A species · Marine life
Leafy seadragon
A seahorse relative dressed as a scrap of drifting weed, found nowhere on Earth but the cool southern-Australian seas — South Australia's marine emblem, and a slow, improbable life lived inside the gulf's seagrass meadows.
- On the gradient
- Below the gradient — out in the open, saline water of the backwards sea itself, among the gulf's seagrass meadows
- Rock
- Nearshore sand and shell-grit seabed over the drowned shelf of Gulf St Vincent
- Soil
- Fine sand and rubble bound by seagrass rhizome mats — no soil, but a comparable living substrate
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What you'll take away
- Recognise the leafy seadragon by what it is doing, not what it looks like — a slow-sculling fish shaped and coloured to read as a torn scrap of seagrass or drift-weed, not an animal at all.
- Explain why it lives nowhere but the cool temperate seas of southern Australia — a signature of a marine fauna that has evolved in long isolation, in seas cut off from the rest of the world's oceans.
- Connect the seadragon's fate to the meadow beneath it: the *Posidonia* and *Amphibolis* seagrass of Gulf St Vincent that has been lost, and partly recovered, around metropolitan Adelaide.
Every so often, if you are patient and the water is clear and still, a torn-looking scrap of weed will detach itself from the meadow floor and swim.
Look closer and it resolves into an animal: a slim, armour-plated fish, dressed head to tail in trailing, leaf-shaped fronds that do nothing at all for swimming and everything for disappearing.
This is the leafy seadragon, a close cousin of the seahorse that has taken its already odd body plan — bony rings instead of scales, a tube for a mouth, no chewing gear worth the name — and pushed it one step further into disguise.
It has almost no fins to speak of, just a small dorsal fin and tiny pectoral fins so nearly transparent they seem to have been left off by mistake, and it doesn’t so much swim as scull, drifting at the pace of the current, looking exactly like a piece of its litter.
It is one of the more improbable animals in the sea, and for that South Australia adopted it as the state’s official marine emblem in February 2001 — it lives nowhere on Earth but the cool temperate waters of southern Australia (Symbols of South Australia 2001).
That confinement is a clue to something larger.
Southern Australia’s temperate seas have been isolated from the world’s other oceans for a very long time, and long isolation is exactly the recipe that produces animals found nowhere else — the seadragon is the most photogenic evidence of a marine fauna built the same way, much of it still being worked out.
Here, its stage is Gulf St Vincent itself: the “backwards sea,” a broad, shallow, evaporation-driven inverse estuary where salinity climbs the further inshore you go (Lennon 1987) — and whose cover, the Posidonia and wire-weed Amphibolis meadows of the gulf floor, also nurses whiting and prawns and stores a substantial buried “blue” carbon (Tanner et al. 2014).
None of which has made the meadow safe.
Since the 1930s more than five thousand hectares of that seagrass have vanished off metropolitan Adelaide, fed to algal blooms by decades of nutrient-rich sewage and stormwater, and though rehabilitation has since clawed some of it back, a seadragon with no meadow left to impersonate has nowhere to hide (Tanner et al. 2014).
To find one, then, is to be shown two things at once: how strange and specific southern Australia’s marine life can be, and how entirely its strangest residents depend on a green, unglamorous carpet of grass that is easy to overlook and easier still to lose.
Sources for this guide · 3
Every claim here is traceable. Each citation links to its entry in the bibliography.
- Symbols of South Australia (Government of SA). Leafy seadragon (marine emblem, proclaimed 8 Feb 2001). — Leafy seadragon proclaimed South Australia's marine emblem, 8 February 2001; framed as an icon of the region's high-endemism temperate marine fauna.
- Tanner, J.E., Irving, A.D., Fernandes, M., Fotheringham, D., McArdle, A. & Murray-Jones, S. (2014). Seagrass rehabilitation off metropolitan Adelaide: a case study of loss, action, failure and success. Ecological Management & Restoration 15(2): 168–179. DOI: 10.1111/emr.12133. https://onlinelibrary.wiley.com/doi/10.1111/emr.12133 — Posidonia/Amphibolis seagrass loss and rehabilitation off metropolitan Adelaide — the meadow habitat the seadragon depends on.
- de Silva Samarasinghe, J.R. & Lennon, G.W. (1987). Hypersalinity, flushing and transient salt-wedges in a tidal gulf — an inverse estuary. Estuarine, Coastal and Shelf Science 24(4): 483–498. https://www.sciencedirect.com/science/article/abs/pii/0272771487901296 — The hypersaline, inverse-estuary gulf whose seagrass meadows the seadragon drifts through.
Concepts this teaches — follow a thread
The inverse estuary (the backwards sea)Blue carbon (the gulf's quiet climate work)
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Test yourself →
The leafy seadragon's survival is tied to one specific feature of the Gulf St Vincent seabed. What is it, and why does losing it leave the seadragon with nowhere to hide?
The seadragon's trailing, leaf-shaped fronds only work as camouflage against a background of actual weed and seagrass — take the meadow away and the disguise has nothing left to hide inside. That is not a hypothetical risk: more than five thousand hectares of Posidonia and Amphibolis seagrass vanished off metropolitan Adelaide from the 1930s onward, fed to algal blooms by nutrient-rich sewage and stormwater, and although rehabilitation has since clawed some of it back, the loss shows how directly the seadragon's fate rides on a carbon-storing 'blue carbon' habitat that is easy to overlook and easier to lose. (Tanner et al. 2014.)