Hippocampus hippocampus

Researched ByMarisa Sabatini & Susie BallerstedtData Supplied ByMarLIN
Refereed byNeil Garrick-Maidment
Taxonomy
Scientific nameHippocampus hippocampusCommon nameShort snouted seahorse
MCS CodeZG239Recent SynonymsNone
PhylumChordataSubphylumPisces
SuperclassGnathostomataClassOsteichthyes
SubclassTeleosteiOrderSyngnathiformes
SuborderFamilySyngnathidae
GenusHippocampusSpecieshippocampus
Subspecies  
Additional Information

Hippocampus hippocampus is one of two species of seahorses found in the British Isles, the other is Hippocampus guttulatus, which has a longer snout and elongated protuberances along the back of the neck, giving the impression of a 'horses mane'.

The exact size and distribution of the population of seahorses around the British Isles is not known at present. The British Seahorse survey is collating records currently and can be found at:www.britishseahorsesurvey.org.

Please note: the biology of seahorses is poorly known and little information on Hippocampus hippocampus was found. Therefore, the following review is based in part on reviews of the biology of seahorses by Vincent (1996), Garrick-Maidment (1997) and Lourie et al, (1999). See also the British Seahorse Survey Report 2004 (Garrick-Maidment & Jones, 2004).

Taxonomy References Hayward et al., 1996, Froese & Pauly (ed.), 2004, Whitehead et al., 1986, Wheeler, 1969, Howson & Picton, 1997, Vincent, 1996, Garrick-Maidment, 1997, Garrick-Maidment & Jones, 2004, FishBase, 2000
General Biology
Growth formSee additional informationFeeding methodPredator
Mobility/MovementSwimmerEnvironmental positionDemersal
Typical food typesOrganic debris, plankton, brine shrimp, small crustaceans and small fishHabitFree living
BioturbatorNot relevantFlexibilityHigh (>45 degrees)
FragilityFragileSizeMedium(11-20 cm)
HeightNot relevantGrowth RateInsufficient information
Adult dispersal potential100-1000mDependencyIndependent
SociabilitySolitary
Toxic/Poisonous?No
Additional InformationGrowth form
All seahorses have the same basic body shape, that is, a horse-like head held at right angles to an erect body.

Body flexibility
The tail is highly flexible although it cannot bend directly backwards very far (N. Garrick-Maidment, pers. comm.).

Abundance
Seahorse population density tends to be low (Vincent, 1996). However there are no published data about population trends or total numbers of mature animals for this species.

Camouflage
Hippocampus hippocampus has the potential (like all seahorses) to grow appendages on its body for camouflage and protection. However, none have ever been identified (Garrick-Maidment, 1998).

Mobility
Hippocampus hippocampus is better suited to manoeuvrability than speed (Blake, 1976). Only the dorsal fin on their back provides propulsion, while the 'ear-like' pectoral fins below the gill openings are used for stability and steering. Hippocampus hippocampus is able to use its prehensile tail as an anchor, wrapping it around the stems of seagrass, coral heads or any suitable object. It uses its tail to hold on in strong currents (N. Garrick-Maidment, pers. comm.) and the tail is used by both sexes to grasp a partner in mating and greeting rituals. The tail is also used a great deal for climbing and is used as a hand when grasping for climbing (N. Garrick-Maidment, pers. comm.).

Growth rates
Growth rates have not been investigated in any detail but young fry are known to exhibit growth inflection points as they switch between prey types (Boisseau, 1967; cited in Vincent, 1996). Adults are known to grow more slowly as they grow larger (Vincent & Sadler unpublished; cited in Vincent, 1996).

Feeding
On average, an adult seahorse will eat between 30-50 mysid shrimp a day (Garrick-Maidment, 1997). Hippocampus hippocampus is an ambush predator that feeds on live, moving food. Hippocampus hippocampus will remain motionless until a small animal such as a mysid shrimp passes within reach. Within a second, the seahorse will flick its head and suck its prey out of the water column through its long tubular snout. Hippocampus hippocampus has no teeth or stomach, therefore prey that are caught are swallowed whole and pass rapidly through the digestive system.

Predators
Few predators appear to target adult seahorses. Lourie et al. (1999) suggested that this could be due to camouflage and immobility that makes the seahorse difficult to detect. They are, however, taken by crabs, and large pelagic fish (Lourie et al., 1999). There are also records of sea gulls and penguins eating seahorses of which the former appear to eat them commonly (N. Garrick-Maidment, pers. comm.).

Biology References Vincent et al., 1992, Garrick-Maidment, 1998, Lourie et al., 1999, Vincent, 1996, Blake, 1976, Garrick-Maidment, 1997, Garrick-Maidment & Jones, 2004, FishBase, 2000
Distribution and Habitat
Distribution in Britain & IrelandDistributed along the south coast of England, with substantial populations around the Channel Islands and Ireland (Garrick-Maidment & Jones, 2004).
Global distributionReported from the Netherlands, Belgium, the east Atlantic coast of Europe, Algeria, Italy, Malta and Greece.
Biogeographic rangeNot researchedDepth rangeSublittoral to 77 m
MigratoryNon-migratory / Resident
Distribution Additional InformationSeahorses are often found in water less than one metre deep. Most species of seahorse live at depths between 1-15 metres and as deep as 77 m when they move out into deeper waters over winter (Garrick-Maidment, 1998) but the depth varies throughout its range according to habitat (N. Garrick-Maidment, pers. comm.). In the Channel Islands, they are found in much deeper water because here they have a wide tidal range and deep gullies (N. Garrick-Maidment, pers. comm.). On the whole, Hippocampus hippocampus is found below 5 m whereas %Hippocampus guttulatus% is found in shallower depths (N. Garrick-Maidment, pers. comm.). They occupy only certain parts of seemingly suitable habitats, for example sticking to the edge of seagrass beds leaving large areas unoccupied. These microhabitats have not been investigated but is has been suggested that seahorses find more food in areas of good water exchange (Vincent, 1996). Habitat / substratum preferences may be seasonal and related to seasonal migration (N. Garrick-Maidment, pers. comm.).

Adults may disperse during short range migrations but most movement to new areas happens when adults are cast adrift by storms or carried away while grasping floating debris (Vincent, 1999). They can cope with strong tidal strength for varying periods of time (N. Garrick-Maidment, pers. comm.).

Substratum preferencesAlgae, Seagrass, Bedrock, MudPhysiographic preferencesOffshore seabed, Strait / sound, Estuary
Biological zoneUpper Eulittoral, Mid Eulittoral, Lower Eulittoral, Sublittoral FringeWave exposureModerately Exposed, Sheltered, Very Sheltered, Extremely Sheltered, Ultra Sheltered
Tidal stream strength/Water flowWeak (<1 kn), Very Weak (negligible)SalinityVariable (18-40 psu)
Habitat Additional Information
Distribution References Hayward et al., 1996, Froese & Pauly (ed.), 2004, Whitehead et al., 1986, Wheeler, 1969, Vincent, 1996, Garrick-Maidment & Jones, 2004, FishBase, 2000
Reproduction/Life History
Reproductive typeGonochoristic Developmental mechanismViviparous (Parental Care)
Reproductive SeasonApril to NovemberReproductive LocationBrood chamber / Pouch
Reproductive frequencyAnnual episodic Regeneration potentialNo
Life span3-5 yearsAge at reproductive maturity<1 year
Generation timeInsufficient informationFecundityUp to 250 young
Egg/propagule sizeNot relevantFertilization typeInternal
Larvae/Juveniles
Larval/Juvenile dispersal potential100-1000mLarval settlement periodNot relevant
Duration of larval stageNot relevant  
Additional Information
Sexual maturity
Hippocampus hippocampus becomes sexually mature during the first reproductive season after birth i.e. at age six to twelve months (Lourie et al., 1999). Sexual maturity in males can be recognized by the presence of a brood pouch, although the size of the pouch will vary with its reproductive state. Any physical manifestation of sexual maturity is less obvious in females (Vincent, 1996).

The length and timing of the reproductive season varies with location, and will be influenced by light, temperature and water turbulence. It was originally thought that Hippocampus hippocampus and Hippocampus guttulatus only bred from Spring (April) to the Autumn (October) (Lourie et al., 1999) but recent work (Garrick-Maidment, British Seahorse Survey 2004) has found juveniles in February that would have been born in November (N. Garrick-Maidment, pers. comm.).

Pair bonding
Hippocampus hippocampus like all seahorses are monogamous and form faithful pair bonds. The male seahorses are the predominant competitors for access to mates as they compete more to get pregnant than females do to give their eggs away (Vincent, 1994a). In courtship, males exhibit higher levels of aggressive competitive behaviour patterns (wrestling and snapping) than females. Competitive wrestling and snapping are described below.
  • Snapping occurs when a male aims his snout at his competitors opercular flap before flicking his snout hard in order to propel the rival male away. A well aimed powerful snap could propel a seahorse as much as 10 cm and lead to the recipient darkening and flattening in a submissive posture (Vincent et al., 1994a).
  • When two seahorses are wrestling, one male will refuse to release his competitor from its grasp. Both males may then tumble with locked tails. Eventually, the submissive male will darken and flatten in a submissive posture until it is released by the dominant male (Vincent et al., 1994a).
Tail wrestling and snapping with the snout is confined to males (Vincent et al., 1992). Although females do compete for access to males (in the sense that they exhibit higher levels of courtship when in the presence of another member of the same sex than when courting alone) but to a far lesser extent than males (Vincent, 1994a). Courtship included the following behaviour.
  • Both seahorses may grasp a common holdfast with their tails and rotate around it.
  • Male and female may promenade together often holding tails.
  • Head tilting and quivering (Vincent, 1994a).

    • Hippocampus hippocampus forms sexually faithful pairs that endure through multiple mating and breeding seasons. Pair bonding is more on a seasonal basis in temperate species and there is no evidence from the wild to prove that they pair for life (N. Garrick-Maidment, pers. comm.). The male and female reproductive state changes are always synchronized within a pair and only within a pair, confirming that they are faithful to each other (Vincent, 1996). Pair bonding is reinforced by daily greetings performed only with an individuals partner. These greetings commonly last six to eight minutes (Vincent et al., 1992).

    Reproduction
    The seahorse has a unique method of reproduction in which the male plays the dominant role. It is the male rather than the female that becomes pregnant (Vincent, 1994a). At the beginning of the mating season the males prepare their brood pouches. Females eggs ripen in an assembly line ovary, throughout the reproductive season. In order to mate males must eliminate the previous brood from the pouch and females must hydrate their eggs. Each sex provides a signal of readiness to mate, males pump water in and out of their pouch and females point their head towards the water surface (Fiedler, 1954; cited in Vincent, 1994b). Females also have a trunk which is visibly distended with hydrated eggs, which becomes concave after the egg transfer (Vincent, 1994b). The female aligns the base of her trunk with the males pouch opening and inserts her ovipositor into the pouch. The female then deposits her eggs into the brood pouch where they are fertilized (Dipper, 2001). Egg transfer takes about 6-10 seconds (Vincent, 1994a).

    Once the eggs are fertilized the brood pouch then seals up. The pear shaped eggs become embedded into the pouch wall and enveloped by tissues. Oxygen is provided via capillaries in the pouch wall (Carcupino et al., 2002; Dipper, 2001). The pouch acts like the womb of a female mammal, complete with a placental fluid that bathes the eggs and provides nutrients and oxygen to the developing embryos while removing waste products. Nevertheless most of the nutrition comes from the mother. The egg yolk provides nutrients, while the male secretes a hormone called prolactin (Boisseau, 1967b; cited in Lourie et al., 1999; Dipper, 2001). Prolactin induces enzymatic breakdown of the outer part of the egg (chorion) to produce a placental fluid (Boisseau, 1967b; cited in Lourie et al., 1999). The pouch fluid is altered during pregnancy from being similar to body fluids to being more like the surrounding seawater. This helps reduce the stress on the offspring at birth (Dipper, 2001).
    Pregnancy in male Hippocampus hippocampus can last for about 20-21 days (Garrick-Maidment, 1998). At the end of this period the male goes into labour (usually at night), which can last for hours. The brood pouch opens and fully formed young are pushed out by vigorous pumping movements. Male seahorses have relatively small broods (TRAFFIC, 2002). Brood size depends largely but not entirely on the size of the adults (Lourie et al., 2002). The number of young produced by Hippocampus hippocampus can range from 50-100 (Garrick-Maidment, 1998) although recent information shows that the number can be as high as 250 (N. Garrick-Maidment, pers. comm.). Fecundity depends on the age of the male, older males producing a larger number of offspring (N. Garrick-Maidment, pers. comm.). In pairs that are familiar with each other the male is able to mate again within a few hours of emptying his brood pouch without any decline in his physical condition (Carcupino et al., 2002)

    Dispersal potential
    It has been suggested that young fry are more likely to colonize new or depleted areas because they are often carried away from natal habitats despite attempts to settle into the substrata (Vincent, 1996). The extent of dispersal by this mechanism is unknown (Vincent et al., 1999).
    Longevity
    The natural lifespan and mortality rates of seahorses and the parameters that define them are virtually unknown and in need of research (Anon, 1990a; cited in Vincent, 1996). It has been suggested that the lifespan is only about 1 year in very small species such as Hippocampus zosterae (~ 2.5 cm) (Strawn 1953; cited in Vincent, 1996) whereas larger seahorses, such as Hippocampus hippocampus, have a lifespan of 1-5 years (Lourie et al., 1999). Hippocampus hippocampus has been regularly recorded living up to 5-6 years in captivity (N. Garrick-Maidment, pers. comm.).

    Natural adult mortality rates are likely to be low but the data on mortality rates is very limited (Vincent, 1996). Fry are released at an advanced stage of development, which probably gives them a higher chance of survival than in many other species (TRAFFIC, 2002). However, it has been suggested that mortality is probably highest in young fry, as they are highly vulnerable to piscivorous fish (Vincent, 1996).
    Reproduction References Dipper, 2001, Vincent et al., 1992, Vincent, 1994a, Vincent, 1994b, Garrick-Maidment, 1998, Carcupino et al., 2002, Lourie et al., 1999, Vincent, 1996, TRAFFIC, 2002, Garrick-Maidment & Jones, 2004, FishBase, 2000