Two new species of Dictyota - Phycology Research Group, Ghent ...

Phycologia (1999) Volume 38 (3), 184-194 Published 16 August 1999
Two new species of Dictyota (Dictyotales, Phaeophyta) from the
Indo-Malayan region
O. DE CLERCK* AND E. COPPEJANS
Laboratory of Botany, University of Gent, K.L. Ledeganckstraat 35, 9000 Gent, Belgium
O. DE CLERCK AND E. COPPEJANS. 1999. Two new species of Dictyota (Dictyotales, Phaeophyta) fram the Indo-Malayan
region. Phycologia 38: 184-194.
Two new species of Dictyota (Dictyotales, Phaeophyta) are described fram the Indian Ocean, Indonesia, and Papua New
Guinea. Dictyota grossedentata is a pracumbent species with large, irregularly placed marginal teeth, differing from aH other
prastrate species by its stout habit and conspicuous dentation. Dictyota grossedentata differs from other species characterized
by dentate margins by its prastrate habit and by the lack of a single, conspicuous stupose base. Dictyota rigida is a stiff,
completely erect species with a single point of attachment. Because of its stiff texture, it resembles D. jastigiata Sonder
and D. suhrii Murray but differs by its uni1ayered meduHa. Dictyota rigida also is compared with the other slender taxa.
Taxonomic prablems of the genus, such as substantial morphological variation and a lack of clearly discriminating characters,
are considered.
INTRODUCTION
Species of Dictyota are cornmon components of tropical and
subtropical marine fioras, but many species lack easily defined
presence-absence characters and demonstrate substantial morphological
variability [e.g. Dictyota dichotoma (Hudson) Lamouroux].
These problems have hampered taxonomic and fioristic
studies over the past two centuries. The lack of understanding
of the variability of vegetative characters has resulted
in developmental and morphological forms being described as
different species (Phillips 1992). With the work of Weber­
Peukert (1985), Hornig & Schnetter (1988), and Hornig et al.
(1992a, 1992b), taxonomists again showed an interest in the
genus, culminating in a revision of the north Atlantic Ocean
species. Phillips (1992) made a detailed study of the Australian
representatives of the genus Dilophus. These studies have
led to a better understanding of the morphological variability
of the characters within this group and their proper application
in distinguishing species. Weber-Peukert (1985) emphasized
the use of cell dimensions and ratios of dimensions to distinguish
certain species. All species of the genus have a single
meristematic apical cell, and vegetative cells do not undergo
further division once the final strap dimensions are reached in
the subapical regions. Hence, elongation of the interdichotomies
is exclusively the result of cell elongation. For this reason,
cell sizes could represent a good taxonomic character in
mature parts of the thallus. Phillips (1992) proved the value
of the number of sporangial stalk cells, the diameter of the
divided sporangium, and the size of the spores as taxonomic
characters. The number of sporangial stalk cells was especially
useful for discriminating among notoriously difficult
species [e.g. D. fastigiata Sonder and D. gunniana (J. Agardh)
Hornig, Schnetter et Prud'homme van Reine]. De Clerck &
Coppejans (1997a, 1997b) gave these new systematic insights
a more stable nomenclatural basis after the study of several
* Correspondence and requests for reprints should go to this author.
E-mail: olivier.declerck@rug.ac.be
type collections (Herb. Agardh, LD; Herb. Kützing, L; Herb.
Lamouroux, CN; Herb. Sonder, MEL).
The concept of Dictyota used in this paper is that of Hornig
et al. (1992a), which includes species characterized by a single
lenticular, transversely orientated apical cell and a parenchymatous
thallus composed of a cortex and a medulla. The
number of medullary layers shows considerable variation between
the species. Sorne species may consist entirely of a
unilayered or multilayered medulla, whereas in other species,
a multilayered medulla is restricted to the base, the stoloniferous
holdfasts, or the thallus margins. In the past, taxa with
a multilayered medulla were referred to the genus Dilophus.
However, numerous species were difficult to assign to either
Dictyota or Dilophus because of the presence of a multilayered
medulla in a specific part of the thallus (e.g. stoloniferous
holdfasts) or sporadic duplications of medullary cells (Setchell
& Gardner 1925; Dawson 1950). Eventually, both genera were
merged, based on culture experiments demonstrating that the
number of medullary layers can be altered in many species
depending on the culture conditions (Hornig et al. 1992a).
Phillips (1992) stated that Dilophus should be merged with
Dictyota based on morphological and reproductive characters
but preferred to retain Dilophus as a separate genus until the
taxonomy of Dictyota was clarified.
Silva et al. (1996) include Dilophus in Dictyota and list a
total of 35 previously reported species for the Indian Ocean;
many of these may be nothing more than misapplied names.
Most publications on Dictyota from this region provide short,
uninformative descriptions without indication of voucher
specimens. The only detailed accounts for the Indian Ocean
were by Jaasund (1970) who recognized nine species for Tanzania,
Womersley (1987) who listed many collections from
the west coast of Australia, and Phillips (1992) on Australian
species characterized by a multilayered medulla. The first author
of this paper has prepared a revision of the genus in the
Indian Ocean based on a morphological-anatomical analysis
of all representatives combined with molecular data of certain
184

De Clerck & Coppejans: Two new species of Dictyota 185
2
1 cm
-
Figs 1-4. External morphology of Dictyota grossedentata.
Fig. 1. Holotype of D. grossedentata (HEC 12060) from Tanzania.
Fig. 2. Habit of a broad specimen from Papua New Guinea (ODC 252).
Fig. 3. Detail of an interdichotomy with sporangia and marginal teeth (ODC 252).
Fig. 4. Detail of an apical segment with lateral patches of rhizoids and rounded apices (ODC 252).
species (De Clerck 1999). This revision has lead to the recognition
of 23 species of Dictyota for the Indian Ocean. Two
species are new to science and are presented in this papero
MATERIALS AND METHODS
The specimens were collected by SCDBA or snorkeling during
several collecting trips in the Indian Ocean, Indonesia, and
Papua New Guinea since 1980. Material was prepared as herbarium
specimens or preserved in 4% formalin/seawater.
Vouchers are lodged in GENT. Resin-embedded material was
used for transverse sections. Small pieces of thallus were dehydrated
in alcohol using 20% increments and subsequently
infiltrated with Technovit 7100. Sections were cut on a Reichert-Jung
Autocut 2040 microtome, mounted on Vectabond
slides and stained with toluidine blue O.
The morphological data are the result of an extensive morphometric
analysis. Length and width of the interdichotomies,
the part of the thallus between two successive dichotomies,
were measured up to 0.5 mm. Approximately five to 25 interdichotomies
were measured per specimen. This procedure
was repeated for each specimen listed. From the combined
data, minimum and maximum sizes were determined as well
as the 95% confidence limits. For each specimen, the width
and length of 50 cortical and medullary cells were measured
from surface view of an interdichotomy situated well below
the apex. Minimum and maximum sizes and the 95% confidence
limits were determined in a way similar to that used
for interdichotomies. The height of cortical and medullary
cells was measured from transverse sections. The diameter of
the sporangia and the size of the gametangia were measured
from surface view.
OBSERVATIONS
Dictyota grossedentata De Clerck et Coppejans sp. nov.
Figs 1-9
DIAGNOSIS: Planta ad 3-6 cm langa, prostrata, valde crispata; pagina
inferiore rhizoideis ad substratum adhaerenti, sine baside conspicua;
ramificatio regulariter dichotoma, angulis (60-)80(-90t; segmenta
latitudine constanti, interdum basin versus paulo angustata: (5.5-)
8.3-9.6(-13.0) mm langa, (1.8-)3.3-4.5(-7.3) mm lata, ratione:
(2.1-)2.3-2.8(-3.6); apicibus rotundatis ad obtusis; marginibus
grosse dentatis; dentibus triangularibus, non directus apicem versus;
color fuscus, iridescens; cortex monostromaticus, cellulis (17-)43­
46(-77) j.Lm longis, (12-)23-25(-41) j.Lm latis, ratione: (1.6-)1.8­
2.0(-2.2), et (20-)21-25(-28) j.Lm altis; medulla ubique monostromatica,
cellulis (53-)199-211(-312) j.Lm longis, (98-)127-138
(-230) j.Lm latis, ratione: (1.4-)1.5-1.6(-1.7), ad 390 j.Lm altis; te-

186 Phycologia, Vol. 38 (3), 1999
trasporangia in tota pagina superiore dispersa, sine cellulis involucralibus,
ad 95-120 fLm diametra; antheridia in tota pagina superiore,
in soris disposita, sori 3-4 seriebus paraphysium unicellularum
circumcincti, antheridia 50 f.Lm alta, 24 f.Lm lata; oogonia observata
unica immatura.
Thalli 3-6 cm long, prostrate, very crisp; attached by means
of patches of rhizoids wherever the thallus is in contact with
the substrate, lacking a conspicuous base; branching regularly
dichotomous, in a single plane, angle (60-)80(-90t; interdichotomies
of constant width, sometimes slightly tapering toward
the base: (5.5-)8.3-9.6(-13.0) mm long, (1.8-)3.3-4.5
(-7.3) mm wide, ratio: (2.1-)2.3-2.8(-3.6); apices rounded to
obtuse; margins coarsely dentate; teeth triangular and not apically
directed; color brown, iridescent; cortex unilayered, cells
(17-)43-46(-77) /-Lm long, (12-)23-25(-41) f.Lm wide, ratio:
(1.6-)1.8-2.0(-2.2), and (20-)21-25(-28) f.Lm high; medulla
unilayered throughout the whole thallus, cells (53-)199-211
(-312) f.Lm long, (98-)127-138(-230) /-Lm wide, ratio
(1.4-)1.5-1.6(-1.7), to 390 f.Lm high; tetrasporangia exclusively
on the upper surface of the thallus, not grouped in sori,
not surrounded by an involucrum, to 95-120 f.Lm in diameter;
antheridia grouped in sori, sunounded by three or four rows
of paraphyses, antheridia 50 /-Lm high, 24 f.Lm wide; oogonia
only observed in an immature state.
HOLOTYPE: Kombe South, Matemwe, Zanzibar, Tanzania,
10-15 m, on vertical walls of coral heads, outer reef slope
(Coppejans & Dargent, 26 July 1997; GENT HEC 12060)
(Fig. 1).
ETYMOLOGY: Named for the conspicuous, coarse marginal
teeth.
DISTRIBUTION AND HABITAT: Recorded from severallocalities
in the Indian Ocean (Maldives, Sri Lanka, Tanzania), Indonesia,
and Papua New Guinea. Usually growing on coral or
algal turf in deep water on the outer reef slope, 5-25 m deep.
Sorne specimens were collected in shallower water (0.5-3 m
at low tide) and many of these were growing epiphytically.
OTHER SPECIMENS EXAMINED: Maldives: Male Atoll, Bi Ya
Doo, 0.5 m depth at low tide (Coppejans, 7 April 1986; HEC
6164); Tari Atoll, Ellaidho, 20-30 m (Coppejans, 16 January
1999; HEC 12563). Papua New Guinea, Port Moresby area:
Bootless Inlet, Osborne Point, 1 m (De Clerck & Coppejans,
2 August 1994; ODC 271); Loloata Island, 10 m (De Clerck
& Coppejans, 27 July 1994; ODC 237); Loloata Island, 10 m
(De Clerck & Coppejans, 31 July 1994; ODC 261); Loloata
Island, 5-14 m (De Clerck & Coppejans, 4 August 1994;
ODC 278); Motupore Island, 1-3 m (Coppejans, 5 Ju1y 1986;
HEC 6307); Motupore Island, 7 m (De Clerck & Coppejans,
19 July 1994; ODC 212); Patch reef E of Loloata Island, 7 m
(De Clerck & Coppejans, 29 July 1994; ODC 252); South
Patch Reef, 5-25 m (De Clerck & Coppejans, 28 July 1994;
ODC 244). Sri Lanka: Negombo, coral reef, 25 m (Coppejans,
29 January 1997; HEC 11858). Tanzania: Zanzibar, Chumbe
Island, 10 m (De Clerck & Coppejans, 5 September 94; ODC
367). Indonesia: Selayar (Coppejans, Prud'homme van Reine
& Heijs, 28-29 September 1984; Snellius-II 11554C).
THALLUS STRUCTURE: Plants are procumbent and grow mainly
on algal turf or coral branches. Thalli are crisp and exhibit
a vivid greyish blue iridescence in situ. The iridescence is not
evenly dispersed over the whole thallus surface but forms a
banded pattern of narrow transverse lines. Upon drying, the
color changes from medium brown to chestnut brown. The
exact size of the thallus is difficult to assess because of the
prostrate habit but is generally 3-6 cm long. Branching is in
a single plane and regularly dichotomous. Only when the
growth of the thallus is physically hindered by the substrate
or when the plant is damaged will the outline of the thallus
become inegular. In such cases marginal teeth will form proliferations
that grow into new straps. The width of the straps
is almost uniform from the apical to the basal parts, but may
vary between specimens. The nanowest specimen (ODC 252)
is only 2 mm wide; an exceptionally broad specimen (Fig. 2)
(HEC 6307) is up to 7.5 mm wide, but the average is between
3.3 and 4.5 mm. The average length of the interdichotomies
is more constant, most of the interdichotomies being between
8.3 and 9.6 mm long. The combination of a variable width
and a more constant length of the interdichotomies results in
a rather variable length : width ratio of 2.3-2.8, with extremes
of 2.1 and 3.6 in very broad and narrow thalli, respectively.
The shape of the apices (Fig. 4) also differs between nanow
and broad specimens, being rounded in the former and more
obtuse in the latter. Teeth are strongly developed along the
apical interdichotomies. Toward the lower parts of the thallus,
the dentation is less conspicuous or even absent. Teeth are
broadly triangular (c. 700 f.Lm wide at the base, 320 f.Lm high)
and not apically directed (Fig. 3). A transverse section of the
thallus reveals a typical Dictyota structure composed of a unilayered
cortex sunounding a unilayered medulla (Figs 5, 6).
The cortex cells are rather thick-walled and possess a conspicuous
belt-like thickening running around the cell pm:allel
to the thallus surface (Fig. 7). Although present in all Dictyota
species, these thickenings are rarely as pronounced as in D.
grossedentata. Medullary cells are large compared to those of
other species of similar habit size, being 199-211 /-Lm long,
127-138 /-Lm wide, and up to 390 f.Lm high near the basal
parts of the thallus. Sporangia only develop from the upper
surface, which is typical of prostrate species. They are mainly
dispersed over the middle and lower parts of the thallus, leaving
a nanow sterile band near the margins. The apical and
subapical interdichotomies are generally devoid of sporangia.
Sporangia are not grouped in sori, are borne on a single stalk
cell, and lack an involucrum (Fig. 8). Mature sporangia are
up to 95-120 f.Lm in diameter. A single male herbarium specimen
was found (ODC 244), embedded and sectioned. Sori of
antheridia are distributed evenly over the upper thallus surface
and measure 280-450 f.Lm long and 155-230 f.Lm wide (Fig.
9). Adjacent sori generally do not fuse. Each sorus contains
lOto 25 antheridia and is sunounded by three or four rows
of unicellular paraphyses, which are approximately as long as
the antheridia. Antheridia are 50 f.Lm high and 24 f.Lm wide.
A single young female gametophyte (ODC 237) was observed,
the dimensions of its sori and oogonia were not representative
because of the thallus' irnmaturity. Each sorus contained
15-25 oogonia ananged in longitudinal lines.
REMARKS: The most conspicuous characters defining D.
grossedentata are the dentate margin, the shape of the teeth,
and the prostrate habito The combination of these characters
makes this species clearly distinct from all other dentate species,
such as D. ciliolata Sonder ex Kützing, D. crenulata J.
Agardh, D. hauckiana Nizamuddin, and D. spinulosa Harvey.
Besides the fact that all these species are characterized by an
exclusively erect thallus having a single point of attachment,
other characters help to distinguish them from D. grosseden-

De Clerck & Coppejans: Two new species of Dictyota 187
---
50 IJm
9
Figs 5-9. Anatomy of Dictyota grossedentata.
Fig. 5. Transverse section showing medullary and cortical cells (REC 10260).
Fig. 6. Transverse section of the margin (ODC 271).
Fig. 7. Detail of the belt-like thickenings of the cortical cells (ODC 271).
Fig. 8. Transverse section of a sporangium (ODC 271).
Fig. 9. Transverse section of a male antheridial sorus, with sUlTounding paraphyses (ODC 244).
tata (Table 1). Dictyota hauckiana Nizamuddin, a species endemic
to the northem Indian Ocean (Bangladesh, Burma, India,
Pakistan, and Sri Lanka), is up to 50 cm long, with conspicuous
ciliate teeth directed toward the apices (Hauck 1884,
as D. atomaria Hauck; B0rgesen 1932, as D. atomaria
Hauck). Whereas the teeth of D. grossedentata are up to 320
/Lm long, teeth of D. hauckiana reach a length of 2-3 mm.
Furthermore, sporangia of D. hauckiana are grouped in small
sori containing four to eight sporangia. Besides marginal
teeth, D. spinulosa, a species from southem Japan (Harvey in
Hooker & Amott 1838), also contains spinose surface proliferations
(Kützing 1859; Okamura 1915). This character makes
D. spinulosa distinct from the other dentate Dictyota species.
The marginal teeth of D. crenulata are somewhat similar to
those of D. grossedentata, but the former is characterized by
a multilayered medulla near the stupose base (Nizamuddin &
Gerloff 1979, as Dil. crenulatus). Dictyota grossedentata has
neither a stupose base nor a multilayered medulla. Furthermore,
the apical dichotomies of D. crenulata are typically
spathulate (Vickers 1908; Hornig et al. 1992b), which makes
it distinct from both D. ciliolata and D. grossedentata. Dictyota
crenulata J. Agardh, a common species in the Atlantic
Ocean and the Pacific coast of Central America, has been
reported a few times from Sri Lanka but is probably not present
in the Indian Ocean. The record dates back to a D. ciliolata
specimen collected by Ferguson (Ferguson's Algae
Ceylonensis No. 428), identified by Grunow as D. crenulata
and reported by MUITay (1887). Despite the examination of
both recent and historical collections, no D. crenulata specimens
from Sri Lanka were identified. The species has also
been recorded from the Hawaiian Archipelago (Neil 1930;
Buggeln 1965), but every herbmium specimen identified as
D. crenulata from that region proved to belong to D. sandvicensis
Sonder ex Kützing, which is easily distinguished by
its spathulate marginal proliferations. Verheij & Prud'homme
van Reine (1993) reported D. crenulata from Indonesia, but
all voucher specimens in L belong to other species: D. bartayresiana
Lamouroux, D. cervicornis Kützing, and D. ciliolata.
Apmt from its erect habit and the presence of a stupose
base, D. ciliolata is clearly distinct from D. grossedentata by
its gross morphology. The most conspicuous difference is the
thallus length. Dictyota ciliolata is (8-)12-20(-30) cm long,
whereas specimens of D. grossedentata are 3-6 cm long. The
divarication angle of D. ciliolata (30-50°) is also smaller than
that of D. grossedentata (60-90°). The dentation of D. ciliolata
is variable, often within a single thallus, ranging from
microscopic teeth to being conspicuously ciliate. Dentation in
D. grossedentata is invariable and never absent or ciliate. Dictyota
serrulata Lamouroux (1809), another dentate species
that was described from the Indian Ocean but neglected ever
since, remains problematic. Unfortunately the holotype (CN
C7F73) is fragmentary; therefore, an accurate identification
remains extremely difficult. Dictyota serrulata could be conspecific
with either D. ciliolata or D. crenulata, and a proposal
for rejection is being considered.
Dictyota grossedentata also resembles two other prostrate
species, D. friabilis Setchell and D. canaliculata De Clerck et
Coppejans. These species share the same habitat, often growing
together. However, D. grossedentata is easily distinguished
by its dentation, which neither D. friabilis nor D.

Table 1. Comparison of D. grossedentata with species of similar morphology.
D. ciliolata D. crenlllata D. grossedentata D. hallckiana D. spinlllosa D. canaliclllata D. friabilis
"'c:l
Size (8-)12-20(-30) cm (5-)8-14(-23) cm 3-6 cm 15-50 cm 10-15 cm 8-10 cm 3-5 cm ~
("')
Texture crisp crisp crisp supp1e unknown crisp supp1e
c::>
Rabit erect erect prostrate erect erect prostrate to resupinate prostrate O-
Apices rounded spathulate rounded to obtuse e10ngate and spathu- rounded rounded rounded Ocl
·late
JS'
Dentation absent absent

De Clerck & Coppejans: Two new species of Dictyota 189
canaliculata possesses (Table 1). Dictyota friabilis occasionally
forms marginal proliferations that may be misinterpreted
as teeth in an early developmental stage. Moreover, specimens
of D. friabilis grow in extremely dense mats with several
overlapping layers, resulting in the characteristic jigsaw pattern
(Setchell 1926). The branching pattern of D. grossedentata
is more lax and regular. Dictyota canaliculata is another
prostrate species that usually occurs in deeper water, but the
margins are smooth and the straps grooved (De Clerck & Coppejans
1997b). Iridescence has been demonstrated to be a variable
character in several species. However, all specimens of
D. grossedentata exhibit a bluish grey iridescence in narrow,
transverse bands. The iridescence in D. canaliculata is inconspicuous
and evenly dispersed over the whole surface. Generally,
D. friabilis is also evenly iridescent, but sometimes
broad bands may occur. The latter species may also form
small, noniridescent longitudinal stripes that are conspicuous
in situ (Magruder & Hunt 1979). To avoid confusion, it should
be pointed out that D. friabilis sensu Jaasund (1970) is a misapplied
name for D. crispata Lamouroux. The specimens of
D. friabilis examined in the Herb. Jaasund (DSM Jaasund
93A; BM Jaasund 63D and 93A) were all characterized by
surface proliferations and involucrate spores, characters typical
of D. crispata Lamouroux (De Clerck & Coppejans
1997b). The holotype of D. friabilis (VC 261252) did not
reveal any of these characters. From his description, it is clear
that Jaasund misinterpreted D. friabilis specimens as young
growth forms of D. crispata.
'"d Q.)
::1
.S ¡:;:
o
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Dictyota rigida sp. nov. De Clerck et Coppejans sp. nov.
Figs 10-17
DIAGNOSIS: Planta ad 5-9 cm longa, erecta, rigida, non crispata; basis
conspicua, circularis, parva; ramificatio regulariter dichotoma, angulis
45-80°; segmenta latitudine constanti, interdum basin versus
angustata, (5.5-)7.7-14.0(-18.5) mm longa, (1.5-)1.8-2.2(-2.6) mm
lata, ratione: (2.8-)4.2-5.6(-7.2); apicibus rotundatis; marginibus
laevibus; color fuscus, iridescens; cortex monostromaticus, cellulis
(21-)27-30(-38) ¡.Lm longis, (11-)15-17(-24) ¡.Lm latis, ratione:
(1.5-)1.7-1.8(-2.1), et (14-)16-19(-21) ¡.Lm altis; medulla ubique
monostromatica, cellulis (98-)133-159(-185) ¡.Lm longis, (56-)72­
87(-105) ¡.Lm latis, ratione: (1.5-)2.0-2.1(-2.4), ad 170 ¡.Lm altis;
tetrasporangia in duo paginis, dispersa, sine cellulis involucralibus,
ad 80-100 ¡.Lm diametra; antheridia et oogonia non vidi.
Thallus up to 5-9 cm high, erect, stiff, not crisp; base a
conspicuous small disc; branching regularly dichotomous, angle
45-80°, interdichotomies of constant width, basal segment
tapering toward the point of attachment, (5.5-)7.7-14.0(-18.5)
mm long, (1.5-)1.8-2.2(-2.6) mm wide, ratio: (2.8-)4.2-5.6(­
7.2); apices rounded; margins smooth; color brown, iridescent;
cortex unilayered, cells (21-)27-30(-38) fJ-m long, (11-)15­
17(-24) fJ-m wide, ratio: (1.5-)1.7-1.8(-2.1), and (14-)16­
19(-21) fJ-m high; medulla unilayered throughout the whole
thallus, cells (98-)133-159(-185) fJ-m long, (56-)72-87(-105)
fJ-m wide, ratio: (1.5-)2.0-2.1(-2.4), to 170 fJ-m high; tetrasporangia
on both surfaces of the thallus, not grouped in sori, not
surrounded by an involucrum, 80-100 fJ-m in diameter; antheridia
and oogonia not observed.
HOLOTYPE: Kunduchi, Dar es Salaam, epiphytic on Thalassodendron
stipes, 2.5 m deep (De Clerck & Coppejans, 17
January 1996; GENT, ODC 460). (Fig. 10).
ETYMOLOGY: Named for its stiff habit.

190 Phycologia, Vol. 38 (3), 1999
10
-
1 cm
10-17. Morphology and anatorny of Dictyota rigida.
Fig. 10. Holotype of D. rigida (OOC 460) from Tanzania.
Fig. 11. Oetail of the stupose base (OOC 460).
Fig. 12. Detail of an interdichotorny with sporangia (OOC 399).
Fig. 13. Habit of a young specirnen (OOC 465).
Fig. 14. Detail of the rounded apex (OOC 399).
Fig. 15. Transverse section of a sporangium (OOC 460).
Fig. 16. Transverse section showing rnedullary and cortical cells (OOC 460).
Fig. 17. Transverse section showing young adjacent sporangia (OOC 460).
~'igs

De Clerek & Coppejans: Two new species of Dietyota 191
DISTRIBUTION AND HABITAT: Recorded from two localities in
Tanzania, the Kunduchi area near Dar es Salaam and Kiwengwa
on the east coast of Zanzibar. Invariably found in the
shallow water of the lagoon that separates the fringing reef
from the beach. Growing epiphytically on the stipes of Thalassodendron
ciliatum or epilithic on vertical walls of coral
heads, 1-3 m.
OTHER SPECIMENS EXAMINED: Tanzania: Dar es Salaam, Kunduchi,
2-3 m (Coppejans & De Clerek, 3 January 1996; HEC
11110); Dar es Salaam, Kunduchi, (De Clerek & Coppejans,
4 January 1996; ODC 399); Dar es Salaam, Kunduchi, Mbudya
Island, 3 m (De Clerek & Coppejans, 18 January 1996;
ODC 465); Zanzibar, Kiwengwa, Cairo, 2 m (Coppejans &
Dargent, 22 July 1997; HEC 11959).
THALLUS STRUCTURE: Dietyota rigida is completely erect
(Figs 10, 13), reaches a height of 5-9 cm, and is attached at
a single, small rhizoidal disc (Fig. 11). Often several fronds
arise from a single base. The thallus is stiff but not crisp, and
the branches are regularly dichotomous. Young specimens
(Fig. 13) are often fiabellate, whereas older specimens branch
in all directions. The branching angle varies between 45 and
80°. The interdichotomies, 7-14 mm long and 1.8-2.4 mm
wide, are of constant width throughout the thallus, but the
lowermost segment tapers toward the small basal holdfast.
The width and length of the interdichotomies varies little, and
the overall morphological aspect of the thallus is always clearly
recognizable. Apices are rounded (Fig. 14) and the margins
smooth (Fig. 12). The thallus in situ is medium brown and
exhibits a vivid orangy iridescence in narrow, transverse
stripes. Tetrasporangia develop from cortex cells and are distributed
over both thallus surfaces (Fig. 12). In general, sporangia
are not present in the apical straps, but are most common
in the middle part of the thallus, leaving a sterile band
along the margins. They are not surrounded by an involucrum
and are borne on a single stalk cell (Figs 15, 17). They are
80-100 /-lm in diameter. Although divided sporangia were observed
frequently, antheridia and oogonia have not been
found.
REMARKS: In general appearance and externa! morphology,
D. rigida closely resembles D. fastigiata Sonder from Australia
and D. suhrii Murray from South Africa. However, both
species are characterized by a multilayered medulla and were
formerly placed in the genus Dilophus. Most of the Dietyota
species characterized by a multilayered medulla are rather stiff
compared to many unilayered species. Also, the presence of
a single holdfast is characteristic for (but not restricted to)
many multilayered species. Nonetheless, a multilayered medulla
has never been observed in D. rigida. Further characters
separating D. rigida from D. fastigiata are the involucrum
surrounding the sporangia and the presence of two sporangial
stalk cells in the latter (Phillips 1992). Dietyota rigida is also
somewhat similar to the South African D. suhrii Murray, but
the latter possesses a multilayered medulla, forms terete stoloniferous
holdfasts near the base, has wider straps (3-6 mm),
more spathulate apical segments, and narrower divarication
angles (30-50°). Furthermore, the sporangia of D. suhrii form
block-like patches, separated from each other by a transverse
sterile bando The sporangia of D. spiralis are arranged in a
way similar to D. suhrii, but the multilayered medulla is restricted
to the basal parts of the thallus and the stoloniferous
holdfasts, whereas in D. suhrii, the medulla remains multilayered
near the margins throughout the whole thallus.
Few tropical species possess a single attachment point similar
to that of D. rigida, the exceptions being those species
characterized by dentate margins (see Table 1). Most species
present along tropical coasts are either completely prostrate
(D. adnata Zanardini, D. eanalieulata, D. friabilis, and D.
grossedentata) or are at least attached at several points near
the base or wherever the thallus is in contact with any substrate
(D. bartayresiana Larnouroux, D. eellJieornis Kützing,
D. eeylaniea Kützing, D. crispata Lamouroux, and D. humifusa
Hornig, Schnetter et Coppejans) (personal observation).
A third group of species is attached by means of stoloniferous
holdfasts [D. fasciola (Roth) Lamouroux, D. radieans Harvey,
D. stolonifera Dawson, and D. suhrii].
Apart from the typical base and the stiff upright habit, D.
rigida may superficially resemble D. eeylaniea Kützing, a
very common species of the Indian Ocean. The concept of D.
eeylaniea applied here is that of De Clerck & Coppejans
(1997b) and includes the tropical Indo-Pacific specimens of
the D. diehotoma complex, which in the past often were referred
to as D. divarieata Larnouroux. Dietyota eeylaniea is
characterized by its straps, which are extremely variable in
width and length. This variability is often noticeable within a
single specimen and is refiected in a broad 95% confidence
limit of the average width of the interdichotomies (Table 2).
Although the interdichotomies of D. eeylaniea are generally
shorter and less broad than those of D. rigida, a separation of
these taxa based on interdichotomy sizes is not possible because
of the extreme variation in the morphology of D. eeylaniea.
Narrow forms of D. eeylaniea-that is, forms of similar
width to specimens of D. rigida-however, have typical
acute apices with a surmounting apical cell. The apices of D.
rigida are invariably rounded, and the apical cell is level with
the surrounding cells (Fig. 14). Furthermore, the height of the
medullary cells never exceeds 70 /-lm in D. eeylaniea, whereas
in D. rigida these cells attain a height of 170 /-lm. The cortical
cells are similar in width and length in both species but are
somewhat higher in D. rigida than in D. eeylaniea (16-19 /-lm
compared to 11-15 /-lm).
DISCUSSION
The two species described in this paper fall completely within
the limits of the genus Dietyota in its narrow sense, because
neither of them exhibits a multilayered medulla in any pmt of
the thallus. They are distinguished from similar species by a
combination of overall growth form, holdfast, sizes of the interdichotomies,
and to a lesser extent, cell dimensions and
reproductive characters. Recently, several authors investigated
the use of certain hitherto neglected characters in the genus
Dietyota. Phillips (1992), studying the Australian Dietyota
species characterized by a multilayered medulla, emphasized
the use of a combination of characters to delineate species:
sporangial characters, the number of medullary layers, the surface
morphology, and to a lesser extent, the garnetangial structure.
The variation in these characters among temperate representatives
of the genus, however, seems to be much more
pronounced when compared to most tropical Dietyota species.
Whereas Australian and South African species from temperate

192 Phycologia, Vol. 38 (3), 1999
Table 2. Comparison of D. rigida with species of similar morphology.
Size (cm)
Texture
Rabit
Apices
Margins and surface
Interdichotomies
1ength (mm)
width (mm)
ratio
Cortical cells
1ength (fLm)
width (fLm)
ratio
height (fLm)
Medullary cells
1ayers
1ength (fLm)
width (fLm)
ratio
height (fLm)
Iridescence
Sporangia
aggregation
invo1ucrum
sta1k cells
diameter (fLm)
Oogonia
sori
oogonia
Antheridia
sori (fLm)
paraphyses
antheridia
Distribution
References
D. rigida D. ceylanica D. fastigiata D. su!zrii
5-9 4-16 (4-)8-15(-30) (3-)5-9(-15)
stiff not crisp supple stiff stiff
erect, single point repent erect, stoloniferous erect, stoloniferous
rounded acute to rounded rounded rounded
smooth smooth smooth smooth
(5.5-)7.7-14.0(-18) (1.0-)3.1-4.2(-11.0) (8.2-)16.5-28.9(-46) (5.2-)9.5-12.6(-20.5)
(1.5-)1.8-2.2(-2.6) (0.3-)0.5-3.1 (-6.5) (1.5-)1.9-2.3(-3.2) (2.2-)3.0-4.3(-5.8)
(2.8-)4.2-5.6(-7.2) (2.3-)3.1-4.9(-8.5) (4.1-)7.3-11.8(-17.2) (2.3-)2.8-3.3(-4.5)
(21-)27-30(-38) (21-)33-39(-72) (15-)22-27(-50) (31-)48-51(-71)
(11-)15-17(-24) (7-)15-17(-26) (12-)18-20(-25) (12-)15-17(-25)
(1.5-)1.7-1.8(-2.1) (1.8-)2.1-2.4(-3.0) (0.8-)1.2-1.4(-1.9) (2.1-)2.9-3.2(-4.2)
(14-) 16-19(-21) (12-) 14-15(-16) (13-)14-16(-18.0) (13-)15-18(-21)
single single variable, 1-7 centrally, 1 centrally, 2-4 margin-
4-7 marginally ally
(98-) 133-159(-185) (38-)117-133(-292) (65-)89-100(-135) (65-)109-119(-156)
(56-)72-87(-105) (29-)56-69(-146) (41-)56-60(-79) (45-)71-78(-107)
(1.5-)2.0-2.1(-2.4) (1.3-)1.9-2.2(-2.5) (1.2-)1.5-1.7(-2.3) (1.4-)1.5-1.8(-3.1)
(86-)125-155(-170) (41-)46-58(-72) (38-)45-62(-85) (56-)86-105(-130)
orangy, narrow transverse absent to b1uish absent absent
1ines
solitary
absent
1
80-100
unknown
unknown
Tanzania
solitary
absent
1
90-120
180-236 fLm wide, 288­
345 fLm long, 12-58
sporangia per sorus
50-62 fLm high, 1 sta1k
cell
unknown
Indo-Pacitic
De C1erck (1999)
solitary
present
2
185-210
160-500 fLm wide, 290­
700 fLm long, 4-32
sporangia per sorus
140-165 fLm high, 1 sta1k
cell
280-1000 long, 250-500
wide
3-5 rows
140 fLm high, 1 sta1k cell
Australia
Phillips (1992); De
C1erck (1999)
b1ock-1ike sori separated
by a transverse steri1e
band
present
1
125-140
unknown
unknown
South Africa
Papenfuss (1940); De
C1erck (1999)
regions (e.g. D. fastigiata and D. suhrii) often exhibit differences
in the number of medu1lary layers, surface morphology,
the number of sporangial stalk ce1ls, the presence or absence
of an involucrum, and the extent to which sporangia are united
in sori, such variation is rarely seen among tropical lndian
Ocean representatives of Dictyota. Of the tropical species discussed
in this artic1e, only D. hauckiana and, to a lesser extent,
D. crenulata have sporangia aggregated in sma1l sori;
moreover, a1l species are characterized by noninvolucrate sporangia
borne on a single stalk ce1l. For the tropical lndian
Ocean, an involucrum surrounding the sporangia is present1y
known only for D. cervicornis and D. crispata (Jaasund 1970;
De Clerck & Coppejans 1997b).
Sexual reproductive thalli are rare among the Dictyotales.
Several suggestions have been put forward to explain the
dominance of sporophytes over gametophytes (Mathieson
1966; Barlotti 1971; Liddle 1971, 1972; A1lender 1977).
Among the species discussed in this paper, gametangia are
known only in a limited number of species (D. ceylanica, D.
ciliolata, D. fastigiata, D. grossedentata, and D. hauckiana).
Sizes of oogonial or antheridial sori and the number of 00­
gonia or antheridia per sorus varied strongly within individual
specimens. On the other hand, the amount of interspecific variation
was sma1l, thus limiting the use of such characters to
distinguish species. Phillips et al. (1990) and Phillips (1992)
also observed no or little variation in the gametangial structure
among several Australian Dictyota species. Unlike some other
dictyotalean genera, Exallosorus, Lobophora, Homoeostrichus,
and Zonaria, (Phillips & Clayton 1991; Phillips 1997;
Phillips & Clayton 1997), the gametangial structure was fairly
uniform among a1l species except D. rugulosa (as Dil. marginatus
J. Agardh). Howevel~ the latter species was already
c1early distinguished based on gross tha1lus morphology, surface
structure, and sporangial stalk ce1ls. In this respect, the

De Clerck & Coppejans: Two new species of Dictyota 193
lack of gametophytes is not thought to hamper the taxonomy
of the genus Dictyota.
As stated by Weber-Peukert (1985) and Hornig et al.
(1992b), cell sizes and their ratios could represent a good
character to define and discriminate species. However, the interspecific
variation of cortical and medullary cells is limited.
C0l1ical and medullary cells exhibit a broad range in their
sizes within a single thallus, which is reflected in the difference
between the absolute minimum and maximum values.
Therefore, 95% confidence intervals are more robust, but a
huge number of cells should be measured to obtain representative
values. In terms of confidence intervals, cortical cells
are generally between 20 and 60 /-Lm long and 19-30 /-Lm
broad. Medullary cells exhibit larger differences between species
(50-500 /-Lm long and 50-150 /-Lm broad), but the variation
within a single specimen is also much larger. In sorne
cases cell dimensions can represent a valuable character to
distinguish species, but certain taxa (e.g. D. crispata and D.
bartayresiana), which are distinct on the basis of morphological
and reproductive characters, cannot be separated statistically
by cell sizes (Hornig et al. 1992b, as D. bartayresiana
and D. neglecta, respectively). Furthermore, Weber-Peukert
(1985) did not investigate the amount of intraspecific variation.
Cornhaire (1998) shows a clear relationship between the
width and length of the interdichotomies and the width and
length of the c011ical and medullary cells in the extremely
valiable D. dichotoma along the European coasts. This lowers
the taxonomic value of the cell sizes, especially in species
subject to considerable morphological variation.
A morphological character that has been undemtilized is
the means of attachment. Most tropical species have several
points of attachment, which may be restricted to the basal
parts of the thallus or occur from the basal to the apical parts.
In the first case, this results in an ascendant thallus, lacking a
well-defined base (e.g. D. alternifida J. Agardh, D. bartayresiana
Lamouroux, D. cervicornis, D. ceylanica, D. crispata,
D. dumosa B0rgesen, and D. humifusa Hornig, Schnetter et
Coppejans). The relative dominance of the prostrate part of
the thallus ayer the erect pm1 is subject to substantial vm-iation.
Specimens of D. bartayresiana, D. ceylanica, D. crispata,
and D. cervicornis often have a prostrate growth phase,
which, depending on the ecological conditions, may be dominant
01' nearly absent. Specimens growing epiphytically on
stipes of seagrasses lack an obvious prostrate base, whereas
specimens growing on surf-exposed reef flats often possess an
elaborate prostrate base with only a limited number of erect
straps. Similar variation in growth forms, related to ecological
conditions, has been described for other algae, e.g. Hypnea
spinella (e. Agm"dh) Kützing and H. cervicornis J. Agardh
(Haroun & Pmd'hornme van Reine 1993). Other species are
always completely prostrate (e.g. D. adnata, D. canaliculata,
D. friabilis, and D. grossedentata) and are attached by patches
of rhizoids occuning ayer the complete length of the thallus.
In yet another group of species, the thalli are attached at a
single point. The base is often conspicuous and stupose. This
difference in attachment is thought to be important because
no transitions exist within a single species. Dictyota binghamiae
J. Agm"dh, D. ciliolata, D. crenulata, D. flabellata
(Collins) Setchell et Gardener, D. hauckiana, D. liturata J.
Agardh, D. naevosa (Suhr) Montagne, D. mertensii (Martius)
Kützing, D. fastigiata Sonder, D. spinulosa, and D. robusta J.
Agm"dh each attach at a single point and the basal segments
are often covered with a velvet, rufous rhizoidal patch. Another
group of species is characterized by the formation of
terete stoloniferous holdfasts, which are mostly limited to the
basal segments [D. fasciola (Roth) Lamouroux, D. kohlmeyeri
(Nizamuddin et Gerloff) Homig, Schnetter et Prud'hornme
van Reine, D. intermedia Zanm"dini, D. pinnatifida Kützing,
D. rugulosa Lucas, D. spiralis Montagne, D. stolonifera Dawson,
D. suhrii Munay], but in D. radicans, apical stolonoidal
fibers often originate from deformed apical dichotomies
(Womersley 1987, as D. prolifera Lamouroux).
Despite the value of sporangial characters, cell dimensions,
the overall growth form, and means of attachment, sorne species
will still be difficult to define unambiguously. In this respect,
it is important that descriptive characters such as sizes
of the interdichotomies, branching pattem, shape of the apices
and cell sizes are unambiguously defined. Data should preferably
be the result of a morphometric analysis comparing
individuals of as many populations as possible. To chm"acterize
the variability, it is important not to restrict data to a simple
mean value. Ranges, 95% confidence limits, and standard
deviations of specific characters often provide useful information
to discliminate particulm" taxa. Using rigid cliteria to
define species would certainly help the taxonomy in this recalcitrant
genus by limiting the number of local growth forms
being described as new species, but it possibly could underestimate
the overall diversity as certain taxa will be difficult
to define unambiguously. Dictyota lnenstrualis (Hoyt) Schnetter,
Homig et Weber-Peuke11 (1987) is notoriously difficult to
tell apm1 from D. dichotoma (Hudson) Lamouroux, of which
it had originally been considered a variety. However, chromosome
counts and interbreeding experiments clearly demonstrated
the status of D. menstrualis as a sepm-ate species. In
the future, molecular data of many species and geographically
isolated populations could resolve the uncertainty about the
existence of cryptic species.
ACKNOWLEDGMENTS
O. De Clerck. was research assistant of the Fund for Scientific
Resem"ch-Flanders (Belgium) (FWO). Financial support was
provided by FKFO projects 2.000.92 and G002496. We are
grateful to DI' Paul Goetghebeur for correcting the Latin diagnoses.
We also acknowledge John Rewald, Director of the
Motupore Island Research Station, DI' Ngoile, Director of the
Institute of Mm"ine Sciences Zanzibar, and Olivier Dargent as
a diving partner.
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