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.
HABITAT AND DISTRIBUTION
Hippocampus zosterae occurs from Bermuda and the Caribbean to
Florida, and throughout the Gulf of Mexico (Hoese and Moore 1977, Robins et
This fish can be encountered in seagrass beds throughout the IRL.
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.
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
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
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
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).
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).
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.
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.
High reproductive rates and a stable population size suggest dwarf
seahorses are an important trophic link in Halodule seagrass
communities (Strawn 1958).
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).
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.
Hippocampus zosterae is a diurnal species (Froese and Pauly 2008).
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.
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