When you think of grazing mammals, you might envisage large herds of antelopes roaming African savannahs. Did you know that there is an equivalent in the ocean, feeding on seagrass?

Dugongs (Dugong dugon) are one of four extant species of sea cows (Sirenia), an order of large, aquatic herbivorous mammals that are quite closely related to the elephants. They are unusual in being the only marine mammalian herbivores that have specialised on seagrass communities. And not only that – their grazing system is highly sophisticated and shows a number of convergences…

Seagrass was an important dietary resource for these animals from early in their evolution, as is evident by the association of sirenian fossils with deposits containing fossilised seagrasses. The sea cows’ use of seagrass meadows probably brought about substantial changes in the structure and dynamics of this ecosystem, and it has been suggested that “the close herbivore-plant connection between sirenians and seagrasses over the past 50 million years may have led to significant co-evolution” (Clementz et al. 2006, Journal of Vertebrate Paleontology, vol. 26, p. 365).

Cultivation grazing

Seagrass meadows are characterised by high spatial heterogeneity, and dugongs also seem to be particularly limited by the availability of nitrogen. As they are hindgut fermenters with a large, heavy colon and food retention times of up to a week, they have to ingest large quantities of nitrogen-rich seagrass. The seagrass furthermore needs to be relatively low in fibre, because dugongs have lost their hard dental surfaces. But the animals have come up with a remarkable solution to all these problems.

Dugongs regularly move between food patches. In Moreton Bay, Australia, for example, large herds were observed to graze at a particular location for several weeks. As these herbivores prefer rapidly growing seagrass species that are high in nitrogen and low in fibre (e.g. Halophila ovalis) at the expense of less nutritious species that are dominant but grow more slowly (e.g. Zostera capricorni), they can change the species composition of seagrass meadows (similar to algae-farming damselfish). But dugong activity affects seagrass communities also in another way. In experiments, simulated intensive grazing boosted the nitrogen concentrations of H. ovalis and Halodule uninervis regrowth by 35% and 25%, respectively. Although starch concentration decreased and fibre content increased over a longer period of grazing, it was concluded that the overall nutritional value of the plants is enhanced. This is supported by the observation that dugongs often re-graze those sites, where the recovering plants contain very high concentrations of nitrogen. So the timing of returning to a previously grazed area is important and depends on how quickly the particular seagrass species recover – returning too early would substantially reduce the benefit. Dugong return times have been shown to be variable, but detailed studies are still lacking. This grazing system that enhances the nutritional quality of the dugongs’ diet has been termed cultivation grazing.

Similar grazing optimisation occurs in terrestrial systems, but the mechanisms are different. While local fertilisation by the animals’ faeces and urine plays a major role for the nutritional value of terrestrial grasses, not surprisingly this seems to be of minor importance for seagrass, probably on account of the water movement and the dugongs’ extremely long gut retention times. Instead, dugong feeding is likely to increase the activity of nitrogen-fixing microorganisms – through the production of detritus and aeration of the sediment – and, consequently, boosts the nitrogen concentration in regrowing plant tissue. Accordingly, a decrease in dugong numbers is likely to have potential effects on the level of the whole ecosystem, such as a degradation of the habitat in terms of its nutritional value.

Convergence with placodontid reptiles

Placodontids (e.g. Placodus gigas), extinct reptiles that roamed the oceans during the Triassic, were long assumed to be shell-crushing durophages. A reanalysis of teeth, general jaw morphology and other features of three placodontid genera, however, painted a rather different picture. These fossil reptiles show a number of striking anatomical convergences with dugongs as well as with Halitherium schinzii, a fossil sirenian from the Tertiary. “Apart from the body shape adapted for aquatic diving, the skull morphology and dentition provide the most important evidence for convergent developments” (Diedrich 2010, Palaeogeography, Palaeoclimatology, Palaeoecology, vol. 285, p. 293). For example, dugongs and Halitherium evolved adaptations to grinding seagrass, such as a horny oral pad with a counterpart and a specialised rasping tongue, while placodontids formed a similar grinding structure from large, flat teeth covering the entire upper and lower jaws. The sirenian tongue is thus functionally convergent with the placodontid lower jaw. Both groups developed similar canal-like structures in the horny pads or teeth in the upper jaw, possibly to flush out sand and help to expel fluid while crushing food. In addition, the anatomy of the thoracic region is very similar, including enlarged ribs, increasing the ballast and so allowing for more efficient diving. All these adaptations strongly suggest that placodontids also fed on sea-plants (probably mainly macroalgae, as seagrass had yet to evolve) and used a similar feeding strategy as modern dugongs. These reptiles have therefore been aptly nicknamed “Triassic sea cows”.