Biogeography of the Pacific Seahorse (Hippocampus ingens)

By Noelle Dames, Student in Biogeography 316, Fall 2000
San Francisco State University Department of Geography
 


Taxonomy
Kingdom: Animal
    Phylum: Chordata
        Subphylum: Vertebrata
            Superclass: Gnathostomata
                Class: Osteichthyes
                    Subclass: Actinopterygii
                        Infraclass: Teleostei
                            Superorder:  
                                    Atherinomorpha

                        Order: Gasterosteiformes
                    Suborder: Syngnathoidei
                Family: Syngnathidae
            Genus: Hippocampus
        Species: Hippocampus Ingens
    Synonyms: H. ecuadorensis,
H. gracilis, H. hildebrandi
(Vincent 1996 and Whitley 1958)                                                                                 
(Stuart Westmoreland @ Corbis.com)

 

Introduction Perhaps like me you once believed seahorses existed only in the imagination.  In mythology, they guided Neptune’s chariot, and sailors once tattooed them on each other for protection from drowning.  But they are quite real, though far more delicate and docile than the “horse sea monster” their Latin name suggests.  Their relatively small size and unique body composition render them barely able to swim, and the only fish capable of holding your hand (Vincent in Lourie 1999).  This stillness and grace cast them as stars of the world’s first wildlife film, made in Italy in the 1890s (Arrigoni 1989).
 

Description Despite previous taxonomic classifications as insects and amphibians, they are fish, cousins to pipefish and sea dragons, from the family Syngnathidae, Greek for “fused jaws.”  Like most fish they have gills for breathing, a swim bladder for buoyancy, two pectoral fins for balance, and dorsal fins that flutter as much as 35 times per second (Breder 1942).  They lack teeth, a stomach, and a caudal fin for speed.  Although beautiful, seahorses are the Frankensteins of the underwater world, seemingly made up of parts of several animals.  As their name suggests, they possess the head of a horse, perpendicular to their upright bodies, on top of which they wear a crown or coronet, as unique as a human fingerprint (Nova 1997).  Their snout and body armor resemble an aardvark’s.  They come in many shades and combinations of yellow, orange, red, brown, black, gray, white, and pink.  As with chameleons, coloration can change to blend with the environment.  Also with lizards, they share independently moving eyes, assisting both survival and predation tactics.  They’ve borrowed prehensile tails from monkeys, and made a major alteration to the brood pouch of marsupials, with male seahorses nurturing their young (Lourie et al 1999).

Anatomy of Pacific Seahorses (Lourie et al 1999)
 

Reproduction The most fascinating aspect of seahorse behavior is the mating ritual.  Like most fish, the male parent assumes prenatal care duties.  But male seahorses go a step further by carrying and birthing babies, even producing the female hormone prolactin (Vincent 1994).  Individuals reach sexual maturity by the next breeding season following birth, at the ripe age of three months to a year, depending on size of the species.  Breeding seasons vary by species and may be restricted by cooler weather or monsoons, though some go year-round.  In captivity, the large H. ingens breeds at 10-12 months, but eschews the infamous pair bonding (Mann 1998).  In the wild, couples are monogamous, at least throughout a season.  It is not yet known if they mate for life, because they spend winters in the open ocean.  It is interesting to note that despite the apparent role reversal, it does not extend to widely held animal behavior theory regarding competition.  Male seahorses are still more aggressive, butting heads and tail wrestling each other into submission over an available female.  And paired females will ignore potential fathers and dispose of eggs if her mate is still pregnant (Vincent 1994).  Fortunately, egg production and male fertility are usually synchronized by the morning greeting (Vincent 1989).  In a gesture of acknowledgment, pairs meet, change colors and may make a clicking sound by rubbing parts of the skull together (Lourie et al 1999).  With tails intertwined around each other and a blade of grass, they perform an elaborate underwater square dance, do-si-do-ing up and down the water column.  The courtship takes place for three consecutive days.  Finally the male displays his empty pouch, and if his partner gives the nod, proceeds to fill it with hundreds of eggs via her ovipositor.  The conditions inside the pouch will gradually alter to match those outside.  The size of offspring is larger furthest the equator, and the number of offspring varies by species from 5 to a record 1572!  Depending on species and water temperature, following a 10 day to 6 week gestation period, the father experiences labor.  Contractions begin before sunrise, often during full moons when tides are highest, maximizing foraging opportunities for the newborns (Vincent 1996).  At birth, babies heads are disproportionately larger than their bodies, making them look even stranger than adult seahorses.  They emerge hungry, yet fully formed and self-sufficient (Lourie at al 1999).  With no further care required, the parents are able repeat the process immediately.  In the Pacific seahorse, centimeter-long fry arrive 200 at a time, after a 14-15 day pregnancy (Lourie et al 1999).

Predator vs. Prey  Anchored by its tail, camouflaged among grasses and corals, and saddled with a voracious appetite, the seahorse is a successful sit and wait predator.  With a jerk of the head, any unsuspecting fish, larvae, plankton, or other living thing small enough to fit, can be sucked into the powerful snout.  In the laboratory however, H. ingens has proven to be a picky eater (Mann 1998).  It has been observed that decreased light intensity negatively impacts the ability of at least one Caribbean species to forage for food (James 1994).  This might explain why most are diurnal.  However, H. ingens and a few others are nocturnal.  This may have been a necessary behavior modification due to increased daytime fishing activity since seahorses are often captured as a by-product of shrimp trawling.  As unappetizing as they may seem, they are sometimes eaten by pelagic fish, crabs, sea birds, and people in Tasmania (Vincent 1994 and 1996).
 

Evolution According to the fossil record, the first fishes appeared almost 500 million years ago, during the Ordovician Period of the Paleozoic Era.  During the Silurian, fish underwent a great radiation, which has led scientists to refer to the following period, the Devonian, as the age of fishes.  By 420 million years ago, primitive forms had evolved of all present day fish including the class Osteichthyan, which most likely originated in the equatorial waters off the north coast of the Early Devonian landmass Euramerica.  This was followed by a period of abrupt extinctions for many species of two of the three ancient subclasses.  The most successful subclass Actinopterygii, or “ray winged,” were once the minority.  They have continued to diversify and now represent at least 23,000 known species.  The majority of modern fish are teleosts, which were contemporaries of dinosaurs in the late Triassic or early Jurassic Period of the Mesozoic era, approximately 200 million years ago.  They are called bony fish due to the presence of bones in the tail supporting dorsal fins for propulsion (Long 1995).  This trait allowed for greater agility.  Combined with a variety of feeding mechanisms permitted by the lack of cheek and jawbones, the means for the specialized family syngnathidae to emerge are evident.  The loss of the caudal fin sacrificed speed but facilitated the evolution of a diverse group of body forms.  Pipefish, the oldest syngnathids, appear in the fossil record during the Eocene, 50 million years ago, the first to exhibit a reedlike body.  This adaptation appears to be linked to the rise of intertidal vegetation, which made way for the vertical seahorse, either as an offshoot of the sea dragon or a separate branch of the evolutionary tree.  Both genera possess the prehensile tail, and the ability to grow appendages for protective purposes.  But male sea dragons carry fertilized eggs on their tails or in skinfolds not in brood pouches (Whitley 1958).  Current research suggests the Atlantic seahorse species are older than those of the Indo-Pacific, despite the fact that the latter now occur in greater numbers (Project Seahorse 1999). Hippocampus ingens, the only eastern Pacific species, is closer genetically to the Caribbean h. reidi than any of its western Pacific counterparts.  The two are believed to have diverged from a single ancestral species as a result of the formation of the Isthmus of Panama, three million years ago (Lourie et al 1999).  Early taxonomic efforts listed over 100 seahorse species (Whitley 1958).  Many of the names described different members of the same species.  Color variations and camouflaging techniques contributed as much to the confusion as the lack of a concerted scientific study.  Prior to 1986, studies of seahorses were conducted mainly in aquaria and academia, not in the wild (Vincent 1994).  Current morphological classifications overlook color and filaments in favor of height, coronet, snout, spine, ray and ring measurements.  Understandably, there is still some debate, but recent research has narrowed the number to between 32 and 35 species, all belonging to the same genus, hippocampus (Lourie et al 1995).
 

 

seahorsefig3.jpg (29038 bytes) Seahorses Worldwide:
Afrikaans: seepferd
Bahasa: ikan kuda or kudu
Chinese: hai mai, shui mai
Hebrew: jamsus
Italian: cavaletto marino
Japanese: tatsu no otoshigo
Korean: haema
Maori: kiore
Mindanao: undok undok
Portuguese: caulinho
Spanish: caballito de mar
Tamil: kadal kudara
Thai: manam
Visayan: cabayo cabayo
(Vincent 1996 and Whitley 1958)
Worldwide distribution of all species in green, Pacific seahorses in yellow (Adapted from Project Seahorse 2000)

Distribution and Habitat 
Seahorses are distributed throughout the world, in both tropical and temperate zones, ranging from about 45 degrees north to 45 degrees south.  They favor saltwater habitats or brackish waters of estuaries, containing mangroves, coral reefs and sea grass beds.  The range varies by gender.  Females may wander as far as 100 square yards, centering around, but reducing feeding competition with their oft-pregnant partner who confines himself to a single square yard (Vincent 1996).  Because prey is abundant in areas with good water exchange, they tend to colonize only edges of suitable habitats (Breder 1942).  As its common name suggests, the Pacific seahorse frequents the shallow, coastal waters of the western Pacific Ocean.  It lives among gorgonians, sea kelp, and eelgrass, typically at depths of 1 to 20 meters, though it has been seen at 60 meters (Lourie et al 1999).  Its disjunct distribution encompasses nine habitat clusters, scattered from central Baja, Mexico in the north, through Guatemala, El Salvador, Panama, Columbia, and Ecuador to Pucusana, Peru in the south, extending as west as the Galapagos islands (Fritzsche 1980).  Individuals or small populations have been known to reach latitudes as high as Caleta Chipana, Chile and San Diego, California (Soto 1985).  From the latter, the first 5 specimens were collected and subsequently named in the 1850s, during a survey for the Pacific Railroad project.  Over the next 130 years, through 1983, only nine additional sightings were recorded, despite numerous attempts.  But between 1984 and 1987, 22 specimens were reported in Southern California waters or on beaches.  Their reoccurrence is attributed to intermittent warming of the Pacific by the El Nino phenomenon, and recently, improved water quality in San Diego Bay (Jones et al 1988).  More research on the Pacific seahorses of this region are needed.  However, the appearance of a lone Pacific seahorse in San Francisco waters in 1960 (Hubbs 1963), might be better explained by a theory that credits seahorse migration on outside forces, rather than instinct (Vincent 1996).

Seahorses Past and Future 
Seahorses suffer from relatively low fertility, averaging 200 offspring per brood, plus infant mortality as high as 99%, and adult life spans believed to range from 1 to 5 years, depending on species (Lourie et al 1999).  The fact that partners are monogamous and males both fertilize and carry their unborn, makes their loss particularly devastating.  Since they display strong site fidelity under favorable conditions, but are sensitive to their environmental and slow to recolonise disturbed areas, they are a good indicator species.  Their habitats are at risk from natural and human phenomena such as freshwater flooding, pollution, dredging, dynamiting, silting, and logging.  Besides these threats, seahorses are faced with over fishing for home aquariums, ornamental tchotchkes and traditional folk medicines.  At various times throughout Western medical history, seahorses were thought to aid production of mother's milk, cause death if mixed with wine, or cure hairloss, rabies, leprosy, and hydrophobia (Whitley 1958).  Currently Eastern medicines dry and grind them up for use as an aphrodisiac or treat symptoms ranging from impotence, asthma, fractures, and diseases of the heart, kidneys, skin and thyroid (Lourie et al 1999).  At least 47 countries participate in the legal trade of seahorses, either living or dead.  The United States both imports and exports them.  In the mid 1990s, conservative estimates of the amount of dried seahorses imported worldwide totaled 56 thousand metric tons, numbering 20 million individuals (Vincent 1996).  Without even taking into account those traded alive and/or illegally, this marked a dramatic increase that is expected to continue.  All of these factors have led the World Conservation Union to list 18 species, including Hippocampus ingens, on their Red List of Threatened Animals, as either endangered or vulnerable, predicting a 20% or more decline in population over the next decade, or three generations (IUCN 2000).  Both seahorses and their respective ecosystems can benefit from further research and conservation efforts.
 


 

What we can do to prevent  seahorse extinction:
1. Resist the temptation to buy or replace pet seahorses.
2. Develop sustainable fisheries.
2. Seek alternative ingredients or mature members of less endangered species for medicines.
3. Make or collect only cruelty-free souvenirs that do not use dried seahorses.
4. Encourage seahorse-friendly legislation.
5. Contribute time or money to a conservation or research                                                          organization.
                                         6. Boycott any joint that features seahorse stirfry on the                                                          menu.
 

Well Camouflaged Seahorses at Cal Academy
 

Bibliography
Arrigoni, W.  1989. “Seahorses.”  Sea Frontiers  35(6): 358-65.

Breder, C.M., Jr. and H.E. Edgarton.  1942.  “An analysis of the locomotion of the seahorse, hippocampus, by means of high speed cinematography.”  Annals of the New York Academy of Sciences  43: 145-172.

Fritzsche, R.A. 1980.  “Revision of the eastern Pacific Syngnathidae (Pisces: Syngnathiformes), including both recent and fossil forms.” Proceedings of the California Academy of Sciences  42(6): 181-227.

Hubbs, C.L., and S.D. Hinton.  1963.  “The giant seahorse returns.”  Pacific Discovery  16(5):  12-15.

IUCN.  (2000).  Red List of Threatened Species.  [Online].  Available at:  http://www.redlist.org
[8 December 2000]

James, P.L. and K.L. Heck, Jr. 1994.  “The effects of habitat complexity and light intensity on ambush predation within a simulated seagrass habitat.”  Journal of Experimental Marine Biology and Ecology  176(2): 187-200.

Jones, A.T., P. Dutton, and R.E. Snodgrass.  1988.  “Reoccurrence of the Pacific Seahorse, Hippocampus ingens, in San Diego Bay.”  California Fish and Game  74(4): 236-238.

Long, J.A.  1995.  The Rise of Fishes.  Baltimore and London.  The Johns Hopkins University Press.

Lourie, S.A., A.C.J. Vincent, H.J. Hall.  1999.  Seahorses: An Identification Guide to the World’s Species and Their Conservation.  London, UK.  Project Seahorse.

Mann, R.H.  1998.  "Guiding Giant Seahorses."  California Wild - Here at the Academy.  http://www.calacademy.org/calwild/archives/seahorse.htm

Nova.  (April 15, 1997).  Kingdom of the Seahorse.  [Online and televised].  Available: http://www.pbs.org/wgbh/nova/seahorse/  [10 October, 2000].

Project Seahorse.  (1999).  “Studying the genetics of seahorses" (in progress).  [Online].  Available at: http://www.seahorse.mcgill.ca  [21 September, 2000].

Soto, R.  1985.  “Effects of the 1982-88 El Nino phenomena on the ecosystems of the region.”  pp. 199-206.  In Instituto Formento Pesquero Chile ed. Tailler Nacional Fenomen El Nino, 1982-83.  Santiago: Invest. Pesq.

Vincent, A.C.J.  1996.  International Trade in Seahorses.  Cambridge, UK.  TRAFFIC International.

Vincent, A.C.J.  October 1994.  “The improbable seahorse.”  National Geographic  186(4): 126-140.

Vincent, A.C.J.  February 1994.  “Seahorses exhibit conventional sex roles in mating competition, despite male pregnancy.”  Behaviour 128(1-2): 135-151.

Vincent, A.C.J.  1989.  “Pregnant males and seahorse tails.”  Australian Natural History  23(2): 122-129.

Whitley, G. and J. Allan.  1958.  The Sea-Horse and its Relatives.  Adelaide.  The Griffin Press.
 

Other Seahorse Sites:
Caballitos de Mar. [in spanish]  Gema Dominguez. http://www.marenostrum.org/caballitos/primera.html

Ocean Oasis Field Guide.  San Diego Natural History Museum.  http://www.oceanoasis.org/fieldguide/hipp-ing.html

Seahorse Symphony.  Animals and Exhibits.  John Shedd Aquarium.  http://www.sheddnet.org/

Seahorses: A Piece of Everything.  Sea Critters.  http://www.geocities.com/Rainforest/Canopy/7897/page2.html
 

Send comments to bholzman@sfsu.edu
 

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Yee-haw and thanks to everyone at Project Seahorse and Cal Academy for their help.
No seahorses were harmed in the making of this photo by Maria DeAngelo.