Journal of Plankton Research Vol.19 no.6 pp.749-757, 1997
SHORT COMMUNICATION
Morphological variability in three populations of the dinoflagellate
Alexandrium taylori
Maximino Delgado, Esther Garc6s, Magda Vila and Jordi Camp
Institute) de Ciencias del Mar, Ps. Joan de Borbd s/n, 08039 Barcelona, Spain
Abstract. Water discolourations were observed in two localities of the NW Mediterranean (La Fosca
and Palmira, Spain) during the summer of 1994 and 1995 caused by a non-toxic dinoflagellate belonging to the genus Alexandrium, subgenus Gessnerium. The organism was identified as Alexandrium
taylori Balech, although differences were encountered compared to the original description from
Arcachon (France). Detailed studies of the 1' plate were conducted on the Spanish material and compared with specimens from the type locality. Differences reported here between the Spanish and
French populations are considered to be within the range of the morphological variability of this
taxon.
During the summer of 1994, a green patch was seen off the tourist beach La Fosca
(Costa Brava, Spain) caused by a dinoflagellate belonging to the genus Alexandrium. The waters were characterized by high temperatures and salinities (26°C
and 38 p.s.u., respectively). Owing to the occurrence of previous toxic events of
paralytic shellfish poisoning caused by Alexandrium species off the Catalan coast
(Delgado et al, 1990), mussels were immediately collected from wild banks for
toxicity analysis. The result of toxicity analysis by mouse bioassay (Association of
Official Analytical Chemists, 1990) in two samples was negative. The dinoflagellate was kindly identified as Alexandrium taylori by Professor E.Balech. This
species is included in the subgenus Gessnerium, characterized by the V plate disconnected from the apical pore complex.
The summer of the next year (1995), green patches were seen again off the same
locality (La Fosca) and also off the beach of Palmira (Mallorca), both caused by
the same species as previously. The plate pattern of specimens from the Spanish
localities revealed some differences with regard to the original description of
A.taylori (Balech, 1994). Further detailed studies of the 1' plate were conducted
on the Spanish samples in comparison to specimens from the type locality of the
species (Arcachon, France). In addition, we present scanning electron microscope
(SEM) images of A.taylori from the type locality, because of the lack of published
photographs of this species, which complement the description.
The material studied came from three locations, coinciding with water discolourations: La Fosca (Costa Brava, Spain) collected on 12 July 1994, Palmira
(Mallorca, Spain) collected on 4 August 1995 and Arcachon (France) collected on
22 August 1985. The samples were centrifuged at 1000 r.p.m. for 5 min to concentrate the dinoflagellates. The resulting pellet was suspended in a solution of the
fluorescent dye calcofluor white (Fritz and Triemer, 1985). A small amount of the
cell suspensions was placed on microscope slides. One drop of sodium hypochlorite was added to separate the thecal plates. The slides were examined under
1500 X magnification in a Leitz DM EL inverted microscope with fluorescence
© Oxford University Press
749
MJ>dgado el al.
illumination. A fixed number (20) of 1' plates (first apical) were drawn using a
camera lucida for each of the three samples. Measurements of the length and transdiameter of the cells were also made on each sample using an image-analysis
system (Microm) attached to a Nikon Diaphot TMD microscope. In these
measurements, the width of the cells was excluded due to the variability in the ends
of the cingular lists, depending on the robustness of the theca. For SEM examination, water samples fixed with glutaraldehyde (2%) were filtered through Nuclepore polycarbonate filters (2 um pore, 13 mm diameter) at low pressure. The filters
were dehydrated according to Paerl and Shimp (1973), critical point dried,
mounted on stubs and coated with gold-paladium. The examination was carried
out at 15 kV using an Hitachi S-570 apparatus The terminology used in this work
on the morphology of the organisms is adopted from Balech (1995).
Figures 1-3 present photographs of specimens from La Fosca. In Figures 1 and
2, the 1' plate is four sided, lacks the left anterior border, and the ventral pore is
located at the confluence of the 1', 1", 2' and 4' plates. The 6" plate is longer than
wide in bothfigures.The Sa plate (Figure 2) has a left margin which ends no longer
than the left posterior margin of the 1' plate in the complete theca. The presence
of a very small plate in the ventral pore is shown in Figure 2. The Sp plate (Figure
3), which is important to differentiate A.taylori from other species of the subgenus
Gessnerium, agrees with the type description (Balech, 1994). The specimens of La
Fosca (and also of Palmira) showed a thin easily collapsible theca, which makes
the examination of plates very difficult.
As a part of its life cycle, this dinoflagellate produces two types of cysts. Temporary cysts (Figure 4a) are usually found in the water column and the sediment
from La Fosca, germinating into motile vegetative cells in a short period of time
(days). They are usually spherical, 35-40 um diameter, but elongated forms have
also been seen (mainly in the laboratory after some days). The cell contents are
similar to vegetative cells. The wall of the cyst is very thin and without markings,
lacking plates. Sexual resting cysts have recently been observed (Figure 4b) in a
sample of sediment from La Fosca maintained in the laboratory for 1 year. The
resting cysts are spherical, 40-45 (i.m in diameter, with clear globular content, with
a thicker wall than the temporary cysts, and with a red accumulation body. The
walls of the resting cysts stain weakly with calcofluor dye.
The outline of the 1' plate in specimens from all three localities exhibited significant morphological variability (Figure 5). This variability involved the size of
the plate, the number of margins, the angles between the margins and the presence or absence of a small notch related with the ventral pore. Particularly important was the presence of a left anterior margin, which was present in few
specimens from the Spanish localities (eight in La Fosca and two in Palmira),
whereas it was present in the majority of specimens from Arcachon (17), out of
a total of 20 cases studied in each locality. The specimens of Arcachon showed
larger 1' plates. This result also agrees with the measurements of cell dimensions
conducted on samples from the three localities (Table I) showing smaller cell sizes
in the two Spanish localities compared to Arcachon.
A summary of the variability found in the three populations is presented in
Figure 6 as drawings with camera lucida on selected squashed specimens
750
Morphological variability of Alexandrium taylori
Figs 1-4. (LM) Alexandrium taylori from La Fosca. Fig. 1: detail of the outline of the 1' plate, connecting plates and the location of the ventral pore (arrow). Calcofluor stain. Fig. 2z squashed epithecal
plates. Calcofluor stain and hypochlorite treatment Fig. 3: detail of the Sp plate. Calcofluor stain and
hypochlorite treatment. Fig. 4: two types of cysts, temporary cysts (a) and resting cysts (b). All scale
bars are 10 um.
involving the 1' plate and adjacent plates. These drawings include the 1' and 6"
plates, the location of the ventral pore, the lines of connection between 1", 2' and
4' plates and the Sa plate. When a left anterior margin on the 1' plate is present,
the ventral pore does not contact the V plate (Figure 6b, c, f, h, i), but when this
margin is absent the ventral pore is located at the confluence of the 1', 1", 2' and
4' plates (Figure 6a, d, e, g).
751
MDdgado et al.
La Fosca
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3
4
5
6
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8
9
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12
13
14
15
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Palmira
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Fig. 5. Drawings of the Y plate of A.taylori dissected from 20 specimens of the three studied localities: La Fosca (1-20), Palmira (21^0) and the type locality of Arcachon (41-60). Calcofluor stain and
hypochlorite treatment. Scale bar 10 urn.
Table L Measurements (urn) of cell length and transdiameter of A.taylori from the three localities
Arcachon
Palmira
La Fosca
Number of cells
58
50
50
Length
Minimum
Maximum
Mean
SD
Median
29.5
44.1
36.3
3.2
36.5
26.8
42.7
33J
3.7
33.0
26.3
42.7
33.6
3.8
33.1
Transdiameter
Minimum
Maximum
Mean
SD
Median
28.5
44.0
34.8
3.7
34.9
24.5
433
33.1
3.8
32.7
24.9
42.1
32.9
3.6
32.4
Figures 7-12 show SEM micrographs of the theca of A.taylori from the type
locality. TTie plates are perforated by numerous densely distributed minute pores
(also present in the specimens of La Fosca and Palmira), which are difficult to
752
Morphological variabffity of Alexandrium lay tori
La Fosca
Palmira
Fig. 6. Drawings of the 1', 6' and Sa plates, the location of the ventral pore and lines of connection
with adjacent plates of A.taylori in specimens from the three studied localities. Selected specimens
which illustrate the summary of the variability. Scale bar 10 um.
resolve by light microscopy (LM). In Figure 7, it is possible to recognize the plate
pattern of the epitheca. A large displacement between the 1' plate and Po is shown.
Plates l'and 6" are rather long, and plate 3" is the smallest among the precingular series. Although the ventral pore is usually present, and excavated into the 4'
plate (Figure 9), it can be absent in some specimens as presented in Figure 7. The
hypotheca (Figure 8) shows a characteristic Sp plate, which is long and narrow,
with a V-shaped anterior notch that slopes to the right. A large left anterior border
of the 1' plate is present in the specimen in Figure 9 (with thicker and wider 1' and
6" plates) when compared to the specimen in Figure 7. Hie Sa plate is very narrow
(Figure 10), with a thickening in the middle of the plate, often observed in LM as
a line of brightness with calcofluor staining. A small notch is present in the anterior
portion of 1' plate (Figure 11), associated with the location of the ventral pore. The
Po plate (Figure 12) has a well-delineated comma shape, covered by a canopy, with
marginal pores of the same size as the pores in other plates.
753
M.Delgado el al.
Figs 7 and 8. (SEM) Alexandrium taylori from the type locality of Arcachon. Fig. 7: plate pattern of
the epitheca. A large displacement of the 1' plate from Po is shown. The 3" plate is the smaller of the
precingular series. In this specimen, the ventral pore is absent. Fig. & plate pattern of the hypothecs,
showing a characteristic Sp plate, which is long and narrow, with a V-shaped anterior notch that slopes
to the right Scale bars 10 |im.
754
Morphological variability at Alexandrium taylori
Figs 9-12. (SEM) Alexandrium taylori from the type locality of Arcachon. Fig. 9: detail of the 1' plate
and adjacent plates. A large left anterior margin of the 1' plate is shown. A large ventral pore (arrow)
is present, notched mainly in the 4' plate. Fig. 10: detail of the Sa plate, with a thickening in the middle
of the plate, lacking a straight and hori2ontal left portion. Fig. 11: detail of the 1' and 6* plates. A small
notch is present in the anterior vertex of the 1' plate, related to the location of the ventral pore. Fig.
11 detail of the Po plate, with a well-delineated comma, covered by a canopy, and showing the marginal pores. Scale bars 5 urn in Figures 9 and 11,1 (im in Figures 10 and 12.
755
MDelgado el al.
The present study of the variability in the 1' plate has been essential to elucidate the taxonomic position of this dinoflagellate in the Spanish localities (La
Fosca and Palmira). Most of the 1' plates from the Spanish localities lacked the
left anterior margin (60% in La Fosca and 90% in Palmira). This morphology
differs from the original description of A.taylori (Balech, 1994) in which the 1'
plate is pentagonal, with two anterior margins and the ventral pore located at the
anterior portion of the plate without contact with the 2' plate. Nevertheless, the
presence of 1' plates without the left anterior margin was also observed in a few
specimens of A.taylori from the type locality. Other differences were found in the
three studied populations compared to the type description (Balech, 1994).
Although the 3" plate is the smallest plate in the precingular series, in the present
work this plate was not as small as in the type description. The 6' plate was also
narrower than in the first description. Descriptions of both plates 3" and 6" have
been modified in the later Alexandrium monograph (Balech, 1995). The Sa plate
in our observations was always very short in the three localities, with a thickening at the junction of the elements, as the SEM photographs document, and
lacking a straight and horizontal portion as described by Balech (1994).
The location of the ventral pore is another valuable morphological taxonomic
character used to differentiate A.taylori from other species. Although the ventral
pore is large, and easy to observe in most of the cells, in some specimens we were
not able to see it despite the staining with calcofluor. This occurred mainly in
Spanish specimens (La Fosca and Palmira), but also in some specimens of the type
locality, as Figure 7 illustrates. Moreover, the location of the ventral pore in relation
to the 1" plate depends on the presence of a left anterior margin on the 1' plate. The
existence of cells of A.taylori with two ventral pores, illustrated by Balech (1995)
and observed by us, and the presence of a very small plate in the ventral pore (we
have only observed this in two specimens), are considered isolated exceptions.
It is noteworthy that the higher proportion of cells with a left anterior margin
of plate 1' in the sample from Arcachon, which is the type locality of the species,
was related to a large size of the 1' plate and also larger sized cells. This result
leads us to consider that the degree of robustness of the cell cover can be an
important source of the morphological variability observed. When a theca is
robust, the plates are wider and also increase the overlap between plates. Owing
to the overlapping of therims,the outline of the plates can thus be different when
observing the complete theca (i.e. as observed with phase contrast or with calcofluor staining) than when the plates are disjoined (i.e. after hypochlorite treatment). One peculiarity of the 1' plate is the location under the contiguous plates,
except for the posterior margin which is over the Sa plate.
Furthermore, the weakness of the theca in the Spanish specimens can be related
to the production of temporary cysts. In La Fosca, the vegetative cells were abundant in the water column in the morning and during the afternoon temporary cysts
were produced, without plates, cingulum and sulcus. The rapid turnover between
vegetative cells and temporary cysts could be the cause of the weakness of the theca.
Such temporary cysts have been described previously in Alexandrium hiranoi (Kita
et al, 1985; Kita and Fukuyo, 1988) and Alexandrium pseudogonyaulax (Montresor, 1995), both species also included in the subgenus Gessnerium.
756
Morphological variability of Alexandrium taylori
Acknowledgements
We acknowledge Prof. Enrique Balech for the identification of the species and
Santiago Fraga for comments on the manuscript. Thanks to Dr Elizabeth Nezan
for the sample from Arcachon, to Vicens Forteza for the sample from Palmira,
and to Dr Carles Sanvicente and Mariona de Torres for the samples from La
Fosca. Jos£-Manuel Fortufio and Joan Biosca provided technical assistance. This
work was funded by Junta de Sanejament (Generalitat de Catalunya) and C.S.I.C.
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Received on July 22,1996; accepted on January 23,1997
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