Invasive Marine Seaweeds: Pest or Prize? - Seaweed Biology - page 233

12.3 The Identification of Introduced Seaweeds
Is Not Always Easy
Seaweeds have circumnavigated the globe for centuries. Unintentional introductions
by humans have occurred since the beginning of naval explorations and this may
explain a perceived “cosmopolitan distribution” reported in many “hull-fouling”
species (mostly small, ephemeral species that colonize ship's hulls; Godwin 1975 ;
Ruiz and Carlton 2003 ). A species' geographic distribution can be misinterpreted
especially in taxa with few diagnostic morphological characters (e.g., Ulva lactuca
Linnaeus, Bryopsis plumosa (Hudson) C. Agardh) for the following reasons: firstly,
most seaweed taxa have been described from a limited number of geographical
regions and/or expeditions; taxonomists in the past relied heavily on these already
characterized “type species” to identify new taxa. Secondly, although molecular
tools are today able to characterize genealogically distinct units, a significant
number of taxonomists still rely on the use of morphological characters for a routine
identification of species (Sherwood 2007 ).
The identification of even well-studied introduced seaweeds is not always
straightforward and the assessment of an introduction episode is extremely complex.
We will discuss here two important reasons. (1) Due to extreme phenotypic plasticity
of many seaweed genera, closely related algal species can be morphologically
almost identical and therefore difficult to distinguish. Furthermore, cryptic species
with separate geographic distributions have been observed within morphologically
homogeneous taxa. Cryptic invasion episodes may therefore occur and are likely to
lead to (a) misidentified biologically distinct invasive species due to morphological
similarities with natives or (b) misidentified genealogically distinct lineages of
invasive character found within a single morpho-species complex. Although foren-
sic techniques may provide the necessary tools to achieve a reliable and quick
identification of the invader, results are often compromised by the absence of
sufficient DNA sequence data for comparisons and species identification (Box
12.1 ), even in some phylogenetically important groups of seaweeds (e.g., Ulva and
Bryopsis ). Furthermore, a species concept to describe the taxonomical units within
these groups is often nonexistent. (2) A large number of species are today recognized
as “cryptogenic” (i.e., there is no reliable evidence to indicate whether a species is
native or introduced, Carlton 1996 ); because human-mediated relocations of marine
biota have occurred since historical times while archeological or paleontological
evidence related to the geographical origins of many soft-body marine organisms
is largely absent. A number of biogeographical criteria have been proposed to
distinguish between native and introduced taxa (Geller et al. 2010 ) and a species'
cryptogenic history can be resolved in some cases where biological invasions have
been systematically studied, e.g., Ulva flexuosa Wulfen cited as Enteromorpha
lingulata J Agardh, Caloglossa leprieurii (Montagne) G. Martens, Bostrychia
radicans (Montagne) Montagne in southern Brazil (Neves and Rocha 2008 ).
The effective management of a biological invasion requires a solid knowledge of
(1) which species has been introduced, (2) the source of the introduction, and (3) the