Denise’s Pygmy Seahorse (Hippocampus denise) occurs in the Eastern Indian Ocean and Northwest and Western Central Pacific from Sumatra, Indonesia to Vanuatu, and from Kagoshima Prefecture, Japan to the northern Great Barrier Reef, Australia, at depths of 13–102 m. This seahorse relies exclusively on gorgonian corals. Although coral reefs are generally threatened by destructive fishing practices, overfishing, pollution, and climate change, fleshy gorgonian corals have shown some resilience to warming waters and ocean acidification. Overall, it is suspected that gorgonian corals are declining less rapidly than scleractinian hard corals. Aquarium trade is a minor threat for this seahorse and likely occurs at low levels. 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 within its range. However, given the low declines in its habitat and its association with multiple gorgonian coral species, reduction in its population size is suspected to occur at a level that does not approach the threshold for a threatened assessment. Denise’s Pygmy Seahorse is therefore assessed as Least Concern. Further research is needed on population size and trends, and on the trends of its gorgonian coral hosts across its range.

Denise’s Pygmy Seahorse occurs in the Eastern Indian Ocean and Northwest and Western Central Pacific from Sumatra, Indonesia to Vanuatu, and from Kagoshima Prefecture, Japan to the northern Great Barrier Reef, Australia (GBIF 2022, Heard et al. 2019, Lourie et al. 2016, Lourie and Randall 2003).

There are no global population estimates for Denise’s Pygmy Seahorse and there have been few local surveys or population estimates conducted to date. Surveys from Sulawesi, Indonesia in 2007 and 2008 found a low population density of 1.17 individuals per 200 m², which translates to an estimated population of 2,343 individuals (95% CI: 1,266–3,420 individuals) across the 20-km stretch of reef within the study area (Smith et al. 2012). However, this estimate is likely higher than that exhibited across its distribution, given that the study was conducted within the remote and less-populated Wakatobi Marine Protected Area (Smith et al. 2012).

Based on the inferred low levels of decline in its gorgonian coral hosts over the past decade, it is suspected that the population size of this seahorse is declining at a level that has not approached the threshold for a threatened assessment.

Denise's Pygmy Seahorse is found at depths of 13–102 m (Foster et al. 2012, Lourie and Randall 2003, Nishikawa et al. 2011). This seahorse has an obligate association with gorgonian corals and its appearance strongly resembles its coral hosts (Lourie and Randall 2003, Reijnen et al. 2011). It has been found on several gorgonian coral genera, including Annella, Muricella, Villagorgia, Acanthagorgia, Echinogorgia, Melithaea, Verrucella, and Ellisella (De Brauwer et al. 2020, Foster et al. 2012, Heard et al. 2019, Lourie and Kuiter 2008, Lourie and Randall 2003, Nishikawa et al. 2011, Reijnen et al. 2011, Smith et al. 2012). Low occupancy rates of Denise’s Pygmy Seahorse among Annella and Villagorgia corals have been observed in Sulawesi, Indonesia, with Smith et al. (2012) reporting an occupancy rate of 7.8% in A. reticulata and De Brauwer et al. (2020) reporting an occupancy rate of 2.7% and 10.5% in Annella spp. and Villagorgia spp., respectively. Further research is needed to determine the occupancy rates of Denise's Pygmy Seahorse among other gorgonian coral species and to understand the factors causing the low occupancy rates and their effects on the species’ abundance and risk of extinction.

Little is known about the diet of this seahorse, but it is likely similar to other Hippocampus species, which commonly feed on small crustaceans such as copepods and amphipods (Kendrick and Hyndes 2005, Manning et al. 2019).

Denise's Pygmy Seahorse is one of the smallest seahorse species, with a maximum height of 2.4 cm (Lourie and Randall 2003). Based on the length of pregnant specimens, sexual maturity is inferred to occur at a height of less than 1.33 cm (Lourie and Randall 2003). Pregnant specimens were found by Lourie and Randall (2003) in February, May, and October, and a pregnant male was observed by Smith and Tibbetts (2008) in December, which suggest that breeding might occur year-round. Eggs are located in the trunk of males without being enclosed in a separate brood pouch, which potentially reflects an adaptation to its small size (Lourie and Randall 2003; Smith and Tibbetts 2008). Its gestation period lasts for 11 days and births happen at dawn, with 6–16 juveniles being released per pregnancy (Lourie and Randall 2003, Smith and Tibbetts 2008).

Denise’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). Within the Coral Triangle, the decline in coral reefs is projected to continue due to increases in ocean temperature and acidification (Cornwall et al. 2021, McManus et al. 2020). Increased temperatures have resulted in higher frequencies of bleaching events in the Great Barrier Reef, Australia. Four mass bleaching events have occurred over the past decade and affected more than 98% of coral reefs within the region (Great Barrier Reef Marine Park Authority 2022, Hughes et al. 2021), although substantial recovery efforts to restore reefs in the Great Barrier Reef were undertaken in 2022 (Australian Institute of Marine Science 2022).

It is not entirely clear how gorgonian corals are affected by climate change. Findings on coral reefs in Mexico suggest that they may be more tolerant to higher temperatures than scleractinian corals (Goulet et al. 2017, McCauley et al. 2018), but Quintanilla et al. (2019) found reduced growth rates in gorgonian coral population in Malpelo Island, Colombia associated with higher temperatures during ENSO events in 2014–2016. Ocean acidification may affect gorgonian corals due to increased CO₂ concentrations, but the effects may vary depending on species and location. Negative effects on sclerite shapes and skeletal formation were observed in a temperate gorgonian coral under acidic conditions (Bramanti et al. 2013), 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, Gabay et al. 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, although reports from the Indo-Pacific region have been fewer than elsewhere (Sutherland et al. 2004, Weil et al. 2016).

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

No assessments have been done for any of the coral species on which Denise’s Pygmy Seahorse has been found, and little is known about the trends of gorgonian coral populations across the species’ range. However, it is inferred that the gorgonian coral hosts have declined at a level that is not enough to warrant a threatened assessment for this seahorse, considering the potential for gorgonian corals to be relatively resilient against ocean acidification and warming conditions.

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).

Denise's Pygmy Seahorse has been recorded in five aquarium trades between 2004 and 2006, with a total of around 2,100 wild-sourced individuals being exported from Indonesia for commercial purposes during this period (UNEP-WCMC 2022). Aquarium trade of this seahorse likely occurs at low levels given the challenges posed by its diminutive size and cryptic nature.

There are no species-specific management or conservation measures in place for Denise's Pygmy Seahorse. However, all seahorses (Hippocampus spp.) are listed under Appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), which requires exports to be sustainably sourced and subject to regulations (Vincent et al. 2011). Exports of wild seahorses are also banned in Indonesia and Malaysia as a response to the CITES Review of Significant Trade (Foster 2016). All Syngnathids are listed under Australia’s Environment Protection and Biodiversity Conservation Act 1999, in which the killing, taking, trading, keeping, and moving of Syngnathids are considered an offense. Under this Act, permits may only be issued if the aforementioned actions significantly contribute to the species’ conservation and/or will not cause adverse effects on the species. Under the Philippines’ Republic Act No. 10654, it is prohibited to fish, collect, trade, or export all species listed under CITES Appendix II. However, these actions may be allowed for scientific research or where conservation breeding is done simultaneously with commercial breeding.

This species occurs in at least two protected areas within its range (e.g. 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 population trends and the effects of coral decline on Denise’s Pygmy Seahorse. Conservation measures needed for this species include mitigating climate change and regulating destructive fishing practices.