Species Description: The lined seahorse, Hippocampus erectus is a large deep-bodied seahorse (Lourie et al. 1999) with a variable body color. Individuals can be olive-brown, ash gray, orange, red, black or yellow (Lourie et al. 2004, Robins & Ray 1986). Many have white lines following the contour of the neck, contrasting saddles across the dorsal surface and tiny white dots on the tail. Some specimens, especially those living amongst brown Sargassum algae, bare fleshy tabs and protuberances that serve as camouflage (Robins & Ray 1986). The coronet on the top of the head forms a low, triangular wedge with sharp edges. The 1st, 3rd, 5th, 7th and 11th trunk rings may be raised and enlarged. Cheek spines may be single or double, and the snout is usually less than one-half the head length. One tail and two trunk rings support the dorsal fin. Fin rays and other meristic counts are as follows: trunk rings = 11; tail rings = 34-39; dorsal fin rays = 16-20; pectoral fin rays = 14-18 (Lourie et al. 2004).
Potentially Misidentified Species: Two additional seahorse species occur in the IRL and surrounding coastal waters: the longsnout seahorse, H. reidi; and the dwarf seahorse, H. zosterae. The snout of H. reidi is longer than H. erectus, and the body usually has many dark spots evenly scattered over a brown background. Meristic counts are: 16-19 dorsal fin rays; 31 to 39 tail rings; and 15-17 pectoral fin rays. The dwarf seahorse is usually tan and unpatterned, with a dark stripe along the outer edge of the dorsal fin. Adults reach a maximum size of 5 cm, significantly smaller than adult H. erectus. Meristic counts are: 31-32 tail rings; 12 dorsal fin rays; 11-12 pectoral fin rays; and 9-10 trunk rings (Lourie et al. 2004, Robins & Ray 1986).
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.
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 1,000 (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).
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).
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.
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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