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Thomas Wetherill
Literature Review
Due to their environmental adaptations and intricate feeding and mating, seahorses
are one of the many species that are adversely affected by overfishing and habitat change
(Oliveira, T.P.R et al. 2010; Foster, S.J & Vincent, A.C.J. 2004). In the captive rearing of
seahorses it has been found that factors such as background colour and sound can
drastically effect the mortality rate with some colours causing increased mortality and some
sounds stunting growth which shows how changes from the norm can drastically effect
population both captive and in the wild, small changes in stress can vastly affect breeding
potential and population globally if environmental problems become widespread (Pawar,
H.B. 2011; Berzins, I.K et al. 2010). According to Teske, P.R & Beheregaray, L.B. 2009,
seahorses are thought to have diverged from pipefish during the late Oligocene period
around 20 million years ago (Casey, S.P et al. 2004). The seahorses’ habitats, generally sea
grasses in shallow waters were greatly increased allowing for the evolution of the upright
position which would have been favoured in the new habitat without affecting speed or the
ability to remain camouflaged. The pipefish, however never entered this new habitat so
there was no evolutionary need to evolve into an upright position meaning they maintained
their horizontal body shape like the majority of fish found in the ocean. Syngnathids move
using the dorsal and pectoral fins which beat at a high frequency enabling movement in the
water, generally seahorses can beat their fins at a higher rate than pipefish so can move
faster (Ashley-Ross, M.A. 2002).
The Long-snouted seahorse, Hippocampus reidi, is one of the most sought out
species in the international aquarium trade (da Hora, M.D.C. 2009; Diniz, A.D. 2008; Luiz, A
et al. 2008; Rosa, I.L. 2006). Seahorses, along with pipefish, form the family Syngnathidae
which are categorised by male pregnancy (Hoffman, E.A. 2006). The only genus in that
family is Hippocampus (Freret-Meurer, N.V & Andreata, J.V. 2008; Salin, K.R et al. 2005;
Silveira. 2000). The female carries the eggs until courtship and mating occurs, in which time
the eggs are transferred to specialised brooding pouches located on the tail or abdomen of
the male seahorse. The courtship dance and mating can last for several days. All post
fertilisation care such as nourishment and osmoregulation is taken care of by the male
which has evolved morphological traits to do this (Wilson, A.B. 2004). Like all species of the
Syngnathid family, the young are born as miniature replicas of the adult and increase in size
when they mature rather than change appearance (Garrick-Maidment, N. 1998).
Roos et al, 2009 stated that Hippocampus reidi, like all Syngnathid fish, have a very
morphologically different method of feeding compared to other fish. Whilst other fish feed
by creating an anterior to posterior flow of water in their expandable head to create suction
in a uni-directional direction, seahorses utilize their mouths to make rapid sucking
movements close to the prey in order to feed. The maximum flow velocity is not found in
the mouth channel, like with most teleosts, but rather in the narrow channel in the buccal
cavity. They have specialised features such as elongated snouts and an immobile pectoral
girdle which allows for this to occur (Leysen, H. 2011). They are visual feeders in that they
have to see their prey before they hunt it rather than relying on scent or chemical indicators
like some fish (Lee, H.R & O’Brien, K.M.D. 2011). Hippocampus reidi, when studied in the
wild, have been found to be rather sedentary predators waiting for their prey to come to
them most of the time rather than stalk them (Felicio, A.K.C. 2006). Their colour varies
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massively from black, yellow, orange, brown and red with numerous white dots all over but
mainly the tail area. Research into seahorse populations have led to the focusing on
different management procedures that could be put in place to ensure the conservation of
the species (Lourie, S.A et al. 1999).
Seahorse populations are a cause for concern due to the vast market for various uses
for them. All seahorses are listed on CITEs as vulnerable on the red list of endangered
species in the International Union for the Conservation of Nature. This puts severe
restrictions on the export and import of all seahorses, alive or dead which dramatically helps
in seahorse conservation as global fishing will reduce as the available market decreases as
well as helping to eliminate the wild seahorse trade in the aquarium trade. (IUCN, 2008;
Olivotto, I et al. 2008). They are a popular ornamental species as well as being traded to be
used in various traditional medicines, remedies and other product forms in various cultures
around the world in high volumes (Zhang, D. 2010; Oliveira, T.P.R et al. 2007; Woods, C.M.C.
2007; McPherson, J.M & Vincent, A.C.J. 2004; Payne, M.F & Rippingale, R.J. 2000). They are
targeted by divers that visit areas high in sea grasses and corals and are commonly poached
along with sea cucumbers and a number of different gastropods (Salin, K.R et al. 2005). This
overlong unsustainable exploitation of the seahorse species has caused vast gaps in our
knowledge of their biology and ecology due to them becoming increasingly difficult to find
(Woods, C.M.C. 2002). This is not helped by the gradual degradation and destruction of their
habitats in coastal areas including coral reefs, sea grasses and mangroves (Olivotto, I et al.
2008). Hippocampus reidi are generally found within the tropics globally and are actively
fished in some parts of the world namely the southern American countries such as Brazil
where they have the common name, Brazilian Seahorse (Rosa, I.L et al. 2005). They have
also been found in Guatemala, Honduras, Mexico, Nicaragua and Panama. Fisherman in
those areas have reported a general decline in all seahorse species caught as by-catch in
trawling and live subjects captured by divers. Hippocampus reidi have been found at depths
as deep as 55m so are not thought to be as affected by accidental by-catch as some other
species of seahorse but are thought to be equally susceptible to environmental changes to
their habitats (IUCN. 2008). In 175 countries the export of seahorses has been limited to
sustainable levels now in place to help protect them and also allow their numbers to
replenish (Vincent, A.C.J et al. 2011).
When keeping seahorses in captivity various problems can arise due to problems
with feeding and disease. Fortunately aside from temperature “most seahorse species have
very similar requirements when kept in captivity” (Baldwin, C. 06/12/2011). This allows for
feeds that can be fed to all Hippocampus species universally. When in a juvenile state many
seahorse species have a low survival rate unless fed the correct live foods and kept in an
immaculate environment to reduce the risk of disease. A typical live feed that is often used
is the zooplankton species artemia nauplii, however if it is entirely utilised there is often a
poor juvenile survival rate as the species stop feeding (Lunn, K.E & Hall, H.J. 1999). Despite
not being a natural food source for most species it has been successfully used for many
years though not with a 100% success rate (Rimmer, M.A et al. 1994; Sorgeloos, P. 1987). It
can also be enriched with highly unsaturated fatty acids (HUFAs) which make them very
suitable for juvenile fish and syngnathids as they require live foods with a high nutritional
content and most can feed on the same day they leave the adult males pouch (Sargent, J.R
et al. 1997; Watanabe, T et al. 1983). Other live prey that seahorses feed on includes
rotifers, mysid, shrimp and copepods (Woods, C.M.C. 2002; Payne, M.F & Rippingale, R.J.
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2000). Rotifers and artemia nauplii are generally the favoured though due being able to be
easily cultured in large quantities (Otero-Ferrer, F et al. 2010; Olivotto, I et al. 2008).
Chang, M.C, 2000, stated that most of the strains of artemia found in the aquatic
sense are deficient in HUFAs making them a poor long term food source for many fish
species. By allowing them to ingest food rich in HUFAs prior to their introduction into the
diet of the cultured fish species, they become enriched in fatty acids essential for growth
and development. They are one of the most cultured live food sources used in the
aquacultural world. During a study conducted by Chang, M.C 2000, the highest and quickest
mortalities were observed in the non-enriched artemia feeding group, once again showing
the lack of nutrition that can be obtained from using ordinary artemia as a long term
standard feed. Studies into enrichment have shown a usually substantial decrease in species
mortality for most fish when an enriching agent such as cold liver oil or squid oil has been
fed to artemia prior to their feeding to the fish (Southgate, P.C & Kavanagh, K. 1999).
Another increasingly popular method of enriching agent is selco, a slurried fish meal based
product that is fed to artemia and other zooplankton for enrichment and is very high in
essential fatty acids (Woods, C.M.C. 2002). There are typically two commercial strains of
artemia that are used; the ‘marine’ species which are generally more expensive as well as
being smaller in size with a larger nutritional profile, and the ‘continental’ species which is
cheaper as well as being larger with a poorer nutritional profile (Mendes, A.C et al. 2010).
In order to maintain water quality and help minimise excess waste products such as
nitrogenous waste it is important to regulate the food intake of the cultured species. It is
now commonly acknowledged that fish aren’t the polluters but rather excess food and over
feeding is the leading contributor to waste; either faecal matter or decomposing organic
matter (Amirkolaie, A.K. 2011). Excess levels of ammonia, even milligrams too much can
cause long term damage to many species and can increase potential stress. The optimum
level of ammonia in a closed system is zero milligrams per litre so it is very important to
continually monitor water quality after feeding the species as that is when the waste levels
will be highest so will give a more accurate reading of the exact amounts of ammonia and
other nitrogenous waste in the system. If water quality shows that the levels of ammonia
have risen above 0.02 milligrams per litre it is essential that a water change is performed as
the levels can change quickly and the higher they rise the more likely it is for damage and
stress to occur (Forteath, N. 2001).
The short-snouted seahorse, Hippocampus hippocampus, is a native British seahorse
that is one of only two species known to occur in Northern Europe (Pinnegar, J.K. 2008). The
other is the spiny seahorse, Hippocampus guttulatus. Both of these species are seagrass
dwelling, spending the warm summer month inhabiting the eelgrass beds around the UK
and then migrating in the cooler months to the boulders and rocks in slightly deeper waters
(Garrick-Maidment, N. 1998). In the Falmouth estuary in Cornwall there has always been
rumours of a secret hotspot of seahorses due to the eelgrass beds present (Deeble, M &
Stone, V. 1985). Seahorses have always held a special place in peoples’ hearts because of
their interesting and aesthetically pleasing appearance and their historical aloofness which
means that many people would want to help boost their numbers especially as their
population is hard to predict due to their migrations into different water depths and ability
to camouflage well (Garrick-Maidment, N. 1998). Researching the essential foods that are
most beneficial to the seahorses when reared in captivity is an important investigation due
to its’ potential to lead to the rerelease of bred native species back into the estuary to boost
their numbers. In order to be able to get permission to rear native seahorses, experience is
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needed raising non-native that are not under the protection of the Wildlife and Country side
Act 1981 that was amended in 2008 to prevent the killing, injuring or taking of Hippocampus
hippocampus or Hippocampus guttulatus (Natural England. 2009). The rearing of a nonnative species such as Hippocampus reidi for a period of time in which a feed trial of
different foods would be tested on separate test groups would be a good way to find the
most beneficial foods for growth and development and also for any bodies hoping to
acquire a licence for keeping natives in the future to gain experience.
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