Bargibant’s Pygmy Seahorse (Hippocampus bargibanti) occurs in the Eastern Indian Ocean and Northwest and Western Central Pacific from Sumatra, Indonesia to Vanuatu and from southeastern Japan to the southern Great Barrier Reef, Australia at depths of 4–40 m. This seahorse is exclusively associated with gorgonian corals of the genus Muricella. There are no published data on population trends and the total number of mature individuals for this seahorse to date, but it has been found to occur in low densities across the coral reefs in Sulawesi, Indonesia. Coral reefs are declining globally due to destructive fishing practices, overfishing, pollution, and climate change. However, given that gorgonian corals may exhibit some tolerance to ocean acidification and increased temperatures, we suspect that Muricella corals have experienced low levels of decline over the past decade. Consequently, it is suspected that Bargibant’s Pygmy Seahorse has experienced a past and ongoing population reduction at a level that has not approached the threshold for a threatened assessment. This seahorse is therefore assessed as Least Concern. Further research is needed on population size and trends, and on the status Muricella populations across its range.

Bargibant’s Pygmy Seahorse occurs in the Eastern Indian Ocean and Northwest and Western Central Pacific from Sumatra, Indonesia to Vanuatu, and from southeastern Japan to the southern Great Barrier Reef, Australia (Heard et al. 2019, Lourie et al. 2016).

There is no global estimate of population size for this seahorse. To date, there have been few dedicated surveys or local population estimates. Surveys by Smith et al. (2012) in Sulawesi, Indonesia between 2007 and 2008 found a low population density of 0.34 individuals per 200 m², translating to an estimated population of 686 individuals (95% CI: 134–1,505) across the 20 km reef within the Wakatobi Marine Protected Area (MPA). However, given the remote location of the MPA, it is likely that this density is higher than that exhibited across its distribution (Smith et al. 2012).

Given that the decline in Muricella corals is suspected to occur at low levels, Bargibant’s Pygmy Seahorse is suspected to have undergone a past and ongoing population reduction, but not to a level that would result in a threatened assessment.

Bargibant’s Pygmy Seahorse is one of the smallest seahorse species with a maximum recorded height of 3 cm (Gomon 1997, Shepherd et al. 2017). It is typically found at depths of 4–40 m (Tackett and Tackett 1997, Reijnen et al. 2011, Smith et al. 2012). This seahorse is a habitat specialist that exclusively inhabits Muricella gorgonian corals, specifically M. plectana and M. paraplectana, with its colour and bumps very closely resembling its hosts (Gomon 1997, Lourie et al. 2004, Reijnen et al. 2011, Smith et al. 2012, Tackett and Tackett 1997). Low occupancy rates of Bargibant’s Pygmy Seahorse in Muricella corals have been observed in Sulawesi, Indonesia, with rates of 20% and 13.7% reported by Smith et al. (2012) and De Brauwer et al. (2020), respectively. Further research is needed to understand the factors causing the low occupancy rates and how they might affect the species’ abundance and risk of extinction.

Little is known about its feeding, but this seahorse is likely similar to others which feed on small crustaceans such as copepods and amphipods (Kendrick and Hyndes 2005, Manning et al. 2019, Tackett and Tackett 1997).

Juveniles and brooding males have been observed by Tackett and Tackett (1997) in March, June, August, and November, which suggests that breeding might happen year-round for this species (Lourie et al. 2004). Eggs are located within the male’s trunk region without being enclosed within a separate brood pouch structure, which might reflect an adaptation to its diminutive size (Lourie et al. 2004). Its gestation period lasts for an average of two weeks, and a male may give birth to 12–65 juveniles in one breeding cycle (Lourie et al. 2004, Shepherd et al. 2017). The average length at birth is 2 mm (Lourie et al. 2004).

Bargibant’s Pygmy Seahorse is primarily threatened by habitat loss and degradation.

Coral reefs in Southeast Asia are threatened by destructive fishing practices (e.g. poison fishing, blast fishing), coastal development, pollution (e.g. oil spills, agricultural run-off), sedimentation, overfishing, and climate change (Burke et al. 2002, Chan and Hodgson 2017, Selgrath et al. 2018). Coral reefs within the Coral Triangle are also projected to continue declining in the future from increases in ocean temperature and acidification (Cornwall et al. 2021, McManus et al. 2020). In Australia, the Great Barrier Reef has undergone four mass bleaching events over the past decade due to warmer temperatures, with more than 98% of coral reefs experiencing some level of bleaching (Great Barrier Reef Marine Park Authority 2022, Hughes et al. 2021), although substantial recovery of reefs in the Great Barrier Reef has been recorded in 2022 (Australian Institute of Marine Science 2022).

It is not entirely clear how gorgonian corals are affected by climate change. Gorgonian corals in Mexico were found to be more tolerant to higher temperatures compared to scleractinian corals (Goulet et al. 2017, McCauley et al. 2018), while gorgonian coral population in Malpelo Island, Colombia experienced reduced growth rates associated with higher temperatures during ENSO events in 2014–2016 (Quintanilla et al. 2019). The effects of increased CO2 concentrations from ocean acidification may vary depending on species and location. Bramanti et al. (2013) found negative impacts of acidic conditions on sclerite shapes and skeletal formation of a temperate gorgonian coral, while other studies found that tropical and subtropical octocorals are less affected by low pH levels, potentially due to the protection of their fleshy tissues (Gabay et al. 2013, 2014; Gómez et al. 2015, Inoue et al. 2013, Januar et al. 2017). Diseases associated with warmer temperatures have been observed in gorgonian coral populations globally, but reports from the Indo-Pacific region have been few in number (Sutherland et al. 2004, Weil et al. 2016).

Gorgonian skeletons have also been traded as curios, jewellery, and ornaments (Cooper et al. 2011, Shepherd et al. 2017).

Little is known about the trends of Muricella corals, and no global assessments have been done for any Muricella species. However, it is suspected that the Muricella corals have declined over the past decade, but at a low level that is not likely to lead to declines in Bargibant's Pygmy Seahorse reaching the thresholds for a threatened assessment, given the potential for higher resilience among gorgonian corals against ocean warming and acidification. Further research is needed to determine how Muricella corals are coping with climate change and what effect this has on populations of this seahorse.

Dive tourism may negatively impact this species (De Brauwer et al. 2019, Giglio et al. 2019), but there is likely refuge in remote parts of its range.

Its naturally low densities on specific coral hosts leaves it particularly vulnerable to localized extinction (De Brauwer et al. 2020).

Bargibant’s Pygmy Seahorse has been recorded in two commercial trades involving seized specimens in 2005 (UNEP-WCMC 2022). The species was successfully raised in the Steinhart Aquarium at the California Academy of Sciences, San Fransisco (Stockton 2014, Shepherd et al. 2017). However, aquarium trade of this seahorse likely occurs at low levels due to its challenging husbandry (Shepherd et al. 2017, Smith 2010), as has been observed from Waikiki Aquarium’s failed attempt in raising the species in 2003 due to a decline in its coral (Stockton 2014). Its low density and strong resemblance to Muricella corals can also make the species extremely difficult to find (Gomon 1997, Shepherd et al. 2017, Smith 2010). It is also unlikely to be used for traditional Chinese medicine as preference is typically given to large, pale, and smooth seahorses (Vincent et al. 2011).

There are no species-specific management or conservation measures in place for Bargibant’s Pygmy Seahorse. However, all Hippocampus species are listed under Appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), requiring their exports to be sustainably sourced and subject to regulations (Vincent et al. 2011). This CITES Appendix II listing also warrants the inclusion of all Hippocampus species under the Phillippines’ Republic Act No. 10654, which prohibits the fishing, collecting, trading, or exporting of all species listed under Appendix II of CITES, except for scientific research or simultaneous conservation and commercial breeding, in which these actions may be allowed. It is also considered an offense to kill, take, trade, keep, or move all Sygnathids species under Australia’s Environment Protection and Biodiversity Conservation Act 1999, unless they significantly contribute to the species’ conservation and/or will not adversely affect the species. Wild seahorse exports are also banned in Indonesia and Malaysia in response to the CITES Review of Significant Trade (Foster 2016).

This species occurs in at least two protected areas across its range (i.e. Wakatobi National Park and Great Barrier Reef Marine Protected Area) (UNEP-WCMC and IUCN 2022).

Tourism operations that seek syngnathid sightings should follow best practices for SCUBA diving with seahorses (IUCN SSC Seahorse, Pipefish, and Seadragon Specialist Group 2023).

Further research is needed to understand the trends in abundance of both the Bargibant’s Pygmy Seahorse and Muricella corals, especially in light of climate change. Climate change mitigations and regulations on destructive fishing practices are necessary for the conservation of this species.