ascidian recruitment with interannual changes in water temperature. This was used to gauge how climate change affected the success of the introduced or invasive species. Three species used in the study were considered non-native, D. listerianum, B. violaceus, and A. aspersa, and three others were considered native, C. intestinalis, M. manhattensis, and B. schlosseri. However, B. schlosseri is generally accepted as an introduced species to the New England coastline. Stachowicz et al. (2002b) propose that due to the length of time B. schlosseri has been in the region, it has formed part of that resident community. This established community is what current invaders must contend with.

This theory, as well as the current hypothesis on the origins of B. schlosseri, has recently been reevaluated by James T. Carlton, a Professor at Williams College in Williamstown, MA. The new hypothesis suggests B. schlosseri as a native to the Pacific Ocean instead of the Mediterranean Sea or Europe (Carlton, written communication). This is supported by the fact that two Botryllus species are endemic to the Atlantic, while there are 13 endemic Botryllus species in the Pacific Ocean (Carlton, written communication). A similar hypothesis by Carlton has been made about, B. leachii, a close relative of B. violaceus.

The biogeography of ascidians has raised questions and concerns about determining the origins of cosmopolitan and cryptogenic species. One issue with unidentified species invasions is the increased possibility of hybridization with native organisms (Groshloz, 2002). The result would be loss of native genotypes, thus loss of biodiversity. Multilocus genotyping, such as microsatellite DNA use, are being developed in order to help define phylogenetic relationships of these cryptogenic species (Stoner et al. 2002; Groshloz, 2002). To make a global impact on the spread of invasive organisms, it would be necessary to evaluate phylogenetic assemblage by way of extensive genetic testing and morphology assays (Carlton, written communication).