II. HABITAT
AND DISTRIBUTION
Regional Occurrence & Habitat
Preference:
The range of H. erectus extends
from Nova Scotia to Argentina and throughout the Gulf of Mexico
(Robins & Ray 1986). The species is also suspected to inhabit
several other Caribbean territories and nations (Lourie et
al. 2004). Individuals are found in algal and coral reefs,
floating Sargassum clumps, mangroves, seagrasses, soft
bottom areas and around sponges to a depth of 73 meters (Foster
& Vincent 2004, Vari 1982).
IRL Distribution:
The lined seahorse is distributed throughout
the IRL in nearly every sheltered habitat. However, most populations
are found in seagrass beds.
III. LIFE HISTORY AND
POPULATION BIOLOGY
Age, Size, Lifespan:
The maximum age of H. erectus
is unknown, but the average lifespan is only about four years
(Lourie et al. 1999). The maximum reported size is
19 cm (Lourie et al. 1999), although most specimens
are smaller (Robins & Ray 1986). In captivity, H. erectus
maintained a linear growth of 0.55 mm per day for a period of
100 days (Scarratt 1996). Specimens larger than 20 mm have been
reported to grow an average of 0.11 mm per day (Matlock 1992).
Abundance:
The abundance of the lined seahorse is
variable, depending on habitat, season, sex and other factors.
Abundance estimates for populations of H. erectus in
Florida Bay exceeded 9.9 individuals 1000 m-2 in
some locations, and was highest in July (Powell et al.
2007). Shrimp trawls off Hernando Beach, Florida in the Gulf
of Mexico have collected 72,000 seahorses annually as bycatch
(Baum et al. 2003). Populations in Chesapeake Bay show
a greater abundance of females (Teixeira & Musik 2001),
a pattern likely repeated in other regions as a result of life
history patterns and mating behaviors.
Courtship & Reproduction:
Seahorses are sexually dimorphic, with
differing structural characteristics. The most obvious of these
is the presence of a brood pouch at the base of the abdomen
in males. Males also have a proportionally longer tail than
females (Lourie et al. 2004). The minimum recorded
size for sexually mature individuals is 5.6 cm (Baum et
al. 2003), and males have been reported to develop brood
pouches at 5 to 7 months of age (Scarratt 1995). Most seahorse
species are sexually and socially monogamous, mating with a
single partner for an entire season or lifetime (Baum et
al. 2003). Before sexual reproduction, mated pairs undergo
a complex courtship process lasting a few days. Both partners
may display color changes, becoming pale to whitish during the
process (Lin et al. 2008, Martinez et al.
2005). The male inflates his pouch and begins to pursue the
female to signal that he is ready to mate. A series of movements
follows, including head pointing and the entwining of tails
(Lin et al. 2008). Mating behavior culminates in copulation,
as the female transfers her eggs to the brood pouch of the male.
The male then seals the pouch and fertilizes the eggs. After
the male gives birth, courtship may resume immediately.
Embryology & Development:
Clutch sizes in females may exceed 1000
(Teixeira & Musik 2001), and the reported brood size in
males ranges from 97 to 1,552 eggs. The average diameter of
eggs is 1.5 mm (Vincent 1990), approximately 2-33% of which
were found to be sterile (Teixeira & Musik 2001). The brood
pouch of the male acts as a marsupium, protecting the developing
embryos and providing them with oxygen through a capillary network.
The pouch also serves as an adaptation chamber, altering sodium
and calcium concentrations as development progresses until they
are similar to the surrounding seawater prior to birth (Linton
& Soloff 1964). The average gestation period for H.
erectus is 20-21 days (Herald & Rakowicz 1951), and
the male gives birth to fry approximately 11 mm in length (eg.
Herald & Rakowicz 1951) over the course of about 3 days
(eg. Lin et al. 2008). Breeding spans from
May to October for populations in the Chesapeake Bay (Teixeira
& Musik 2001), with the largest densities of individuals
occurring in July for south Florida populations (Powell et
al. 2007).
IV. PHYSICAL TOLERANCES
Temperature:
The distribution of the lined seahorse
extends throughout temperate to tropical latitudes, spanning
a range of temperatures. In addition, individuals inhabiting
shallow estuarine habitats are likely subject to large temperature
fluctuations seasonally, during tidal cycles, episodes of heavy
precipitation and terrestrial runoff. Adults may migrate seasonally,
moving to deeper waters during colder months (Hardy 1978). Temperature
also affects gonad development, brood size, and survivorship
and growth of juvenile seahorses (Lin et al. 2006,
2007, 2008; Lockyear et al. 1997; Sheng et al.
2006; Wong & Benzie 2003). In one study, the highest growth
rates and survivorship of cultured H. erectus juveniles
occurred at 28-29 °C (Lin et al. 2008).
Salinity:
Common in both estuarine and marine environments,
H. erectus is likely tolerant of a wide range of salinities.
The most commonly encountered salinity range for this species
is probably 25-35 ppt. Broodstock and juveniles have been kept
in captivity at 35 ppt (Lin et al. 2008).
V. COMMUNITY ECOLOGY
Trophic Mode:
Seahorses are predatory fishes, preying
on a variety of small crustaceans, mollusks and various zooplankton.
Prey items are captured via a unique suction feeding behavior.
Once food is located, a sudden upswing of the head draws it
into the mouth, followed by pipette-like suction transport into
the buccal cavity (Bergert & Wainwright 1997). The entire
prey capture process for each strike is quite rapid, with the
total feeding and recovery time lasting less than one second
(Bergert & Wainwright 1997). The origin of the clicking
sounds produced during the feeding process in seahorses is controversial.
Some studies suggest that cavitation occurs during prey capture,
producing sound from the collapse of vapor bubbles in the water,
which is caused by rapid pressure changes in the buccal cavity
(James & Heck 1994). Other experiments support the hypothesis
that the sound actually originates from the articulation or
contact of two bones in the head, the supraoccipital and the
coronet (Colson et al. 1998; Fish et al. 1952;
Fish 1953, 1954; Fish & Mowbray 1970).
Gut content analysis for individuals in Chesapeake Bay shows
a varied diet for H. erectus (Teixeira & Musik
2001). The most common prey items appear to be amphipods, especially
Ampithoe longimana, Gammarus
mucronatus, Stenothoe minuta and Caprella
penantis. Other foods included: copepods; polychaetes;
gastropods; and grass shrimp in the family Palaemonidae. In
captivity, juveniles and adults have been reared on a variety
of foods, including: live and frozen nauplius and adult stages
of the brine shrimp, Artemia spp.; live and frozen
Mysis shrimp; grass shrimp; copepods; gammarid and
caprellid amphipods; fry of the killifish, Poecilia
sp.; and frozen krill, Euphausia pacifica (Lin et
al. 2008, Martinez et al. 2005).
Predators:
Information on specific predators of the
lined seahorse is scarce, but the camouflage behavior of this
species among seagrass blades, algae and mangrove roots reduces
predation risk. However, mobility in H. erectus is
limited and larger fishes likely prey on adults and juveniles.
In addition, captive parental males have been documented to
cannibalize small numbers of their own fry following release
into the water column (Lin et al. 2008).
Parasites:
The lined seahorse is vulnerable to several
parasitic infections, especially in captive adults and aquacultured
juveniles. Documented parasites include: microsporidians, including
Glugea heraldi (Blasiola 1979, Vincent & Clifton-Hadley
1989); a myxosporidian of the genus Sphaeromyxa (Vincent
& Clifton-Hadley 1989); fungi (Blazer & Wolke 1979);
ciliates, including Uronema marinum (Cheung et
al. 1980); and nematodes (Vincent & Clifton-Hadley
1989).
Activity Time:
Like most other syngnathids, H. erectus
is diurnal, actively feeding and engaging in other behaviors
during the day.
Associated Species:
The lined seahorse has no known
obligate associations. However, as inhabitants of a variety
of coastal ecosystems, these seahorses are associated with several
organisms common to mangroves, seagrass beds and other habitats.
For extensive lists of other species found throughout the ecosystems
in which H. erectus occurs, please refer to the “Habitats
of the IRL” link at the left of this page.
VI. SPECIAL STATUS
Special Status:
All 33 species of Hippocampus
are listed on Appendix II of the Convention on International
Trade in Endangered Species of Wild Fauna and Flora (CITES 2004).
The lined seahorse is listed as “Vulnerable”
by the International Union for Conservation of Nature and Natural
Resources (IUCN 2002).
Mexican populations of H. erectus
are included on the country’s endangered species list (Norma
Oficial Mexicana NOM-059-SEMARNAT-2001). In addition, Mexico
prohibits the intentional capture and trade of wild seahorses,
permitting only incidental catches and culturing of captive
populations (Lourie et al. 2004).
Economic Importance:
Live seahorses are frequently collected
for the aquarium trade, and demand is high for dried specimens
used in traditional medicine and curios (Lourie et al.
2004, Vincent 1996). The global trade of these fishes is estimated
at 20 million seahorses per year (Vincent 1996), involving at
least 50 nations and territories (Job et al. 2002).
Brazil is one of the leading exporters of ornamental fishes
used in the aquarium trade. Over a five year period, an excess
of 12,000 H. erectus were traded through one Brazilian
market alone (Monteiro-Neto et al. 2003). Florida is
considered the primary source for live seahorses in the United
States. During the 1990s, seahorses ranked as the seventh most
economically important fish group, with landings increasing
by 184%; whereas, landings of all other valuable groups declined
(Adams et al. 2001). The commercial demand for seahorses
in many countries is met by the incidental bycatch of individuals
in shrimp trawls (Baum et al. 2003, Lourie et al.
2004). Because of the average mesh size of these nets, fishes
measuring 10-20 cm are most commonly collected using this method
(Baum et al. 2003).
Threats & Conservation:
Due to overexploitation of wild stocks
and habitat degradation, seahorse populations are declining
globally (Baum et al. 2003, Lourie et al.
2004). Overfishing may affect this fish group more than others
because shrimp trawls operate throughout the seagrass habitats
in which many hippocampids reside, and because life history
and behavioral traits of seahorses reduce their ability to recover
from disturbances (Vincent 1996). Examples of such traits include:
reduction of reproductive output when a monogamous, mated pair
is separated (eg. Vincent 1995, Vincent & Sadler
1995, Kvarnemo et al. 2000, Perante et al.
2002); and the prolonged recolonization of overfished areas
due to sparse populations and low mobility of individuals (Perante
et al. 2002, Vincent et al. 2005). In addition
to attempting protection of wild populations through legislation,
the interest in seahorse aquaculture continues to grow as a
means of reducing fishing pressure (eg. Job et
al. 2002). Recent studies on the captive breeding of H.
erectus suggest that it is a suitable candidate for commercial
aquaculture (Lin et al. 2008).
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Report by: LH Sweat, Smithsonian Marine Station
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