The Natural History of the Common Seahorse, Hippocampus kuda
In 1852, an influential ichthyologist named Pieter Bleeker described Hippocampus kuda, the common seahorse. The word “hippocampus” literally means “horse sea monster” in Greek, and it was also used in Greek mythology as a half-horse, half-fish steed for the gods (Salin, 2003). Since Bleeker was in Indonesia at the time, he named the species “kuda”, which is part of the Indonesian word for “seahorse” (Anderson, 2000). Although Bleeker is the father of Hippocampus kuda, Carl Linnaeus was the first to formally describe a seahorse in 1758 in Systema Naturae (Salin, 2003). This fish has a recognizable shape with a thick-snouted deep head that sits at a 90-degree angle to its upright body that tapers into a curling, prehensile tail. At the end of the trunk and before the tail is a tiny anal fin. While seahorses do not have caudal or pelvic fins, they do have one dorsal fin that provides propulsion and two small pectoral fins behind the head that help the fish stabilize itself and maneuver through its environment (Salin, 2003). Another unique feature of seahorses that sets them apart from other fishes is that they have no teeth or stomach (Salin, 2003). Male seahorses also have an obvious brood pouch below the trunk that is used to carry eggs of their young.
Although there are many species of seahorses that may look similar, this one may be recognized by its lack of spines (Fig. 1). Instead of spines, H. kuda has 11 low, rounded rings down its body, 36 similar rings down its tail, and a rounded coronet at the top of its head (Lourie et al., 2004). The rings are the result of the seahorse’s scaleless skin being stretched tight over the bony plates that protect its body. The largest H. kuda recorded was 17 cm, and they can live up to five years (Lourie et al., 2004). Also called the yellow seahorse or spotted seahorse, the typical “default” colors of H. kuda can be black, light yellow with dark spots, or sand-colored, according to Lourie et al. (2004). However, since seahorses are masters of camouflage, they can change color and grow accessory filaments meant to imitate the surrounding flora and fauna or to assist in intra-species interactions, so their appearance is not consistent (Salin, 2003; Lourie et al., 2004).
Systematics and paleontology
Hippocampus kuda is in the superclass Osteicthyes and the class Actinopterygii. According to a genetic study done by Kawahara et al. (2008), seahorses are in the order Gasterosteiformes, the suborder Syngnathoidei, the family Syngnathidae, and the subfamily Hippocampinae. Also in the family Syngnathidae, and the closest living relatives to seahorses, are other fish with “fused jaws,” including pipefishes, pipehorses, and seadragons (Salin, 2003). This family is interesting because it appears to represent an evolutionary gradient from pipefishes to seahorses. Pipefishes are long, thin, and straight fish with little tail fins; they look a little bit like a flattened out seahorse. Pipehorses are next in the gradient, as their heads sit at around a 30-degree angle to their body and their tails are slightly prehensile. Seahorses complete the gradient with a fully prehensile tail and a head that sits at a 90-degree angle to the trunk. Fossils from the Syngnathidae family have been found as far back as the Eocene epoch, around 52 million years ago (Teske et al., 2004).
There has been some confusion over the monophyly of the order Gasterosteiformes, and whether or not seahorses are in that order or in the order Syngnathiformes (Kawahara et al., 2008). However, it seems that for the time being, the accepted order is Gasterosteiformes, even though that order will have to be split up at some point in the future to reflect groups based on monophyletic traits (Fig. 2). Also in the order Gasterosteiformes are cornetfishes, pegasids, snipefishes, sticklebacks, and trumpetfishes, but until more precise groups and relationships can be established, it is difficult to say how closely seahorses are related to those fishes (Lourie et al., 2004).
Within the genus Hippocampus, it can be extremely difficult to determine the species of a seahorse. In the past, there have been over 100 seahorse species named; however, careful studies have determined that many of those species were synonymous and estimate that there are really about 32 distinct described species (Fig. 3) (Salin, 2003). Because seahorses of the same species can vary so drastically in color and skin filaments due to extraordinary camouflaging capabilities, using appearance alone can lead one astray while attempting to identify a species. In addition, the size of a seahorse can also be misleading. Young seahorses can be easily mistaken for smaller species, and as fishing pressures increase, the adult sizes of seahorses are also decreasing (Salin, 2003).
Luckily, there are more reliable methods to use when identifying a seahorse, such as length of snout in relation to the length of the head, the number of tail rings, the number of trunk rings, the number of cheek spines, the number of eye spines, the number of trunk and tail rings that support the dorsal fin, and the number of dorsal and pectoral fin rays (Lourie et al., 2004). In addition, there are a few species that are similar to H. kuda, but that have distinguishing characteristics that can be used to tell them apart if one knows what to look for. For example, H. algiricus differs from H. kuda because it has very broad eye and cheek spines; H. ingens has more dorsal fin and tail rings; and H. kelloggi has a narrower body with a higher coronet, as well as more tail rings and larger spines (Lourie et al., 2004). In the past, the naming and taxonomic confusion surrounding seahorses has hindered communication between scientists and slowed research considerably, but now that scientists have access to methods such as genetic sequencing and are becoming more consistent with terminology in the growing number of seahorse studies, research on the fish may be better able to move forward.
Hippocampus kuda can be found in many temperate and coastal waters. Their geographical distribution spans from between 50 degrees North to 50 degrees South (Lourie et al., 2004). H. kuda has been identified and confirmed in the following countries: Australia, Cambodia, China, Fiji, France, India, Indonesia, Japan, Malaysia, Pakistan, Papua New Guinea, the Philippines, Micronesia, Singapore, the Solomon Islands, Thailand, Tonga, and the United States (Lourie et al., 2004). Furthermore, according to Lourie et al. (2004), it is suspected that the common seahorse may also be found in Bangladesh, Brunei Darussalam, Kiribati, Myanmar, Nauru, Palau, Samoa, Sri Lanka, Tuvalu, and Vanuatu. It can be challenging to determine the exact distribution of H. kuda because of their elusive nature, their excellent camouflage, and the difficulty of identifying the exact species. However, by looking at the declining volume of seahorses caught, as well as the smaller sizes of those caught, it is safe to say that the distribution has been decreasing due to the overfishing of the delicate populations (Salin, 2003; Pawar, 2011).
Hippocampus kuda typically inhabits temperate or coastal marine environments between 0-8 meters deep, although it has been found as deep as 55 meters (Lourie et al., 2004). They tend to choose environments with coral, seagrass, weeds, macroalgae, or mangroves, as this provides them a safe place to anchor with their tails. H. kuda has been found in coastal bays, harbors, lagoons, sandy areas in rocky littoral zones, muddy bottoms, estuaries, and even lower brackish areas of rivers (Lourie et al., 2004). Both Lourie et al. (2004) and Salin (2003) acknowledge that seahorses show high site-fidelity, meaning they tend to stay in their established environment, especially during breeding season. They also have small home ranges and live at very low population densities. For example, it is not unusual for a male H. kuda to have a home range of just 1m2, with the maximum range for a female being 100m2 (Salin, 2003). In addition, seahorses have not been observed competing for space, further indicating low population densities. As Salin (2003) points out, there is some debate as to whether very short-range seasonal migrations occur, but it is widely accepted that the majority of movement occurs when seahorses are carried to a new location by storms or floating debris, or when young seahorses are carried away by currents. Because seahorses have such low species populations, Lourie et al. (2004) argues that the decline in numbers caused by overfishing does not increase juvenile survival; in fact, since seahorses have such low densities, overfishing actually hurts the species even more because it is more difficult for an abandoned partner to find another of the same species.
According to Job et al. (2002) and Salin (2003), seahorses have a significant impact on the ecology of some benthic environments by preying on large amounts of organisms, including crustaceans such as copepods and amphipods, small juvenile fish, and invertebrates. For the most part, seahorses are diurnal rather than nocturnal, which can have an influence on what organisms they encounter throughout their day. Since these fish are easily camouflaged and are covered in unpalatable bony plates and sometimes spines, they have few natural predators. Seahorses have been found in the stomachs of tuna, redsnappers, dolphin fish, flat heads, anglerfish, skates, rays, other large pelagic fish, crabs, penguins, and other water birds (Lourie et al., 2004). As communicated by Salin (2003), bryozoans, algae, and hydroids sometimes settle on the skin of seahorses, which is ultimately helpful because it helps to camouflage the fish and further prevent them from becoming prey.
As described previously, a seahorse’s only method of propulsion is through its dorsal fin, so it makes sense that seahorses are relatively immobile creatures. Although they are able to swim quickly for a short distance, seahorses are more apt to find an object to anchor to with their muscular tails (Salin, 2003). Rather than going after prey, these fish are able to blend into their environment and wait for mobile food to come to them. According to Salin (2003), seahorses suck in food through their long, tubular snout, and will eat anything that fits in their mouth, including crustaceans, small fish, and invertebrates. Since they do not have any teeth or a stomach, prey must be swallowed whole; it is then passed quickly through the digestive system to extract nutrients (Salin, 2003). For such small animals, seahorses must consume a large amount of food, over 1,100 calories per day, in order to function successfully (Salin, 2003).
In one of the most extreme cases of paternal care known on Earth, the male seahorses become pregnant. While the female seahorses still are the ones to produce large, energy rich eggs and males produce mobile sperm, the female seahorse will actually deposit her eggs into the male’s brood pouch via a prehensile ovipositor (Salin, 2003). These eggs are red or orange, contain oil droplets, and are pear-shaped (Salin, 2003). According to Lourie et al. (2004) and Salin (2003), the eggs, which average 1.8 mm in diameter, are also yolky, and they may receive even more nutrients through the male’s placenta. Although typically a job for females, the male provides protection, osmoregulation, and gas exchange for the eggs within his brood pouch (Laksanawimol et al., 2006). After a 17-day gestation period, male H. kuda release 100-300 fully independent young (Lourie et al., 2004). These young are around 7 mm long, and look like a small version of fully formed adults, although they may have slightly different proportions, such as larger heads (Lourie et al., 2004; Job et al., 2002). After the young are born, the female will impregnate the male the same day. This happens in a continuous cycle, as the breeding season is year-round, and the young will receive no further parental care after birth.
Another unique aspect of seahorses is their faithful courtship habits. According to Salin (2003), seahorses are the only known example of monogamy in fish species inhabiting sea grasses or flooded mangroves. In order to achieve this monogamy, male seahorses may wrestle with their tails to win over a female (Salin, 2003). These males are also typically more colorful, vocal, and aggressive than females, which, like many animals, works to their advantage when attracting a mate (Salin, 2003). Once the male finds a mate, he performs a greeting ritual every morning with the female. This ritual consists of the seahorses grasping each other’s tail, with the male leading the female in a twirling pattern and sometimes changing colors (Salin, 2003). According to Salin (2003), while the males are in charge of the movements in the ritual, the female controls the length and timing of the interaction, which typically lasts between 6-10 minutes. After the morning ritual, the pair goes their separate ways and spends the day independently until the next morning ritual. The pair bonds are reinforced every day, even once the male gives birth (Lourie et al., 2004). From what scientists understand, it seems that the ritual helps to synchronize the male and female’s reproductive cycles. By starting their relationship with the pair bonding rituals, the male is ready to give birth and then accept more eggs at the same time that the female is ready to give them (Salin, 2003). According to Salin (2003), seahorses are among the most sexually monogamous animals, and it has been shown that a pair bonded male and female will mate repeatedly and even give up opportunities to mate with or interact with any seahorses other than their partner. This faithfulness is unwavering, even if one partner becomes injured or unable to reproduce. If one partner dies, it may take the living seahorse many weeks to find and pair bond with a new mate (Salin, 2003).
Seahorses rely on several senses to communicate and interact with others of their species and with their environment. They have two eyes that can move independently of one another, allowing the fish to search for food and look out for predators more efficiently (Salin, 2003). Seahorses are also able to change color, as previously mentioned, which not only allows them to blend in with their environment, but also assists in social interactions as a way to express interest, recognition, or vitality (Salin, 2003). The third, and perhaps most important, example of seahorses interacting with their environment and communicating with members of their species is the ability to make clicking noises by moving two parts of their skull together (Anderson, 2009). These noises have been observed in many situations, such as while the fish were being fed, during pair bonding rituals, and when the fish explored a new environment for the first time (Anderson, 2009). According to Anderson (2009), the clicking communication was even observed between two seahorses that were in adjacent tanks, which indicates that the fish have a good sense of auditory perception.
According to the International Union for Conservation of Nature, Hippocampus kuda is listed as a vulnerable species due to a few reasons. The first is the loss of habitats. Many of the shallow, coastal areas where the seahorse resides are being destroyed either by pollution or by human interaction, most notably the coral reefs. Another way that both the habitats are being destroyed and the seahorses are being killed is by trawlers, which drag nets on the bottom of the ocean floor (Lourie et al., 2004). Many times, seahorses are unintentionally caught by trawlers as bycatch, and then are sold internationally. A third way in which seahorses are being threatened is through direct exploitation, especially by poor fishermen catching them with the intention to sell them (Lourie et al., 2004).
Seahorses are in high demand in at least 77 countries, as reported by Pawar et al. (2011). Pawar et al. (2011) claims that over 70 metric tonnes, or 25 million seahorses, are traded around the world. For the past 600 years, seahorses have been used in traditional Chinese medicine, which represents the largest consumption of the fish at around 30% of internationally traded seahorses for patented Chinese medicine (Lourie et al., 2004; Salin, 2003). These medicines are used for many purposes, including treatment of asthma, arteriosclerosis, impotence, incontinence, thyroid disorders, broken bones, skin ailments, and heart diseases (Salin, 2003). Seahorses are also eaten in China, as they are thought to be a tonic food (Salin, 2003). Many seahorses are also bought as “curiosities,” for use in jewelry, paperweights, and other souvenirs. They are valued in this market because of their interesting appearance, the ease with which to dry and sell them, and the way their bony plates let them hold their shape even after they are dead (Salin, 2003). More than one million seahorses are also sold internationally for aquariums. The majority of aquarium seahorses are caught in Indonesia and the Philippines and then sent to developed areas such as North America, Europe, Japan, and Taiwan (Salin, 2003; Pawar, 2011).
A major problem with the high demand for seahorses around the world is the difficulty of raising them in aquaculture. They are a very fragile fish with unique and inconvenient needs, especially when they are young. An example of this is the huge amount of tiny live fish and crustaceans that seahorses need in their diet; this problem becomes even more complicated with young seahorses, since acquiring enough small prey that are still mobile enough to satiate the juveniles is usually very impractical (Job et al., 2002). Since seahorses are so difficult to breed and raise, the majority of them are taken out of wild, which ruins ecosystems and puts species at risk of eventually going extinct. Although it is currently nearly impossible to know how many seahorses live in the wild, Lourie et al. (2004) claims that the population of seahorses has declined somewhere between 15-50% over the past five years. It is for this reason that many recent studies of seahorses have revolved around conservation of the fish, as well as ways to make aquaculture more realistic, in order to reduce the amount of wild seahorses being taken out of their habitat (Salin, 2003). Going forward, one can hope that ichthyologists will continue to learn about this unique species in an effort to discover ways in which the world can continue to enjoy the special properties of seahorses without threatening their existence.
Anderson, P. A. 2009. The functions of sound production in the lined seahorse, Hippocampus erectus, and effects of loud ambient noise on its behavior and physiology in captive environments (Doctoral dissertation, University of Florida).
Anderson, R. C. 2000. An Underwater Guide to Indonesia. Honolulu, Hawaii: University of Hawaii Press.
Job, S. D., H. H. Do, J. J. Meeuwig, and H. J. Hall 2002. Culturing the oceanic seahorse, Hippocampus kuda. Aquaculture, 214(1), 333-341.
Kawahara, R., M. Miya, K. Mabuchi, S. Lavoue, J. G. Inoue, T. P. Satoh, A. Kawaguchi, and M. Nishida. 2008. Interrelationships of the 11 gasterosteiform families (sticklebacks, pipefishes, and their relatives): A new perspective based on whole mitogenome sequences from 75 higher teleosts. Molecular phylogenetics and evolution, 46(1), 224-236.
Laksanawimol, P., P. Damrongphol, and M. Kruatrachue. 2006. Alteration of the brood pouch morphology during gestation of male seahorses, Hippocampus kuda. Marine and Freshwater Research 57, 497–502.
Lourie, S.A., S. J. Foster, E. W. T. Cooper, and A. C. J. Vincent. 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.
Pawar, H. B., S. V. Sanaye, R. A. Sreepada, V. Harish, U. Suryavanshi, and Z. A. Ansari. 2011. Comparative efficacy of four anesthetic agents in the yellow seahorse, Hippocampus kuda (Bleeker, 1852). Aquaculture,311(1), 155-161.
Salin, K. R. 2003. Reproductive biology and larval rearing of Hippocampus kuda, and the taxonomy of seahorses (hippocampus spp.) along the southern coast of India (TH 124) (Doctoral dissertation, Central Institute of Fisheries Education, Mumbai).
Teske, P. R., M. I. Cherry, and C. A. Matthee. 2004. The evolutionary history of seahorses (Syngnathidae: Hippocampus): molecular data suggest a West Pacific origin and two invasions of the Atlantic Ocean. Molecular phylogenetics and evolution, 30(2), 273-286.
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