Zonaria diesingiana, copyright 黃淑真.

Belongs within: Phaeophyceae.
Contains: Padina, Dictyota, Dictyopteris.

On Dictyotales
Published 17 July 2021

Most of the various ‘seaweeds’ found around the world can be assigned to one of three major groups, each named for their most characteristic pigments: green algae, red algae and brown algae. Of these, green algae are the closest relatives of land plants, and red algae are the most taxonomically diverse. But for many people, the most familiar of the three will be brown algae. Owing to their often relatively large size and predilection for growing in visible locations, brown algae are likely to be the first examples to come to mind when one thinks of seaweed. For this post, I’m examining a particular subgroup of the brown algae, the family Dictyotaceae.

Forkweed Dictyota dichotoma, copyright Ria Tan.

Representatives of the Dictyotaceae can be found around the world but are more diverse in warmer tropical and subtropical waters. They seem to be particularly diverse in the Australasian region. Dictyotaceae are moderately sized seaweeds with flattened thalli that may grow as branching ribbons or radiating fans. One fan-shaped species of Dictyotaceae, Padina pavonica, has earned itself the vernacular name of ‘peacock’s tail'(this species is also notable for being one of the few calcified brown algae). These thalli grow apically from meristematic cells. Dictyotaceae have an isomorphic life cycle with the alternating sexually and asexually reproducing generations being similar in overall appearance. Sporangia in asexual individuals grow as superficial nodules scattered over the surface of the thallus; the resulting spores usually differ from those of other brown algae in lacking flagella. The less abundant sexual individuals are mostly divided between separate males and females (Bittner et al. 2008).

Peacock’s tail Padina pavonica, copyright Diego Delso.

Dictyotaceae are distinct enough from other brown algae to have consistently been treated as their own order (indeed, their sporangia are unique enough that some very early authors did not even regard them as brown algae). Two species found around Australasia, Dictyotopsis propagulifera and Scoresbyella profunda, have previously been considered distinct enough to warrant their own separate families within this order Dictyotales. Dictyotopsis propagulifera has a monostromatic thallus (that is, the thallus is only one layer of cells thick). Scoresbyella profunda has an apical growing cell that divides lengthwise to the thallus instead of transversely as in other Dictyotales. However, molecular data have indicated that these two genera are nested within Dictyotaceae and so only the single family is currently recognised. Dictyotaceae has also been divided in the past between tribes Dictyoteae and Zonarieae based on the nature of the apical growing cells (Dictyoteae have a single meristematic cell whereas Zonarieae have a cluster or row of cells) and some authors have even treated them as distinct families. Again, however, molecular data have not corroborated this division (Bittner et al. 2008).

Lobophora variegata, copyright John Turnbull.

For most species of Dictyotaceae, their greatest significance to humans probably comes from the role they play in providing habitats to fish and other marine animals. As with other algae, Dictyotaceae produce a range of secondary metabolites that serve functions such as protecting them from grazers, and some of these may prove to have economic applications. Some species of Dictyotaceae, on the other hand, have become significant invasive species. A dramatic recent example has been provided by the northern Pacific species Rugulopteryx okamurae which was probably first imported to the Mediterranean as a contaminant on farmed oysters (García-Gómez et al. 2020). This species was recorded on the southern coast of France in 2002 and was later recorded on the coast of Ceuta in 2015. Within a year of the latter record, its presence in Ceuta had reached absolute plague proportions. Most of the illuminated rocky sea bottom was covered by R okamurae, up to about 90% coverage at optimal depths about ten to twenty metres. Over 5000 tons of washed-up seaweed was removed from the beaches of Ceuta in 2016. Needless to say, native seaweeds, and other sessile marine organisms such as corals, would have been severely impacted by this spread.

Rugulopteryx okamurae in Morocco, from El Aamri et al. (2018).

What caused this dramatic invasion? It would have certainly been a factor that defensive metabolites produced by Rugulopteryx okamurae had a negative impact on competitors. But perhaps even more significant a factor was climate change. Rising sea temperatures in the Straits of Gibraltar would have made things uncomfortable for native marine life used to cooler conditions. Meanwhile, the subtropical immigrant would have found things increasingly to its liking. With its competition hobbled and nothing to hold it back, R. okamurae was set to take over.

Systematics of Dictyotaceae
<==Dictyotaceae [Dictyotales, Dictyoteae, Dictyoti, Scoresbyellaceae, Zonarieae]SRR14
| i. s.: Gymnosorus nigrescens (Sonder) Agardh 1894 [=Zonaria nigrescens]L27
| Chlanidophora Agardh 1894SRR14
| `--*C. microphyllaBP08
| Dictyotopsis Troll 1931 [Dictyotopsidaceae]SRR14
| `--*D. propaguliferaBP08
| Herringtonia Kraft 2009SRR14
| Padinopsis Ercegovic 1955SRR14
| Lobospira Areschoug 1854C-SC06, SRR14
| `--*L. bicuspidataBP08
|--Exallosorus Phillips 1997BP08, SRR14
| |--*E. olsenii [=Homoeostrichus olsenii]BP08
| |--‘Homoeostrichus’ canaliculatusBP08
| `--E. harveyanus [=Homoeostrichus harveyanus]BP08
`--+--+--Lobophora Agardh 1894 non Agassiz 1841 (ICZN)BP08, SRR14 [incl. Pocockiella Papenfuss 1943SRR14]
| | |--L. variegata (Lamouroux) Womersley ex Oliviera 1977HL09 (see below for synonymy)
| | `--L. papenfussiiBP08
| `--+--Newhousia Kraft, Saunders et al. 2004BP08, SRR14
| | `--*N. imbricataBP08
| `--Zonaria Agardh 1817BP08, N10
| | i. s.: Z. collarisG64
| | Z. farlowii Setchell & Gardner 1924AH96
| | Z. parvulaG64
| | Z. sinclairii Hooker & Harvey 1845L27
| | Z. turneriana Agardh 1870L27
| | Z. velutina Hooker & Harvey 1855L27
| |--Z. crenataBP08
| `--+--Z. diesingianaBP08
| `--+--Z. subarticulataBP08
| `--+--Z. tournefortii [incl. *Z. flava]BP08
| `--+--Z. angustataBP08
| `--Z. spiralisBP08
`--+--Homoeostrichus Agardh 1894BP08, SRR14
| |--*H. sinclairii (Hooker & Harvey) Agardh 1894SRR14
| `--H. multifidusBP08
| `--Distromium Levring 1940BP08, SRR14
| | i. s.: D. flabellatumG05
| |--D. didymothrixBP08
| `--+--*D. skottsbergiiBP08
| `--D. decumbensBP08
`--+--+--Rugulopteryx De Clerck & Coppejans in De Clerck, Leliaert et al. 2006BP08, N10
| | |--*R. radicansBP08
| | |--R. marginata (Agardh) De Clerck & Coppejans in De Clerck et al. 2006 (see below for synonymy)N10
| | `--R. okamurae (Dawson) Hwang, Lee & Kim in Hwang, Lee et al. 2009 (see below for synonymy)N10
| `--+--Stoechospermum Kützing 1843BP08, SRR14
| | `--S. polypodioides (Lamouroux) Agardh 1848N10 (see below for synonymy)
| `--+--Scoresbyella Womersley 1987BP08, SRR14
| | `--*S. profundaBP08
| `--+--DictyotaBP08
| `--Canistrocarpus De Paula & De Clerck in De Clerck et al. 2006BP08, SRR14
| |--*C. crispatus (Lamouroux) De Paula & De Clerck in De Clerck et al. 2006BP08, N10
| `--C. cervicornis (Kützing) De Paula & De Clerck 2006HS14 (see below for synonymy)
`--+--+--‘Dictyopteris’ crassinervia (Zanardini) Schmidt 1939BP08, P00 (see below for synonymy)
| `--Stypopodium Kützing 1843BP08, HL09
| | i. s.: S. zonale [incl. *S. fuliginosum]BP08
| |--S. schimperiBP08
| `--+--S. australasicumBP08
| `--+--S. flabelliforme Weber-van Bosse 1913BP08, HL09
| `--S. hawaiiensisBP08
`--+--Taonia Agardh 1848BP08, N10
| |--*T. atomariaBP08
| |--T. australasica (Kützing) Agardh 1894L27
| `--T. lennebackerae Farlow ex Agardh 1894N10
`--Spatoglossum Kützing 1843BP08, N10
| i. s.: *S. solieriiBP08
| S. chapmaniiBP08
| S. howellii Setchell & Gardner 1937N10
| S. lanceolatum Dawson 1954N10
| S. macrodontum Agardh 1882HS14
| S. schroederi (Agardh) Kützing 1859N10
| S. subflabellatum Dawson 1954N10
|--S. stipitatum (Tanaka & Nozawa) Bittner, Payri et al. 2008 (see below for synonymy)BP08
`--+--S. asperum Agardh 1894BP08, HS14
`--S. crassumBP08

Canistrocarpus cervicornis (Kützing) De Paula & De Clerck 2006HS14 [=Dictyota cervicornis Kützing 1859N10]

‘Dictyopteris’ crassinervia (Zanardini) Schmidt 1939BP08, P00 [=Haliseris crassinervia Zanardini 1874P00]

Lobophora variegata (Lamouroux) Womersley ex Oliviera 1977HL09 [=Dictyota variegata Lamouroux 1809HL09, Pocockiella vareigataN10; incl. *Lobophora nigrescensBP08]

Rugulopteryx marginata (Agardh) De Clerck & Coppejans in De Clerck et al. 2006 [=Dilophus marginatus Agardh 1894]N10

Rugulopteryx okamurae (Dawson) Hwang, Lee & Kim in Hwang, Lee et al. 2009 [=Dilophus okamurae Dawson 1950, Dilophus okamurai, Dictyota okamurae (Dawson) Hörnig, Schnetter & Prud’homme van Reine 1992; incl. Dictyota marginata Okamura 1913 non (Agardh) Greville 1830, Dilophus marginatus (Okamura) Okamura 1915 non Agardh 1894]N10

Spatoglossum stipitatum (Tanaka & Nozawa) Bittner, Payri et al. 2008 [=Zonaria stipitata Tanaka & Nozawa 1962]BP08

Stoechospermum polypodioides (Lamouroux) Agardh 1848N10 [incl. Dictyota marginata (Agardh) Greville 1830N10, *S. marginatumBP08]

*Type species of generic name indicated


[BP08] Bittner, L., C. E. Payri, A. Couloux, C. Cruaud, B. de Reviers & F. Rousseau. 2008. Molecular phylogeny of the Dictyotales and their position within the Phaeophyceae, based on nuclear, plastid and mitochondrial DNA sequence data. Molecular Phylogenetics and Evolution 49: 211–226.

[C-SC06] Cavalier-Smith, T., & E. E.-Y. Chao. 2006. Phylogeny and megasystematics of phagotrophic heterokonts (kingdom Chromista). Journal of Molecular Evolution 62: 388–420.

García-Gómez, J. C., J. Sempere-Valverde, A. R. González, M. Martínez-Chacón, L. Olaya-Ponzone, E. Sánchez-Moyano, E. Ostalé-Valriberas & C. Megina. 2020. From exotic to invasive in record time: the extreme impact of Rugulopteryx okamurae (Dictyotales, Ochrophyta) in the strait of Gibraltar. Science of the Total Environment 704: 135408.

[G05] Goldberg, N. A. 2005. Temporal variation in subtidal macroalgal assemblages at Black Island, Recherche Archipelago. Journal of the Royal Society of Western Australia 88 (2): 65–71.

[G64] Gray, J. E. 1864. Handbook of British Water-weeds or Algae. R. Hardwicke: London.

[HL09] Huisman, J. M., F. Leliaert, H. Veerbruggen & R. A. Townsend. 2009. Marine benthic plants of Western Australia’s shelf-edge atolls. Records of the Western Australian Museum Supplement 77: 50–87.

[HS14] Huisman, J. M., & A. Sampey. 2014. Kimberley marine biota. Historical data: marine plants. Records of the Western Australian Museum Supplement 84: 45–67.

[L27] Laing, R. M. 1927. A reference list of New Zealand marine algae. Transactions and Proceedings of the New Zealand Institute 57: 126–185.

[N10] Norris, J. N. 2010. Marine algae of the northern Gulf of California: Chlorophyta and Phaeophyceae. Smithsonian Contributions to Botany 94: 1–276.

[P00] Phillips, J. A. 2000. Systematics of the Australian species of Dictyopteris (Dictyotales, Phaeophyceae). Australian Systematic Botany 13 (2): 283–324.

[SRR14] Silberfeld, T., F. Rousseau & B. de Reviers. 2014. An updated classification of brown algae (Ochrophyta, Phaeophyceae). Cryptogamie, Algologie 35 (2): 117–156.

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