Seahorses

SeahorsesJienne Al-Haideri, Seema Patel, Chithra Rajasekaran & David Yang AgendaIntroductionOverview of SeahorsesMorphology and Behaviorbrood pouches, snout, body, tail, dorsal fins feeding, Sexual dimorphismConservation ConcernsFuture DirectionsSummary & ConclusionsQ & A

IntroductionSeahorsesOrder: GasterosteiformesFamily: SyngnathidaeGenus: Hippocampus

Ancient Romans/AristotleSize RangeMarine 30-120 species

Brood PouchesHighly involved male parental careCompletely enclosed brood pouchFunction: Osmoregulation, nutrition, respiration, incubation site.ComparisonsPipe fishes: simple, inverted pouches.

All members of the family Sygnathidae (which includes both seahorses and pipefish) have evolved reproductive strategies for highly involved male parental care. The differences in reproductive strategies may arise from anatomical differences in the species. Most seahorses have a completely enclosed brood pouch. This pouch can be located ventrally either on the abdomen region or in the tail. They have a small opening under their anal fin that allows the female to deposit the eggs into the brood pouch. These marsupiums, or brood pouches, protect the eggs and embryos from the outside environment. Previously, due to the work done by Huot (1902), seahorse marsupiums were thought to provide the nutrition needed for the young to survive since they would not survive outside of the brood pouch. But Leiner proved this thought incorrect in 1934 when he investigated the osmotic pressure of the fluid in the pouch of incubating males. Through this investigation, it was found that the osmotic pressure inside the pouch was similar to blood during early incubation period and approached osmotic pressure of seawater in later stages. Leiner was able to get a few embryos to develop in partially dilute seawater. This proved that male seahorses played a more important role in providing the optimum osmotic environments for the developing embryos rather than the nutrients that the embryos needed.Using the results of this research, recent researchers have tried to study the role of the male brood pouch in more detail. In a recent paper in the Journal of Fish Biology, scientists compared the type of brood pouches in three different species of syngnathids (Nerophis ophidion, Syngnathus abaster and Hippocampus hippocampus). They found that the interactions between the male body and the developing embryos are inversely proportional to the degree of egg exposure to the external environment, and directly proportional to the complexity of the pouch (Carcupino et al., 2002). In other words, parental input by the males depended on the structure of the brood pouch.N. ophidion and S. abaster are two species of pipefish which do not have the complete brood pouch that the H. hippocampus has. The N. ophidion has a simple brood pouch in which the egg lacks any kind of protection and most of the egg surface is exposed to the external environment. The egg attaches to the outside of the male abdomen with thick mucous. There is little specialization in the tissue attached to the eggs and the only role the male plays is to provide some respiratory support for the embryos. S. abaster on the other hand has what is called an inverted pouch in which the eggs are deposited on the tail of the male and two folds of skin cover and round the eggs. In this species, the brood pouch acts as an osmoregulatory organ and supplies the embryos with oxygen. Finally, the study investigated H. hippocampus brood pouches. The researchers found that since these pouches completely enclosed the eggs, the pouches acted as an osmoregulator organ, providing the embryos with oxygen and supplying some nutrition to the growing embryos.

Male Reproduction

Sexual Dimorphism

Males/females differ in their external anatomyMonogamous (mate for life; unlike most fish)Dimorphism evolved from sexual selective pressuresEx: longer trunks in females accommodate developing ovaries

longer tails in males aid in courtship (tail grabbing) and gestation (brood pouches)

Note: Seahorses still maintain traditional sex rolesFeedingSit-and-wait predatorsCamouflageRely on site (independently moving eyes)Feed primarily on mobile preymainly crustaceansEvolved from straight bodied swimmers (pipe fish) Pivot feeding (2 step process)

S-shaped body increases strike distanceMain point: S-shaped body and sit-and-wait strategy evolved for better feeding

Snoutelongated snoutvarious snout dimensions depending on speciessnout length/width differences with agepivots head to lessen distance from their snout to the preylift angles allow for a greater distance to be covered by the mouthLeafy Seadragon Short-snouted Seahorse One of the most unique and obvious characteristics of the seahorse is its long snout. The main usage of the elongated snout is for feeding purposes. In order to efficiently use their snout, seahorses pivot their heads to lessen the distance from their snout to the prey (Roos et al. 2010). In order to conserve energy while feeding, seahorses keep their body relatively stationary, while only moving their head and mouth forward (Roos et al. 2010). An adaptation observed in the feeding behavior of seahorses is the use of lift angles. A lift angle, used by species such as Syngnathus acus and Hippocampus reidi, allow a greater distance to be covered by the mouth, rather than just a protrusion of the jaw (Roos et al. 2010). Although the elongated snout is used for the same purpose across all seahorses, there are morphological differences from species to species resulting in various snout dimensions. During the different stages of growth (ontogeny) there are morphological changes in snout length and width, causing differences in their kinematics (Choo and Liew 2006). For instance, when comparing the head movement of a newborn to an adult, a newborn can reach a rotational distance of 40 degrees in 2.5 ms, while for an adult, it would take about twice the time to cover only 25 degrees (Roos et al. 2010). Even though snout morphology may vary across age or species, the general usage of the elongated snout is common among all seahorses.

Anatomy of a Seahorse Head:Body Covering lack scalesBody Armor: thin layer of skin covering several bony plates Trunk rings and coronetCoronet: "crown" structure positioned at head (equivalent to human thumbprint)Camouflage used as a defense mechanism and predatory strategy (ex. Dragon Seahorse)Armor makes them unappetizing prey The body of the seahorse is another morphological feature that separates them from other fish. Unlike most fish, seahorses lack scales and, instead, possess a thin layer of skin covering several bony plates, which act as body armor and appear as rings (the longest being approximately 8 inches) around the trunk and tail (Sedberry, Pearse et al.). Each seahorse is distinguished by the number of trunk rings they possess and a bony projection called a coronet (a crown structure positioned on the top of their head that is almost as unique to each seahorse as a human thumbprint is to each individual human) (Irey, 2004). The ability to camouflage and the body armor serve as defense mechanisms for seahorses: the body armor makes them an unappetizing meal for predators and their camouflaging abilities help them blend in with their environment (Praet, Dominique et al. 2011). Their body camouflage also makes them efficient ambush predators, thus increasing their chance for survival and reproduction. TailSpecialized prehensile (grasping) tailEvolution of bent tailChange in regulation of growth and developmentDaily lifeAttached to algae and coralsSocial BehaviorSeahorses are part of a very small group of syngnathids that can bend the tail in a strict spiral and use it to seize or grasp objects. This has evolved only in Hippocampus and in a few other taxa. The tail has a spiral shape because the vertebral bone sizes decrease as they move from anterior to more dorsal positions. This results in the natural curvature of the tail. This change in proportion of parts in the organism may be explained evolutionarily by the change in the regulation of a common pattern of growth and development. A study by Bruner et al. 2008 shows that seahorse ventral tail bending is not only related to muscle activity, but also the seahorse skeletal structure. Because of this correlation, there is the evolution of bent tail that minimizes muscle strength, which allows the seahorse to bind to complex substrates while still maintaining a vertical posture. New efficient tail bending skills and the posture of seahorses have probably opened up ecological alternatives that may have potentially been advantageous when comparing the high number of species in its genus.The seahorse tail also has many functions in its day-to-day life. It spends most of its time attached to algae and corals by their prehensile (grasping) tail and it has an important function in social behavior. Males strongly hang to each other by the tail when competing for females and couples softly use it during mating activities. (Bruner et al. 2008)

Dorsal FinHighly specialized oscillationsHigh frequency median fin propulsion (unusual)Habitat selected for high degree of maneuverabilityComplex, obstacle-strewn environmentThe caudal region has lost its role in locomotion and instead, the seahorse swims through highly specialized oscillations of the dorsal fin (Bruner et al. 2008). The strong dorsal musculature contributes to high frequency median fin propulsion (Ashley-Ross et al. 2002). This need of locomotion by fin undulation is because of the seahorses habitat; it lives in a complex, obstacle-strewn environment (coral reefs) and therefore needs to have a high degree of maneuverability. This hovering has co-evolved with the new anatomy and camouflage ability (Bruner et al. 2008). What separates seahorse fin movements from most fish is that seahorses are able to maintain high frequency waves in median fin propulsion, whereas most fish can only generate low frequency waves (Ashley-Ross et al. 2002).

Conservation IssuesSimultaneous prey and predatorEconomic valueChinese medicinal uses50% decline in natural populationsVulnerable due to behaviorLive in endangered environmentsBy-catch (e.g., Shrimp Trawlers)

Seahorses are popular aquarium fish, but, because of their diet and sensitivity to changes in the environment, they are hard to raise. Market demands by traditional Chinese medicine, curio, and marine aquarium trade have also deprived the natural stocks of seahorses. Seahorses and relatives have particular life histories and behaviors that make them more vulnerable to endangerment and extinction. Their specialized and K-selected reproductive behavior coupled with their small home ranges and small group sizes are detrimental to their adaptation to environmental changes. They also live in the most endangered environments on the planet, including seagrasses, mangroves, corals, estuaries, lagoons, and macroalgae. Seahorses are also victims of by-catch and the majority come from shrimp trawlers. This is because they live in the same habitats as shrimp, are slow moving, and approximately the same size as the targeted shrimp. In some areas where the demand is high, seahorses are extracted from the catch and in regions of low-demand, some are thrown back or likely processed into low-grade fishmeal with other by-catch. Although indiscriminate gear catches few seahorses per haul, the overall total is enormous. An example from Vietnam illustrates this point. Although shrimp trawlers only caught about one or two seahorses a day per ship, this amounted to 2.2 million animals over the course of the year.(Vincent et al. 2011). For this reason, there is a 50% decline of seahorse population from 1990-1995 making seahorses appear at the brink of extinction. (Irey et al. 2002)

Future Research and DirectionsEvolutionary history is still controversialMystery of bending tail evolutionPhylogenetic relationships are poorly understoodDifficult to culture seahorses due to lack of information/dataIndicator species?Although seahorses are widely researched, the evolutionary history of Gasterosteiformes is controversial and much work has gone into creating an accurate phylogeny. This is due to the highly derived and reduced morphology of its families. There is still much debate over the origins of the spiraled tail. It is believed that the tail bending ability of seahorses is derived in Syngnathids, but how and why it evolved is still a mystery (Bruner et al. 2008). This causes much controversy in taxonomy. Even today, although some sister groups have been evolutionarily linked and their position solidified, relationships among groups in Gasterosteiformes remain poorly understood. (Wilson et al. 2011) Only a few Nerophis, Syngnathus and Hippocampus species have been studied in any detail. It is known, however, that this group of fishes exhibits a remarkable variation in sex roles and ornamentation, making them an ideal group for the study of mating patterns, sexual selection and sexually selected signals (Rosenqvist et al. 2011).The shortfall in knowledge concerning life-history characteristics and the taxonomy results in identification challenges and debate. Many seahorse species are listed in the IUCN Red List of Threatened Species and on CITES, however, there is not enough information on their distribution and population numbers to determine if they are at risk of extinction. Due to their high market demand, seahorse populations worldwide have been subject to intense exploitation, causing all species of seahorses to be among the fourteen bony fishes that are listed in the International Trade of Endangered Species of Wild Fauna and Flora. Because of their endangerment, measures are being taken to mitigate fish mortalities by culturing seahorses in captivity. However, it is especially difficult to culture seahorses due to the scarce amount of biological information on these particular fish (Garcia 2012).Furthermore, additional empirical data is needed on seahorse extraction, by-catch, and returns. It is difficult to know if seahorses are sensitive to environmental change or can be labeled as an indicator species. More research needs to be done to assess their reaction to environmental factors. In order to begin conservation efforts towards seahorses, anthropogenic pressures such as overexploitation in target fisheries, incidental take in non-selective gear, and threats from habitat degradation need to be addressed (Vincent et al. 2011).In conclusion, seahorses are exceptionally diverse creatures that possess many defining characteristics such as unique behavioral and reproductive features, snout, armored body, and dorsal fin. Although much research is still needed, these creatures are an important source of biodiversity that should be protected.SummaryAnthropogenic pressures need to be liftedOverexploitation in target fisheriesThreats from habitat degradationExceptionally uniqueUnique reproduction (male pregnancy)Drastically changed the fish body planVertical swimming styleThey need protection!Dramatic declines worldwideBiggest predator is man

Questions?