The Common Seadragon (or weedy seadragon) Phyllopteryx taeniolatus was first described and illustrated by Lacepède, 1804 who named it as Syngnatus (sic) taeniolatus with a type locality in Bass Strait. The specific name taeniolatus is derived from the Latin word taenia (= ribbon), which is a diminutive of taenia, in reference to the small leaf–like appendages projecting from the body. Also, Lacepède, (1804) gave the new species the common name of syngnathe à banderolles’, referring to the appendages as well. In the same year Syngnathus foliatus was named by Shaw, 1804 from “near the coasts of New Holland”, with an illustration of what is clearly a Common Seadragon. The publication of two species names in the same year raised the question of which name has priority. The genus Phyllopteryx was erected in 1839 by Swainson, who named Syngnathus foliatus as the type species for the genus. Subsequently, most authors have not accepted this and regarded Phyllopteryx taeniolatus (Lacepède, 1804) as the senior synonym rather than Phyllopteryx foliatus. Other species names have been erected, including Phillopteryx (sic) elongatus Castelnau, 1872 from South Australia and Phyllopteryx altus McCoy, 1882 from Tasmania. Whitley, 1931 erected a subspecies, Phyllopteryx taeniolatus lucasi, from Western Australia. With studies on the genetics of Common Seadragons across their range from Western Australia to New South Wales and Tasmania it is now clear that there is only a single species of Common Seadragon along the southern Australian coast (Wilson and Rouse 2010, Wilson et al. 2017, Stiller et al. 2023), which here accepted as Phyllopteryx taeniolatus.

The Common Seadragon (Phyllopteryx taeniolatus) is a distinctive syngnathid fish (to 46 cm in length) that occurs across Australia’s Great Southern Reef (GSR) from Nelson Bay in New South Wales to Cervantes in Western Australia, including Tasmania. They are typically found at depths of <5 to 30 m in algae-covered reefs, at the sandy edges of reefs, or in seagrass beds. This seadragon is primarily threatened by the impacts of climate change, including warming ocean temperatures, increased storm severity, loss of kelp cover, and reduction in suitable habitat. Other minor threats to seadragons’ habitat include pollution near industrial centers and, in some areas, trawling. Range–wide estimates of the Common Seadragon’s population size have not yet been established. However, a study of their population trend over the past decade estimated a concerning decline for this species across their distribution. East coast abundance estimates also show declines at specific sites in Tasmania. Though the western part of the range currently lacks abundance estimates, a southerly range contraction appears to be taking place at the northern extent of the distribution in Western Australia. Common Seadragons are protected throughout their range by the Australian Environmental Protection and Biodiversity Conservation Act (1999) as well as state legislation in each state where they occur. Overall, due to observed range contraction and declines in various parts of its distribution, and the ongoing impacts of climate change on its habitat, it is suspected that this seadragon has undergone a population reduction of >30% over the past three generations. Therefore, the Common Seadragon is assessed as Vulnerable A2bc.

The Common Seadragon (Phyllopteryx taeniolatus) occurs across Australia’s Great Southern Reef (GSR) from Nelson Bay in New South Wales to Cervantes in Western Australia, including Victoria, Tasmania, and South Australia (Global Biodiversity Information Facility (GBIF) 2024, iNaturalist 2024, SeadragonSearch 2024).

Common Seadragon populations occur around coastal southern Australia, with a clear genetic break between those from the region encompassing New South Wales, eastern Victoria, and Tasmania compared to those from Melbourne to the western coast (Stiller et al. 2023). There is very little genetic variability in the east coast group compared to the relatively more diverse populations west of Melbourne, and while there is some evidence of migration between the two groups, it is very limited (Stiller et al. 2023).

Population size estimates for Common Seadragons are only available for a subset of locations on the southeast coast (Martin–Smith 2011, Sanchez–Camara et al. 2011, Tustison et al. unpubl. data 2024, SeadragonSearch 2024). JARA (Just Another Red–List Assessment), a Bayesian state–space framework (Winker et al. 2020), was used to combine these regional abundance datasets to produce estimates of broader population trends. From these analyses, no major changes in abundance were evident in New South Wales over the time period analysed, which included population size estimates from 2001–2007 and 2016–2023. Long term raw count data for the Sydney region (A. Trevor–Jones, M. McFadden and others, unpub. data 2024), showed abundance fluctuations over the years 2011–2023, but no overall trend. In this region, a recent large mortality event occurred (in April 2022) where over 200 seadragon carcasses appeared on Sydney shorelines in the wake of a series of climate–change related east coast low storm events. This appears to have led to the disappearance of seadragons at some exposed sites but not other more protected sites (D. Booth unpubl. data 2024). So around Sydney, there appears to be no substantial change in abundance over the last decade, despite large inter–annual fluctuations.

However, data from Tasmania, from Bicheno and the Derwent River Estuary, indicate substantial decreases over the same period. JARA analysis of available data (Martin–Smith 2011, Sanchez–Camara et al. 2011, Tustison et al. unpub. data 2024, SeadragonSearch 2024) suggested a near 50% decrease in abundance in Bicheno, Tasmania 2011–2021. There is no longer enough data to estimate more contemporary population sizes for the Derwent River Estuary, despite earlier surveys being able to do so (Martin–Smith 2011). Overall, for east coast populations, a clear decline is evident in Tasmania, with probable stability in New South Wales (see the Supplementary Information).

In South Australia, a contraction for the Common Seadragon area of occupancy may have taken place in Gulf St Vincent. “Numerous weedies were observed” in northern Gulf St Vincent during the years 1965–1971 (S. Shepherd pers. comm.), but by the years 1990–2005, no Common Seadragon sightings were being reported from that area (Baker 2009). It is unclear whether this possible contraction is ongoing. Historically, local extirpations have happened in South Australia’s Spencer Gulf as a result of prawn trawling, which still occurs. There are no historical or contemporary population size estimates for South Australia or Western Australia.

A more widespread study across most of the species’ range using Reef Life Survey (RLS) methods (Edgar et al. 2023) reported that the Common Seadragon had “significantly declined by 59% from 2011 to 2021”. However, seadragon abundances appeared very low at these RLS sites, with none recorded for 2021 or 2022. It may be that the sites surveyed are located on the peripheries of main population home ranges, and therefore surveys in those areas may not reliably indicate wider changes in abundance. Mean densities of under 0.2 per 1,000 m² are well below those from other datasets reported here. Surveys of deeper sites (below 15m) across the species' range are rare and also represent a major data gap.

Overall, due to observed range contraction and declines in various parts of its distribution, the ongoing impacts of climate change on its habitat, and the lack of species–specific conservation actions, it is suspected that the Common Seadragon has undergone a population reduction of >30% over the past three generations, the causes of which have not ceased.

Common Seadragons inhabit macroalgal habitats within the Great Southern Reef (GSR), where they exhibit a close association with Sargassum, kelp, and seagrass in rocky reef and sandy habitats (Edgar 2000, Connolly et al. 2002, Baker 2009, Sanchez–Camara et al. 2006, Allan et al. 2022a). They primarily inhabit depths of <5m to 30m but have also been observed at depths up to 50m (Edgar 2000). Research in Botany Bay, New South Wales, found Common Seadragons prefer macroalgal habitats, particularly those with a minimum of 40% Golden Kelp (Ecklonia radiata) cover but they will also utilize sandy habitats if mysid shrimp densities are higher there (Allan et al. 2022a). 

In Common Seadragon reproductive cycles, males brood clutches of 250–300 eggs embedded in the skin on the undersides of their tails (Kuiter 2003) over periods of 4–8 weeks (Edgar 2000, Sanchez–Camara et al. 2005). Juveniles measure 2.9–3.5 cm at hatching (Forsgren and Lowe 2006) and grow to full size within 1–2 years (Sanchez–Camara et al. 2005, SeadragonSearch 2024). Adult seadragon body sizes range from 30–46 cm total length (Edgar 2000, Sanchez–Camara et al. 2005), but appear to be variable across their distribution, with smaller–bodied animals in the west (SeadragonSearch 2024). In Tasmania, Common Seadragons show marked sexual dimorphism, though this is not consistent across the range. Seadragons reach reproductive maturity at around one year of age (Sanchez-Camara et al. 2005). Breeding seasons and numbers of broods per season appear to vary in different parts of the Common Seadragon’s range (depending on water temperatures), but the overall breeding season takes place from June–March, with peak breeding season occurring November–January (Sanchez-Camara et al. 2005, SeadragonSearch 2024). Male Common Seadragons have been documented brooding 1–2 clutches per season in the wild (Sanchez-Camara et al. 2005), though this number may be higher in some regions (SeadragonSearch 2024). There are no range-wide estimates of Common Seadragon longevity, and lifespans seem to vary in different parts of the range. Seadragons in New South Wales appear to live about 6 years (Sanchez–Camara et al. 2011), while Tasmanian individuals have been documented living to 16 years (SeadragonSearch 2024). Due to this variability, two different generation lengths have been used for the calculations in this assessment. Because generation length is defined in Red List assessments as the average age of parents of the current cohort, we have used a generation length of 4 years for New South Wales populations and 8 years for Tasmanian populations. These numbers reflect mid–lifespan for Common Seadragons from each of these regions.

Current threats to the Common Seadragon include habitat degradation resulting from pollution, coastal development, and, increasingly, the effects of climate change. Common Seadragon populations show high site fidelity and typically do not move far outside of their home ranges (Connolly et al. 2002, Sanchez-Camara et al. 2004), making them susceptible to changes in their environments. Seadragons are strongly associated with macroalgae for camouflage, hunting, and feeding; therefore loss of macroalgal cover is likely to impact their survival. 

Algae cover has been retracting in various parts of Australia’s Great Southern Reef (GSR) as climate change results in rising water temperatures, as well as more intensive heat waves and storms (Edyvane 2003, Wernberg et al. 2013, Davis et al. 2022). Tasmania in particular has experienced significant losses of Giant Kelp (Macrocystis pyrifera) (Edyvane 2003), known to provide important seadragon habitat. The combination of warming waters and the spread of range-expanding, overgrazing sea urchins have caused these dramatic losses of kelp, resulting in changes in ecosystem structure (Ling and Keane 2024). This appears likely to continue over the next decade without strong conservation action (Ling and Keane 2024).

In addition to macroalgal loss in Tasmania, climate–related habitat changes are also occurring at the northern extents of the GSR. As northern waters warm, Common Seadragons and other coastal species adapted to shallow water environments will not be able to move south beyond the continental shelf, resulting in contracting ranges for many species (Parker et al. 2019, 2021). The effects of climate change are suspected to continue to compromise habitat quality (Parker et al. 2019, 2021), and may also result in increased competition or predation from warmer-climate species moving south (Shalders et al. 2018). At a physiological level, it is likely that rising water temperatures will have adverse effects on seadragons and other temperate fishes. The capacity of fishes to adapt to temperature changes are unknown for many species (Little et al. 2020), but it is understood that ongoing exposure to higher temperatures can negatively impact stress physiology in fishes, reducing energetic capacity to cope with disease and other stressors (Alfonso et al. 2021). Increasing incidences of disease (e.g. “white wilt”) have been observed across the Common Seadragon’s range, possibly concurrent with warming waters (SeadragonSearch unpub. data 2024).

Further habitat degradation is occurring in some parts of the Common Seadragon’s range due to pollution, coastal development, and trawling. Near major urban centers, sand dredging, coastal development, and discharge of storm water and sewage contribute to increased sedimentation, which can have a deleterious impact on macroalgal systems (Schiel et al. 2006, Shepherd et al. 2009). In a prawn trawling survey of Spencer Gulf in South Australia, Common Seadragons were the most frequently captured syngnathid species, accounting for 41 of the 112 syngnathids in the catch (Currie et al. 2009). Most of the Common Seadragons captured were collected from blocks of low trawl intensity (Currie et al. 2009), implying that seadragons are more abundant in areas with less historical trawling effort. The results of the study suggested that thousands of syngnathids may be captured annually due to Spencer Gulf prawn trawling (Currie et al. 2009). If the ratio from the trawl catch mentioned above is applied, that would mean hundreds of captured Common Seadragons each year. Prawn trawling is currently ongoing in both the Spencer and St Vincent Gulfs. Furthermore, prawn trawling has been shown to have strong impacts on benthic fauna and flora in the region (Svane et al. 2009).

Given the limited dispersal abilities of Common Seadragons, as well as their finely-tuned adaptations to algal habitats, changing environmental conditions and loss of algae cover will likely impact local abundances of this species.

A comprehensive analysis of the trade in Australian syngnathids, including Common Seadragons, was undertaken in the early 2000s (Martin-Smith and Vincent 2006). In this study, the only trade recorded for this species was a small live trade for the commercial aquarium market — mean exports of 155 ± 84 (SD)/year with a maximum of 300/year during the period 1994–2001 (Martin-Smith and Vincent 2006). All of these seadragons were the captive–reared offspring from a small number of wild–caught gravid males (<10/year). In this study, the authors found that references to ‘sea dragons’ in Traditional Chinese Medicine (TCM) referred to other syngnathid species, namely Solegnathus spp. (usually called pipehorses in English) and there was no apparent use of either Common or Leafy Seadragons in the TCM market (Martin-Smith et al. 2003, Martin-Smith and Vincent 2006).

There appears to have been no systematic evaluation of the trade in Common Seadragons since the early 2000s. Only seahorses (Hippocampus spp.) are listed on CITES Appendix II, so there is no centralised database of seadragon exports. However, as all syngnathids are protected species under the Environment Protection & Biodiversity Conservation Act 1999 (EPBC), permits are required for commercial wildlife trade operations involving wild harvest or captive breeding — current and archived permits are listed on the Department of Climate Change, Energy, the Environment and Water (DCCEEW) website.

Over the period 2004–2024, there have been three companies with Wildlife Trade Operation (WTO) approvals to collect Common Seadragons and export captive–reared offspring. Despite some success in egg transfer between captive individuals, there are no known fully–closed life cycles for commercial trade in Common Seadragons. There is some evidence to suggest illegal trade of adults exists, with two adults confiscated in the USA, tracked to a southern Western Australian origin (Stiller et al. 2023).

Common Seadragons are protected throughout Australian waters by the Australian Environmental Protection and Biodiversity Conservation Act (1999) and in state waters by the Fisheries Management Act 1994 (New South Wales), the Fisheries Act 1995 (Victoria), the Fisheries Act 1982 (South Australia), the Fish Resources Management Act 1994 (Western Australia), and the Tasmanian Living Marine Resources Management Act 1995 (Tasmania). Parts of the Common Seadragon’s range overlap with areas that have varying levels of protection, such as Marmion Marine Park in Western Australia, Encounter Marine Park in South Australia, and Cabbage Tree Bay Aquatic Reserve in New South Wales. It is important to note that recreational fishing is allowed in all zones of Australian marine parks except for sanctuary zones. Therefore, only a fraction of those waters are fully shielded from extractive human activities and the resulting impacts on habitats. There is no species–specific or international trade legislation in place for the Common Seadragon. International measures are needed to legislate and implement mitigation of anthropogenic greenhouse gas emissions to limit the impacts of climate change on this species and its habitats.