Morphology and Systematics of Two Aberrant Species of Dictyota (Dictyotaceae, Phaeophyta), Including a Discussion on the Generic Boundaries in the Tribe Dictyoteae
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Morphology and systematics of two aberrant species of Dictyota (Dictyotaceae, Phaeophyta), including a discussion on the generic boundaries in the tribe Dictyoteae O De Clerck* & E Coppejans Research Group Phycology, Biology Department, Ghent University, Krijgslaan 281/58, 9000 Gent, Belgium *Corresponding author Email: [email protected] [email protected] Key words: Dictyota, Dictyotaceae, morphology, systematics, generic boundaries Abstract Dictyota naevosa (Suhr) Montagne and D. radicans Harvey possess characters that do not conform to the generic description of Dictyota. The generic boundaries in the tribe Dictyoteae are thus called into question. Dictyota naevosa differs from other Dictyota species by having more than one layer of cortical cells in the lower part of the thallus. The presence of a multilayered cortex is the only character distinguishing Pachydictyon from Dictyota. Dictyota radicans is characterised by the presence of terete surface proliferations, a feature used to distinguish Glossophora from Dictyota. Both D. naevosa and D. radicans have clear sister species that are currently placed in the genus Dilophus. The segregation of Dilophus from Dictyota, as still advocated by some authors, therefore becomes untenable. The morphological evidence leads us to question the generic status of Pachydictyon, Glossophora and Glossophorella, since the characters used to delineate these genera are commonly found in some species of Dictyota. Introduction In the past, species characterised by a multilay- ered medulla, in at least some part of the thallus, Within the brown algal family Dictyotaceae, two were assigned to Dilophus J. Agardh (1882). The tribes are currently recognised, the Dictyoteae and distinction, however, between Dictyota and Zonarieae, differentiated on the basis of the num- Dilophus, was not clear-cut because the extent to ber of apical cells. In Dictyoteae there is a single which the medulla was multilayered varied sub- apical cell, whereas in Zonarieae there is a margin- stantially and some species were particularly hard al row or cluster of apical cells. Within the tribe to assign to one or the other genus (Setchell & Dictyoteae, J. Agardh (1848, 1882, 1894, 1897) Gardner, 1925; Taylor, 1945; Dawson, 1950). recognised four genera: Dictyota, Dilophus, Eventually the two genera were merged by Hörnig Glossophora and Pachydictyon, distinguished by the et al. (1992a, b) on the basis of culture experiments relative number of cortical and medullary layers that showed that the number of medullary layers and the presence of surface proliferations. Recently can be altered in many species depending on the a new genus was added to the Dictyoteae, culture conditions. Although the merger has been Glossophorella Nizamuddin & Campbell (1995). accepted by several authors (e.g. Silva et al., 1996; Dictyota Lamouroux (1809), as defined by Hörnig Wynne, 1998), some authors still recognise et al. (1992a), includes species characterised by a Dilophus as a separate genus (Phillips, 1992; Millar single lenticular, transversely orientated apical cell, & Kraft, 1994; Huisman, 2000). Pachydictyon J. and a parenchymatous thallus composed of a cor- Agardh (1894) is characterised by a single-layered tex and a medulla. 275 DICTYOTA: MORPHOLOGY AND SYSTEMATICS medulla and multilayered cortex. Although the sta- single stupose base from which one to several tus of Pachydictyon has been questioned in the past fronds (and usually a few narrow proliferations) (Setchell & Gardner, 1925; Dawson, 1950), it is arise; the base often with a velvet patch of rhizoids; still currently recognised. Pachydictyon coriaceum colour in situ medium brown, bluish iridescent; (Holmes) Okamura, a species from Japan and dry specimens medium to dark brown. All straps of California, is characterised by a medulla which the thallus of similar width, reaching their maxi- may comprise two layers of cells at the margins. mum width in the middle part of the thallus, grad- Therefore, the species was transferred to ually tapering towards the base and apices; average Glossophorella Nizamuddin & Campbell (1995), width: (3.5–) 6.3–10.7 (–16.0) mm, proximal which accommodates species with a multilayered width: (2.0–) 3.8–6.5 (–11.0) mm, distal width: cortex as well as a multilayered medulla. (3.0–) 8.7–15.0 (–22.0) mm, Wd/Wp: (1.9–) Glossophora J. Agardh (1882), a genus containing 2.2–2.5 (–2.6); length: (12.0–) 24.2–48.0 (–86.0) three species restricted to southern Australia and mm; L/W: (2.0–) 3.0–6.1 (–8.8). Apices rounded, the Pacific coast of South America, is characterised apical segments very long (up to 135 mm) and by a unilayered cortex and medulla, but the surface spathulate, often eroded; apical cell protruding. is often densely beset with terete proliferations. Branching isotomous dichotomous to somewhat The morphology and taxonomy of D. naevosa irregular (often as a result of damage), sparse, most- (Suhr) Montagne and D. radicans Harvey was stud- ly to 2 or 3 orders; branching angle (15–) 25–40 ied in the framework of a revision of the genus in (–50)°. Margins smooth, proliferations rare. the Indian Ocean (De Clerck, 1999). The results Surface proliferations absent. Hair tufts common. are discussed in this paper. Because these species Cortex and medulla predominantly unilayered, but possess characters that do not conform to the several layers of both cortical and medullary cells generic description of Dictyota, we have been led to frequently observed especially in Australian speci- reassess generic definitions in the tribe Dictyoteae. mens (Figs. 2–4); Cortical cells (14–) 22–23 (–36) µm long, (10–) 14–16 (–24) µm wide, ratio: (1.3–) Materials & Methods 1.4–1.6 (–1.9), and (22–) 23–26 (–29) µm high; medullary cells (77–) 112–120 (–197) µm long, Specimens were collected by SCUBA or (48–) 68–71 (–120) µm wide, ratio: (1.5–) 1.6–1.7 snorkelling during several trips in the Indian (–1.8), and (120–) 130–160 (–188) µm high. Ocean, Indonesia and Papua New Guinea since Sporangia on both surfaces of the thallus, also 1980 by ourselves. Material was prepared as present in the apical segments, grouped in conspic- herbarium specimens or preserved in 4% forma- uous ovate to oblong sori, often surrounding one lin/seawater and lodged in GENT. Additional or two central hair tufts (most apparent in the api- specimens were examined from several herbaria cal segments and becoming more obscure near the (Appendix 1). Herbarium abbreviations follow basal parts of the thallus), hair tufts disappear in Holmgren et al. (1990). older parts; sori (433–) 716 (–1,435) µm wide, Morphological data result from morphometric (340–) 1,640 (–5,740) µm long, ratio: (1.35–) 2.5 analyses as described by De Clerck & Coppejans (–5.1) (Figs. 6, 7); sporangia more or less drop- (1999). shaped, not surrounded by an involucrum, mature sporangia born on a single stalk cell, width: Results 60–100 µm, height: 110–130 µm (Fig. 8); divided sporangia frequently observed. Dictyota naevosa (Suhr) Montagne 1840: 145 Male gametophytes with sori of antheridia evenly dispersed over the whole thallus, also pres- Description ent in the apical segments, often merging with Thallus completely erect, sparsely branched and other sori to form large coenosori (Figs. 9, 10); sori very robust, 10–35 cm long (Fig. 1); attached by a (272–) 325 (–758) µm wide, (400–) 725 (–1,846) 276 DICTYOTA: MORPHOLOGY AND SYSTEMATICS Plate 1. Morphology and anatomy of Dictyota naevosa. Fig. 7. Detail of a tetrasporangial sorus with a central hair tuft Fig. 1. Habit of D. naevosa (HEC 11000) (scale: 1 cm). (arrow) (scale: 500 µm). Fig. 2. Transverse section showing unilayered cortex and medulla Fig. 8. Transverse section of a tetrasporangial sorus (scale: 50 µm). (scale: 100 µm). Fig. 9. Surface view of an antheridial strap (scale: 2.5 mm). Fig. 3. Transverse section showing multiple duplications of the cor- Fig. 10. Detail of an antheridial sorus with a central hair tuft (scale: tical layer (scale: 100 µm) . 500 µm). Fig. 4. Transverse section of a three-layered cortex in the lower part Fig. 11. Transverse section of an antheridial sorus surrounded by 6 of the thallus (scale: 50 µm). unicellular paraphyses (scale: 50 µm). Fig. 5. Transverse section of a thallus margin with multiple duplica- Fig. 12. Surface view of an oogonial strap (scale: 2.5 mm). tions of the cortical layer (scale: 50 µm). Fig. 13. Detail of oogonial sori (scale: 100 µm). Fig. 6. Surface view of tetrasporangial strap (scale: 2.5 mm). Fig. 14. Transverse section of an oogonial sorus (scale: 50 µm). 277 DICTYOTA: MORPHOLOGY AND SYSTEMATICS Plate 2. Morphology and anatomy of D. radicans. Fig. 19. Transverse section of a thallus margin (scale: 50 µm). Fig. 15. Lectotype specimen of D. radicans (TCD Harvey 69A) Fig. 20. Transverse sections of a transition zone between a normal (scale: 1 cm). strap and a terete stolonoidal fibre (scale: 50 µm). Fig. 16. Habit of a broad specimen (MEL 16991) (scale: 1 cm). Fig. 21. Transverse section of a terete stolonoidal fibre (scale: 100 Fig. 17. Detail of an apical portion of the thallus with multiple surface proliferations (scale: 5 mm). µm). Fig. 18. Transverse section through the thallus showing a unilayered Fig. 22. Longitudinal section of a terete surface proliferation cortex and medulla (scale: 50 µm). showing a multilayered medulla (scale: 50 µm). µm long, L/W: (0.9–) 2.3 (–4.3); antheridia, born (–1.7); oogonia 65 µm wide, 140 µm high; oogo- on a single stalk cell, 22 µm wide, 65 µm high; nia per sorus: (39–) 65 (–88) (Fig. 14). antheridia per sorus: (31–) 52 (–94); sori sur- rounded by 3–6 rows of unicellular paraphyses Remarks (Fig. 11). Female gametophytes, with sori of oogonia The distribution of D. naevosa is limited to South evenly distributed over the whole surface, also pres- Africa and Australia.