Dictyotales

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.

REFERENCES

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.

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.