Episode 256: In the Sea, Horses Swim Wild

**The Knysna Seahorse (*Hippocampus capensis***)

The Knysna seahorse, also called the Cape seahorse, is the only fully estuarine species of seahorse in the world with the smallest known geographical range, existing in seagrass belts in just four estuaries off the coast of South Africa. Unfortunately, due to its sensitivity to pollution and its tiny range, the Knysna seahorse was also the first seahorse species to be listed as endangered by the IUCN. They are small delicate creatures, generally a mottled brown or yellow colour but with massive variations in colour depending on the local conditions of the water and even the mood of the individual! As seahorses go it is relatively medium sized with a short snout and is distinctive for its lack of a coronet, the crown like structure made up of the bony plates of the skeleton that most seahorses have.

As the only endemic seahorse to South Africa, it is truly a national treasure, but sadly one that is in imminent danger of going extinct.

Fast evolution: how did seahorses come to be?

Seahorses are teleost fishes, also called the ray finned fishes, a large and very diverse group that contain many species of economically important fish, as well as the beautiful species that have awed divers on coral reefs and visitors of aquariums all over the world. Seahorses are a part of the Syngnathidae family which also includes pipefish and the elaborate sea dragons. Many scientists theorize that this group of fish evolved in response to shallow water habitats developing as tectonic plates shifted, causing vast changes in sea levels. As water grew shallower, sea grass bed developed, taking advantage of the sunlit waters and providing a home for the Syngnathidae family with their unique ecology.

Out of the vast diversity of ray finned fishes, seahorses are extremely unique; not only for their strange shape but also for their unique reproductive physiology, which sees the males nourish the developing embryos made by the females. Scientists investigating the seahorse found the secrets to their strange development in their genes or perhaps more importantly, in the genes that were found to be missing!

The genome of the tiger tailed seahorse (Hippocampus comes) was studied and compared against other fish in the sister group the pecomorph fishes, including species like sticklebacks, fugu and tilapia, which diverged away from seahorses around 103 million years ago in the Cretaceous period. The geneticists found that seahorses have lost a significant number of genes through mutations that have led to the unique fish we see today. These mutations are classed as neutral mutations, meaning they have neither detrimental nor beneficial effects on the species, but over time can be become more frequent, leading to evolution of different body shapes, traits, and abilities. Scientists identified a gene that codes for pelvic fins, and when they removed this gene from experimental zebra fish, they found that these fish lost their pelvic fins, as is seen in seahorses. They also found seahorses lack genes for the enamel that coats teeth, which is likely the evolutionary pressure that led seahorses to develop a long tubular snout to suck up prey like small crustaceans and plankton from the water column. Another key finding was a lack of olfactory genes, which has driven seahorses to become so reliant on their eyesight, since their sense of smell is so poor. This study was vital to understanding seahorse evolution, especially as their fossil record is so sparse.

A diagram showing the evolutionary history of the seahorse and its related species. The numbers on the branches show how many gene mutations there are between the seahorse and the other species displayed. Image from Lin et al, 2016

Range and habitat: a unique South African species

As mentioned previously, this rare species is endemic to the seagrass beds of only four estuaries in South Africa; the Knysna, Swartvlei, Keurbooms and Klein Brak estuary. They can be found between depths of 0.5m to 20m and hang around submerged plants, especially around five species of aquatic plants: dwarf eelgrass (Zostera capensis), spoon seagrass (Halophila ovalis), spiral tasselweed (Ruppia cirrhosa) and the seaweeds Caulerpa filiformis and Codium extricatum. This vegetation is really important for seahorses who rely on this camouflage to ambush their prey, as they aren’t exactly the speediest species! Unfortunately, this love of seagrass fields can make them a challenging species to study, especially given how rare they are. In recent years, fieldwork has identified Knysna seahorses living in artificial habitats, in particular discarded mattresses that have found their way into the estuaries from the significant domestic and industrial pollution in the area. One study identified as many as 1.2 individuals per square metre of mattress, similar to the phenomenon of New Holland seahorses (Hippocampus whitei) choosing artificial habitats like shark nets in Australian bays. This could provide new conservation opportunities to tackle the habitat degradation this species is facing, although many scientists stress we still don’t know what possible long term impacts these artificial habitats might have for the security of the Knysna seahorse.

The locations on the coast of South Africa where the Knysna Seahorse can be found. (Image from Mkare et al, 2021)

An ocean romance: unique reproduction

Seahorses are of course famous for their unusual reproduction; males are the ones that carry the embryos in specialised sacs called brood pouches. Seahorses are known for their elegant mating dances, will culminate in the male and female entwining tails and the female aligning a specialised tube called an ovipositor to the males brood pouch and transferring the eggs. The males will carry these eggs anywhere from 9 to 45 days, depending on the species and the environmental conditions, like salinity, temperature and oxygen levels of the water. Seahorses are ovoviviparous, which means the eggs develop inside the parent and hatch within or immediately after release from the parent. In this case, the baby seahorses hatch inside the male and he pumps his tails to expel them from his brood pouch, where the tiny little seahorses are left to fend for themselves; no parental care from either mum or dad here!

In 1998, a Knysna seahorse captive breeding programme was set up and this gave scientists a chance to understand the reproduction of this endangered species and possible ways it could be helped to increase numbers. It was known previously that the Knysna seahorse only bred through the summer months, so the length of the photoperiod was tested to see if their breeding could be extended into the winter months in captivity. However, they found that the length of the photoperiod and the temperature of the water had no effect at all on the pregnancy rates in the seahorses, suggesting there is some other trigger that causes them to reproduce. In the wild, the numbers of young produced fluctuates massively, which was also noted in captivity. This suggests variation in the number of young is an entirely natural phenomenon, with the exact causes unlikely to be discovered. Just another mystery of nature!

Conserving the Knysna seahorse

The Knysna Basin in South Africa, the key habitat for the Knysna Seahorse (image from the Knysna Basin Project)

The largest population of these rare seahorses is found in the Knysna estuary, which is a massive concern as this is one of the most heavily used bodies of water in South Africa. There is huge urban development leading to significant storm run-off and sedimentation of the waterways, which can reduce oxygen in the water and smother the plants the seahorses rely on, and even their gills. Heavy industrial, domestic and recreational pollution is affecting water quality, and field studies in the area have reported seahorses with lesions caused by the trace metals, hydrocarbons, pesticides and organic waste in the water. Seahorses are fairly delicate species, and they cannot tolerate long term disruptions to their environment. Climate change and increasing water temperatures are also a serious threat to these fish, as evidenced by a mass mortality event in 1991 when 3000 dead seahorses were found washed up on the shore. It was believed that heavy rain caused this when flooding from the land breached the estuarine mouth, displacing water and causing the water levels to drop significantly in some areas. Those seahorses trapped in these shallower pools were killed when the temperature rose sharply when the sun returned after the storm was over.

However, this species hasn’t slipped under the radar, and there is work in place to protect them as a national species of South Africa. The species is protected by the South African Fisheries act of 1973, making it illegal to trade these seahorses commercially. This is particularly important as many seahorse species are frequently used in traditional Chinese medicine. Despite the heavy urban expansion in recent years, all of the estuaries in which the seahorses live are under the control of the National Parks Board, and therefore are given some level of protection. In 1998, when the dire situation was discovered, a captive breeding programme was initiated and is currently still active in the Two Oceans Aquarium in Cape Town (https://www.aquarium.co.za/species/entry/knysna_seahorse) and Antwerp Zoo (https://www.zooantwerpen.be/en/).

New technology has also made it easier for scientists to study this hard-to-find species in the wild. Visible Implant Fluorescent Elastomer (VIFE) tags are small, harmless tags with a fluorescent dye attached that can be injected under the skin, and seahorses can then be identified easily with combinations of different colours, rather than relying on subtle differences in the mottling pattern or using mark and recapture techniques. This makes the job of tracking seahorse populations in the wild far easier.

The injection of VIFE tags into a Knysna seahorse. You can see the pattern of fluorescent spots in the second image (image from ProjectSeahorse.org)

Genetic studies are also allowing decisions to be made about the population management of the species in the future. Translocation of individuals from one population to another has been considered in the past but has always been controversial, with many scientists fearing the effects of outbreeding depression. This is a phenomenon that occurs when populations of the same species become so isolated from each other and so adapted to their ecological niche that if they were to interbreed, the resulting offspring would have reduced fitness; being a mix of both populations, but without the necessary adaptations to survive in either populations niche. Although the realities of outbreeding depression are hotly debated by conservationists, genetic management of populations is something that needs to be carefully considered. In the case of the Knysna seahorse, it was long thought that the populations in the three estuaries had been separated for too long, when in fact genetic analysis has revealed that this is not the case and there is evidence of some gene flow between the populations. The Knysna estuary population is the most stable, so focused protection of this population is key, as it could act as a donor population for any future declines in other populations.

Conservation tips and organizations: ways to help the Knysna seahorse

There are some great organizations out there doing wonderful work not just for the seahorse but for the ecological health of the estuary as a whole.

The IUCN seahorse, pipefish and sea-dragon specialist group

https://www.iucn-seahorse.org/

This is a group of specialists in seahorse, pipefish and sea-dragon biology, ecology and conservation that are working to conserve seahorse populations and their habitats around the world, as well conserving related species like trumpetfish, cornetfish and shrimpfish.
You can visit their page on Knysna seahorse conservation at https://www.iucn-seahorse.org/knysna/
Here they provide helpful tips for protecting the Knysna seahorse including:
Advice on good boating practice for locals, in particular avoiding dropping anchor in seagrass beds which are critical for seahorse survival, as well as many other species.
Don’t litter! This isn’t just good advice for Knysna locals but to everyone across the world!
Practice responsible fishing: the IUCN specialist group provides a breakdown of the governments rules on fishing and catch and release expectations.
For any seahorse species, never buy dried seahorses as mementos and never handle wild seahorses.
There are many great citizen science projects that operate all over the world, in particular iSeahorse, which allows people all over the world to record observations of seahorses for an international database.

Project Seahorse

https://projectseahorse.org/

This is an NGO working on seahorse and coastal ecosystem research and aiming to protect seahorses through collaborating with local communities, managing seahorse populations, establishing protected areas, limiting fisheries and controlling trade in seahorses for aquariums, traditional medicine and mementoes.
You can visit their take action page (https://projectseahorse.org/take-action/) for more information on ways to support the organisation.
Also if you’re a fan of chocolate (who isn’t?!) then you can feel happy buying Guylian chocolates, as they support Project Seahorse as part of their sustainability programme. https://www.guylian.com/us/sustainability/

Knysna Basin Project

http://knysnabasinproject.co.za/

This is an NGO working on research to improve the ecological health of the Knysna basin, protecting the home of the Knysna seahorse.
They have management, conservation and education programmes, in particular the shoresearch project; a citizen science project that relies on local volunteers to monitor the intertidal biodiversity in the Knysna estuary.

Awesome videos!

References

Bell, E.M. Lockyear, J.F. McPherson, J.M. Marsden, A.D. and Vincent, A.C.J. (2003) ‘First field studies of an endangered South African seahorse, Hippocampus capensis.’ Environmental Biology of Fish, 67(1), pp 35-46

Claassens, L. and Harsati, D. (2020) ‘Life history and population dynamics of an endangered seahorse (Hippocampus capensis) within an artificial habitat.’ Journal of Fish Biology, 4, pp 974-986

Galbusera, P.H.A. Gillemot, S. Jouk, P. Teske, P.R. Hellemans, B. and Volckaert, F.A.M.J. (2007) ‘Isolation of microsatellite markers for the endangered Knysna seahorse Hippocampus capensis and their use in the detection of a genetic bottleneck.’ Molecular Ecology Notes, 7(4), pp 638-640

Lin, Q. Fan, S. Zhang, Y. and Xu, M. (2016) ‘The seahorse genome and the evolution of its specialised morphology.’ Nature, 540(7633), pp 395-399

Lockyear, J. Kaiser, H. and Hecht, T. (1997) ‘Studies on the captive breeding of the Knysna seahorse, Hippocampus capensis.’ Aquarium Sciences and Conservation, 1(2), pp 129-136

Mkare, T.K. van Vuuren, B.J. and Teske, P.R. (2021) ‘Conservation priorities in an endangered estuarine seahorse are informed by demographic history.’ Scientific Reports, 11(1)

Teske, P.R. Cherry, M.I. and Matthee, C.A. (2003) ‘Population genetics of the endangered Knysna seahorse, Hippocampus capensis.’ Molecular Ecology, 12(7), pp 1703-1715

Teske, P.R. Lockyear, J.F. Hecht, T. and Kaiser, H. (2009) ‘Does the endangered Knysna seahorse, Hippocampus capensis, have a preference for aquatic vegetation type, cover or height?’ African Zoology, 42, pp 23-30