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Species Name:    Hippocampus zosterae
Common Name:                Dwarf Seahorse

 

I.  TAXONOMY

Kingdom Phylum/Division: Class: Order: Family: Genus:
Animalia Chordata Actinopterygii Gasterosteiformes Syngnathidae Hippocampus



Pinfish, Hippocampus zosterae. Photograph courtesy Harbor Branch Oceanographic. Photographer Tom Smoyer.

Species Name: 
Hippocampus zosterae Jordan and Gilbert, 1882

Common Name(s):
Dwarf Seahorse, Pygmy Seahorse

Species Description:
The dwarf seahorse, Hippocampus zosterae , is a small seahorse common to Florida seagrass flats. It is variable in color, often tan and unpatterned, but individuals can also range in color from green to nearly black. The snout is long relative to body size and the coronet (head projection) is high and knob-like. The dorsal fin has 11-13 fin rays and a dark submarginal stripe that may aid in identification. Body ring counts reveal 10-14 trunk rings and 31-33 tail rings (Hoese and Moore 1977, Robins et al. 1986).

Males and females are sexually dimorphic. Scrutiny reveals the presence of a marsupium (brood pouch) in males, evident by around 16-20 mm as an elevated ventral ridge around the height of the top few caudal segments (Strawn 1958). The marsupium is highly vascularized, representing an extreme specialization for paternal care (Masonjones and Lewis 1996).


Potentially Misidentified Species:
The small size and specific body ring and dorsal ray counts should be sufficient to allow the dwarf seahorse to be distinguished from the larger congener Hippocampus erectus, with which it may co-occur.


II.  HABITAT AND DISTRIBUTION 

Regional Occurrence:
Hippocampus zosterae occurs from Bermuda and the Caribbean to Florida, and throughout the Gulf of Mexico (Hoese and Moore 1977, Robins et al. 1986).

IRL Distribution:
This fish can be encountered in seagrass beds throughout the IRL.


III. LIFE HISTORY AND POPULATION BIOLOGY

Age, Size, Lifespan:
Dwarf seahorses are small. Specimens collected from Cedar Key seagrass beds by Strawn (1958) ranged from 7 to 38 mm from the knob at the top of the head (the coronet) to the tip of the tail. Robbins et al. (1986) indicates large specimens may reach 5 cm.

Overwintering Cedar Key dwarf seahorses disappear from the grass flats by early August and do not become member's of the next year's overwintering population. Strawn (1958) suggested that individuals from this population rarely exceeded one year in age.

Abundance:
Hippocampus zosterae is a common inhabitant of Florida bays and estuaries, although not a numerically dominant community component. Thayer et al. (1999) list H. zostera as among the 30 most numerous fish species collected in Florida bay trawl studies they conducted, although capture densities reported are less than 20 individuals per hectare.

Reproduction:
A unique and well-known aspect of syngnathid reproductive ecology is the brooding of young in the marsupial pouches of males.

Courtship and mating details are provided by Masonjones and Lewis (1996), based on laboratory observations of monogamous pairs. The authors describe four distinct courtship phases occurring. The first phase takes place over one or two mornings prior to the day of copulation and consisted of reciprocal side-to-side body quivering displayed alternately by the pair. The next two phases take place on the day of mating and involve female “pointing” with head raised upward, followed by male pointing in response. The final phase involves repeated rising in tandem within the water column culminating in female transfer of eggs into the male brood pouch during a brief midwater copulation. The authors also noted that males more actively initiated courtship, demonstrating that dwarf seahorses are not courtship-role reversed in this aspect as suggested by prior investigation (e.g., Trivers 1985).

Additional observations on dwarf seahorse reproduction are provided by Strawn (1958). Microscopic examination of ripe ovaries by the author revealed two clutches of eggs, one of which is comprised of large eggs ready to be transferred to the male pouch, the other of the small, yolked eggs of the successive clutch. The largest clutch size (mature eggs only) was 69 eggs, and the largest brood from a male was 55 young. One female usually provided all of the eggs brooded in the male marsupium, and courting males pump their pouches full of water.

Field observations by Strawn (1958) revealed females outnumbered males in Cedar Key collections, from “slightly so” at certain sites and times to as much as 2:1 in other cases. Breeding occurred from mid-February through late October to early November, and males (22-38 mm in size) carrying young were collected from late February to the end of October. Breeding season appears correlated to seasonal changes in day length, e.g., rather than simply with changing water temperature. Breeding occurred in late winter and early spring during periods of extreme tidal exposure of the grass flats and in August when exposure was minimal. A long breeding season and rapid onset of maturity (three months or less in males) allows the production of at least three generations of dwarf seahorses at Cedar Key (Strawn 1958).

Embryology:
In the male marsupium, developing eggs and embryos are osmoregulated, oxygenated, and nourished by specially adapted, placenta-like structures (Wilson et al. 2003). Cedar Key males carry broods for approximately 10 days. Males usually produce two broods per month during the breeding season, with an average of 4.7 days in between broods.

An early description of the process by which male H. zosterae “give birth” to young comes from Breder (1940). Expulsion occurred over approximately 10 minutes amid labor-like contortions by the male. Six and eight young were expelled from a pair of males under observation. This brood size is small compared to many larger seahorse species who may expel 100-200 young at birth (Wilson et al. 1998). The findings of Masonjones and Lewis (1996) confirm that small brood size (3-18 offspring in their study) is the norm for H. zosterae . Newly expelled young were approximately 8.5 mm in length. Young are precocial, capable of swimming, clinging to small bits of seaweed with their tails, and maintaining an upright posture.

Newborn young examined at Cedar Key ranged from 7-9 mm in length (Strawn 1958). Aquarium-raised young fed cultured Artemia nauplii grew to as large as 18 mm within 17 day of expulsion. As with other seahorses, the young are miniature fully formed seahorses, independent from birth, and receiving no further care from either parent (Wilson and Vincent 1998).


IV.  PHYSICAL TOLERANCES

Temperature:
The distribution of Hippocampus zosterae is restricted to tropical and subtropical/warm-temperate waters. Strawn (1958) noted that Hippocampus zosterae from Cedar Key, Florida, concentrated in deeper water in the winter, although this may have been due more to winter die-back of tidally exposed shallow seagrass than strictly to cold temperatures. The breeding season for this species appears correlated to seasonal changes in day length, e.g., rather than simply with changing water temperature (Strawn 1958).

Salinity:
Hoese and Moore (1977) indicate individuals are restricted to high-salinity grass flats, but other authors report dwarf seahorses occupy and appear capable of breeding across a broad range of salinities. Strawn (1958), for example, reported heavy summer breeding following periods of high (33 ppt) and low (9.7 ppt) salinity.


V.  COMMUNITY ECOLOGY

Trophic Mode:
Hippocampus zosterae is a first-order predator, i.e., a species that primarily consumes grazers (Gil et al. 2006). Harpactacoid copepods and other epibenthic invertebrates are the major prey of this species (Tipton and Bell 1988, Kendric and Hyndes 2005).

Tipton and Bell (1988) describe the predation strategy of dwarf sea horses as a “sit-and-wait” ambush strategy. Like other syngnathids, H. zosterae is a pipette feeder (Colson et al. 1998), using suctorial force to capture prey with the fused tube-like jaws.

Predators:
High reproductive rates and a stable population size suggest dwarf seahorses are an important trophic link in Halodule seagrass communities (Strawn 1958).

Parasites:
The isopod Lironeca ovalis and the haematozoan Haemogreyarina bigemina have been reported as important parasites of Lagodon rhomboides (Muncy 1984). Parasite infestation was reported as the most prevalent gross external abnormality in Florida Gulf coast pinfish, whereas ulcers/lesions were most common on the Atlantic coast (FWRI 2006).

Habitats:
Hippocampus zosterae is a common inhabitant of bay and estuarine seagrass beds (Brown-Peterson et al 1993, Tolan et al. 1997). The species exhibits a very pronounced association with marine macrophytes, including seagrasses and macroalgae. Strawn (1958) collected individuals from beds of five different Florida seagrasses: Diplanthera (= Halodule) beaudetteii, Ruppia maritime, Halophila engelmanni, Thalassia testudinum, and Syringodium filiforme. All of these seagrasses occur in the Indian River Lagoon.

Activity Time:
Hippocampus zosterae is a diurnal species (Froese and Pauly 2008).


VI. SPECIAL STATUS

Special Status:
None.

Economic Importance:
As early as the mid-1950s, dwarf seahorses were preserved, dried, and sold to shell dealers for $15.00 to $25.00 per 1,000 (Strawn 1954).

Dwarf seahorses are also collected in Florida for the marine ornamental aquarium industry. Annual seahorse landings vary widely, but Adams et al. (2001) indicates more than 80,000 H. zosterae were collected in Florida in 1992.


VII.  REFERENCES

Adams CM, Larkin SL, and DJ Lee. 2001. Volume and value of marine ornamentals collected in Florida, 1990-98. Aquarium Sciences and Conservation 3: 25-36, 2001.

Breder CM Jr. 1940. The expulsion of young by the male of Hippocampus zosterae . Copeia 1940:137-138.

Colson DJ, Patek SN, Brainerd EL, and SM Lewis. 1998. Sound production during feeding in Hippocampus seahorses (Syngnathidae). Environmental Biology of Fishes 51:221-229.

Froese R and D Pauly (Eds). 2008. FishBase. World Wide Web electronic publication. [www.fishbase.org]

Gil M, Armitage AR, and JW Fourqurean. 2006. Nutrient impacts on epifaunal density and species composition in a subtropical seagrass bed. Hydrobiologia 569:437-447.

Hoese HD and RH Moore. 1977. Fishes of the Gulf of Mexico. Texas, Louisiana, and Adjacent Waters. Texas A&M University Press, College Station TX. 327 p.

Masonjones HD and SM Lewis. 1996. Courtship Behavior in the Dwarf Seahorse, Hippocampus zosterae. Copeia 1996:634-640.

Robins CR, Ray GC, and J Douglas. 1986. A Field Guide to Atlantic Coast Fishes. The Peterson Field Guide Series. Houghton Mifflin Co., Boston. 354 p.

Strawn K. 1954. The pushnet, a one-man net for collecting in attached vegetation. Copeia 1954:195-197.

Strawn K. 1958. Life History of the pigmy seahorse, Hippocampus zosterae Jordan and Gilbert, at Cedar Key, Florida. Copeia 1958:16-22.

Thayer GW, Powell AB, and DE Hoss. 1999. Composition of larval, juvenile, and small adult fishes relative to changes in environmental conditions in Florida Bay. Estuaries 22: 518-533, Dedicated Issue: Florida Bay: A Dynamic Subtropical Estuary.

Tipton K and SS Bell. 1988. Foraging patterns of two syngnathid fishes: Importance of harpacticoid copepods. Marine Ecology Progress Series 47:31-43.

Tolan JM, Holt SA, and CP Onuf. 1997. Distribution and community structure of ichthyoplankton in Laguna Madre seagrass meadows: Potential impact of seagrass species change. Estuaries 20:450-464.

Trivers RL. 1985. Social Evolution. Benjamin Cummings Publishers, Menlo Park, CA. 479 p.

Wilson MJ, and ACJ Vincent. 1998. Preliminary success in closing the life cycle of exploited seahorse species, Hippocampus spp., in captivity. Aquarium Sciences and Conservation 2:179-196.

Wilson AB, Ahnesjo I, Vincent ACJ, and A Meyer. 2003. The dynamics of male brooding, mating patterns, and sex roles in pipefishes and seahorses (Family Syngnathidae). Evolution, Vol. 57:1374-1386.

Report by:  J. Masterson, Smithsonian Marine Station
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Page last updated: October 1, 2008