The Knysna seahorse, Hippocampus capensis, is an endemic fish species in South Africa belonging to the fascinating group of teleost fish renowned for their unique reproductive strategy where males carry and give birth to live young. It is the only known seahorse species to be completely estuarine dependent.
Description
They are identified by their short, tube-like snouts and a reduced number fins with no coronet, as it disappears during maturation, and a smooth curve along the arch of the neck. Along the body is encased in 11 trunk rings, but there are no spines, while the tail consists of 34 rings and short, blunt spines. The colour patterns often resemble the vegetation of their habitat, being mottled greenish or brownish with dark spots scattered along the body. Adult individuals measure up to 12 cm in length with the males being longer, heavier with longer tails and a slight keel above the brood structure.
Distribution
The species is adapted to living only in an estuarine environment and is currently found in the shallow water of three estuaries along the southern Cape coast, namely Knysna, Swartvlei and Keurbooms estuaries. Its common name is derived from its predominant occurrence in the Knysna Estuary.
Habitat
Seahorses are highly adapted to the fluctuations in salinity, which ranges between 1 and 59 g/kg; living in seagrass beds with mainly eelgrass, Zostera capensis, at depths of 0.5–20 m along the shoreline. This provides cover from predation as well as feeding opportunities.
Food
The Knysna seahorse utilises suction feeding by generating pressure inside the snout, drawing in prey such as plankton and benthic crustaceans found on surfaces of aquatic plants and the water column. Their colouration allows them to blend in with the vegetation and to ambush prey.
Sex and life cycles
Sex
Breeding occurs from September to April with the gestation period averaging four weeks. The juveniles are planktonic, as they immediately enter the water column. The female will deposit ripe eggs into the male’s brood structure to fertilise, protect and nourish the eggs. The female will then proceed with the production of new eggs during the male’s pregnancy for a second batch. The male will then brood the young in a pouch before giving live birth. The Knysna seahorse will produce between 25 and 60 young with each breeding cycle.
Family life
The Knysna seahorse is a monogamous species living in pairs, where the male and female mate on multiple occasions. Parental care is also observed within the species.
Smart strategies
The species have slow mobility, and to prevent being washed away by tidal currents, they use their prehensile tails to attach to submerged aquatic vegetation.
People and I
The species are mostly captured and collected for trading in the aquarium market and like most seahorse species worldwide, they may be believed to be of high medicinal value internationally.
Conservation status.
The Knysna seahorse is the first species to be listed in IUCN Red Data List of threatened species among seahorse species. The dwindling populations are attributed to habitat degradation within its limited range that occurs due to the eelgrass beds having a high vulnerability to anthropogenic effects. The ecology of the species, along with their life history, also render them Vulnerable, and the declines in population, as estuaries experience increased recreational activities. Conservation management of this species requires habitat protection and water management, which will ensure adequate habitat quality, as well as monitoring of the population size. Constructed artificial habitats such as marinas and boat harbours using Reno mattresses within the estuaries have increased population numbers and increased the range of the species. The species is protected under the National Environmental Management: Biodiversity Act 10 of 2004, which prohibits the collection of the species without a valid permit, and the three estuaries inhabiting the species are protected by the National Parks Board.
Relatives
The Knysna seahorse is interrelated with other South African Syngnathids species. Among the latter, are Critically Endangered pipefish species, namely the estuarine pipefish, Syngnathus watermeyeri, and the long snout pipefish, S. temminckii, which inhabits the Bushmans and Kariega estuaries in the Eastern Cape.
Scientific Name and Classification:
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Syngathiformes
Family: Syngnathidae
Genus: Hippocampus
Species: H. capensis Boulenger, 1900
Common names: Knysna seahorse, Cape seahorse (Eng.); Knysna-seeperdjie (Afr.)
Authors:
Ofentse Kokoti Ntshudisane, Thembile Khoza & Raesetsa Portia Mailula
References
- Bell, E.M., Lockyear. J.F., McPherson, J.M., Marsden, A.D. & Vincent, A.C.J. 2003. First field studies of Endangered South Africa seahorse, Hippocampus capensis. Environmental Biology of Fishes 67: 35–46.
- Claassens, L., Booth, A.J. & Hodgson, A.N. 2018. An endangered seahorse selectively chooses an artificial structure. Environmental Biology of Fishes 101:723–733.
- Claassens, L. & Harasti, D. 2020. Life history and population dynamics of an Endangered seahorse (Hippocampus capensis) within an artificial habitat. Journal of Fish Biology 97:974–986.
- Hamilton, H., Saarman, N., Short, G., Sellas, A.B., Moore, B., Hoang, T., Grace, C.L., Gomon, M., Crow, K. & Simison, W.B. 2017. Molecular phylogeny and patterns of diversification in syngnathid fishes. Molecular Phylogenetics and Evolution 107: 388–403.
- Lockyear, J.F., Kaiser, H. & Hecht, T. 1997. Studies on the captive breeding of the Knysna seahorse, Hippocampus capensis. Aquarium Sciences and Conservation:129–136.
- Lourie, S.A., Foster, S.J., Cooper, E.W.T., Vinvent, A.C.J. 2004. A guide to the identification of seahorses. Project Seahorse and TRAFFIC North America, Washington D.C.: University of British Columbia and World Wildlife Fund.
- Lourie SA, Pollom RA, Foster SJ. 2016. A global revision of the Seahorses Hippocampus Rafinesque 1810 (Actinopterygii: Syngnathiformes): Taxonomy and biogeography with recommendations for further research. Zootaxa 4146:1–66.
- Pollom, R.A. 2017. Hippocampus capensis. The IUCN Red List of Threatened Species 2017:e.T10056A54903534.
- Teske, P.R., Cherry, M.I. & Matthee, C.A. 2003. Population genetics of the endangered Knysna seahorse, Hippocampus capensis. Molecular Ecology 12:1703–1715.
- Teske, P.R., Lockyear, J.F., Hecht, T. & Kaiser, H. 2007. Does the Endangered Knysna seahorse, Hippocampus capensis, have a preference for aquatic vegetation type, cover or height? African Zoology 42:23–30.