Biology Reference
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distinguish between native and invasive cryptic taxa where more than one lineage
was found in sympatry. For instance, lineages 1, 2, and 4 co-occur in the Hawaiian
archipelago [lineage 2 is considered native and lineage 4 introduced to the islands
as recently as in 1991 (Sherwood
2008
)]. Lineages 1 and 4 co-occur sympatrically
within hundreds of meters in the Pacific side of the Isthmus of Panama and they
may represent multiple introduction events. Lineages 2 and 3, the latter of Atlantic
distribution, are found in South Africa and are considered introduced and invasive,
respectively (Bolton et al.
2011
).
Individuals with a
A. taxiformis
morphology collected in unrelated geographic
locations have in the past been described as separate species. This observation,
together with the multiple ESUs found in
A. taxiformis
, strongly suggests that the
taxonomic status of the species sensu stricto needs to be revisited. For instance, one
of these lineages may correspond to
A. svedelii
W.R. Taylor (1945), an endemic
taxon of the Gal´pagos Islands, considered synonymous to
A. taxiformis
(Taylor
1945
), while cryptic lineage 2 may correspond to
A. sanfordiana
Harvey (Setchell
and Gardner
1924
), another species considered to be a synonym of
A. taxiformis
.
12.3.6 Case Study:
Codium Fragile
ssp.
Tomentosoides
A single strain of the green alga
Codium fragile
sensu stricto known as
Codium
fragile
ssp.
tomentosoides
(Fig.
12.4a
) is recognized as highly invasive. DNA
sequence analysis based on plastid microsatellites and the plastid
psb
J-
psb
L
,
rpl
16-
Rps
3, and
trn
G(UCC)-5 S DNA regions of individuals collected throughout
the strain's known native and introduced distribution range indicated extremely low
levels of genetic variation with only four distinct haplotypes encountered in Japan
where the strain occurs naturally. Despite the limited levels of genetic variability,
the phylogenetic signal was sufficient to uncover at least two separate introductions
in the biogeographic history of
C. fragile
ssp.
tomentosoides
: two of these
haplotypes were highly structured geographically and were found in introduced
populations, one exclusively in the Mediterranean Sea, and the other exclusively in
Northwest Atlantic, Northern European, and South Pacific populations (Provan
et al.
2005
,
2008
).
12.3.7 Case Study:
Caulerpa
spp.
The green algal genus
Caulerpa
comprises morpho-species complexes, species that
may become invasive under certain conditions and ESUs known from the literature
as “invasive strains” (e.g., described in
C. taxifolia
sensu stricto
)
or “invasive
varieties” [e.g., described in
C. racemosa
(Forssk
˚
l) J. Agardh sensu stricto (Stam
et al.
2006
)]. For instance, a potentially invasive species,
Caulerpa prolifera
(Forssk
˚
l) J.V. Lamouroux
,
is widely distributed in tropical and subtropical seas
and has been significantly altering the structure and functioning in its introduced
