Biology Reference
In-Depth Information
various locations worldwide (Stam et al.
2006
).
Caulerpa taxifolia
sensu stricto
cannot be distinguished morphologically from this invasive ESU which, following
accidental introductions, has successfully established in temperate areas such as the
United States, Mediterranean Sea, and southern Australia. Eradication attempts in
Australia and the United States (the latter seems to have been successful) and an
international ban of this species in the aquarium trade were the initial management
responses (Anderson
2007
).
The same nuclear marker has been successfully used to uncover the now well-
known invasive variety of
Caulerpa racemosa
(
C. racemosa
var. cylindracea
(Sonder) Verlaque, Huisman and Boudouresque; Fig.
12.4c
) against two other
morphologically similar, and apparently noninvasive, sister varieties within the
introduced range of the
C. racemosa
complex (
C. racemosa
var.
turbinata-uvifera
(C. Agardh) J. Agardh,
C. racemosa
var.
lamourouxii
(Turner) Weber-van Bosse f.
requienii
(Montagne) Weber-van Bosse; varieties reviewed by Klein and Verlaque
2008
). The approach was particularly useful in the Mediterranean basin where other
C. racemosa
varieties have been erroneously misidentified as the invasive variety
(Verlaque et al.
2003
, Yeh and Chen
2004
; Nuber et al.
2007
). Although the invasive
lineage represents a dominant algal component in the Mediterranean basin, its
identity has been definitely confirmed by combining morphological and molecular
data in only five of the 13Mediterranean countries in which
C. racemosa
sensu stricto
has been reported so far (France, Italy, Greece, Croatia, and Cyprus; Klein and
Verlaque
2008
). This ESU has been introduced into the basin from south-western
Australia. Due to its well-documented negative ecological impact, this invasion is
considered as one of the most serious biological invasions in the history of species
introduced into the basin (Klein and Verlaque
2008
).
12.3.8 Assessing Seaweed Introductions
In recent years, several conspicuous seaweed invasions have been detected. Our
case studies illustrate that introduced seaweeds may be either genetically homoge-
neous or species consisting of multiple cryptic lineages.
Populations of taxa in the former category are characterized by weak phyloge-
netic signals and insufficient phylogeographic structuring between the native and
introduced geographical range of the invasive species. Such introduced species are
only easy to detect based on morphology if detailed floristic records exist in a
particular region. It is however impossible to identify the source populations
and the subsequent direction of the invasion based on either morphological
identifications or genetic approaches.
Morphologically cryptic species often occur within so-called widely distributed
taxa characterized by large geographic disjunctions and apparently low levels
of connectivity among populations. The morphological delineation of these ESUs is
often impossible even for the expert taxonomist; most importantly, it is difficult to
distinguish between a recent introduction and the tendency of the species to disperse
