Morphological Perspectives of the Seahorse Hippocampus kuda

E-International Scientific Research Journal
ISSN: 2094-1749 Volume: 2 Issue: 1, 2010
Morphological Perspectives of the Seahorse
Hippocampus kuda (Bleeler) Vertebral System
K.Kumaravel, E.Rethina Priya,
S.Ravichandran, and T.Balasubramanian
Centre of Advanced Study in Marine Biology,
Annamalai University,
Parangipettai-608-502.
Abstract:
The vertebral system in Hippocampus kuda is highly specialized because of the vertical
locomotion and tail prehensility. The vertebral elements represent a special case of
morphological changes. We investigated the vertebral spine in H. kuda through skeletal
morphometric characters, in order to describe functional and structural patterns. Actually, the
dorso-ventral tail bending ability in the genus Hippocampus is one of the most impressive
morphological modifications in the evolutionary history of fishes respectively. The vertebral
size decreases from the anteriormost element backward, with some local variation at the dorsal
area. The allometric trajectory leads to a natural ventral bending of the tail, promoting its
prehensile function. A fan-shaped array of cartilaginous bones, the pterigiophores, forms the
internal supporting structure of the dorsal fin. Each pterigiophore is composed of a proximal
radial that extends from a vertebra to the dorsal side of the animal, where it fuses to a middle
radial. The middle radials fuse with each other to form a dorsal ridge upon the spheroidal distal
radials. The dorsal and pectoral fins are the primary locomotor organs in seahorses
(Hippocampus) and pipefish (Syngnathus). The small dorsal fins beat at high oscillatory
frequencies against the viscous medium of water.
Key words: Seahorse, Vertebral system, Locomotion, Dorsal fins
INTRODUCTION:
Seahorses are found primarily in warm and coastal waters; they are worldwide
distributed in tropical and temperate regions, roughly from 50 degrees north to 50 degrees
south latitude. Seahorses and few other syngnathids are the only fishes that can bend the tail in
a strict spiral used as a prehensile appendage. Considering the 52 genera of syngnathids
(Nelson, 1994), the dorso-ventral tail bending ability have been evolved only in Hippocampus
and in few other taxa like Acentronura and Amphelikturus (Dawson, 1985). Though
Solenostomidae -the sister-group of Syngnathidae - share many characters and a common
evolutionary history with them, the new movement of seahorse tail can be considered a real
apomorphy into this family (Hale, 1993). Seahorses spend most of time attached to algae and
corals by their prehensile tail, cryptic with the surrounding substratum. Swimming is highly
specialized through rapid oscillations of the dorsal fin (Breder & Edgerton, 1942, Ashley-Ross,
2002), the caudal fin being absent and the whole caudal region having lost its role in
locomotion. Locomotion by fin undulation is widespread among fishes that need of a slow
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speed and high maneuverability within complex, obstacle-strewn environments such as coral
reefs (Lindsey, 1978). A unique way of swimming called hovering (Videler, 1993) evolved
with the seahorse new anatomy and the camouflage ability. Seahorse tail has also an important
function in the social behavior of these animals: males strongly hang each other with the tail
when competing for females and it is softly used by the couple during mating (Vincent, 1994).
The new tail bending skills and posture of seahorses have probably opened ecological
alternatives that revealed advantageous considering the high number of species into this genus.
At least theoretically almost all pipefishes have preserved a lateral movement of the tail and in
most cases the tail fin as well. Compared with the general organization of the biomechanical
system of other fishes, Hippocampus undergoes substantial modifications in relation to the
proportions of the myotome parts (Rauther, 1925, Hale, 1996). Previous experimental
approaches have demonstrated a direct involvement of a couple of cordon like median ventral
muscles, and their connection with hemal spines (Hale, 1996). Nevertheless, significant
innovations in the axial skeleton of seahorses have not been explored. In this paper we explore
the variation of the vertebral metameric series in Hippocampus kuda through geometric
morphometrics. (e.g., Bookstein, 1996, Rohlf & Marcus, 1993, Adams et al., 2004). Metameric
elements represent a specific case of morphological variation. Thus, shape variables can be
regressed and compared by virtue of their serial position.Hence this research paper focuses on
the skeletal morphology of H.kuda based on their vertebral structures.
LOCOMOTION
DORSAL FINS: Locomotion via movements of the median fins has arisen independently
multiple times in the evolution of fishes. The methods of median fin propulsion are as varied as
the taxa that employ them (Lindsey, 1978). Locomotion by dorsal fin undulation is classified
as amiiform exemplified by Amia calva, but also seen in many mormyrids, Gymnarchus, and
syngnathids, including Syngnathus and Hippocampus). Gymnotiform locomotion is
characterized by undulations of the anal fin only (seen in gymnotids and notopterids).
Undulation of both dorsal and anal fins is referred to as balistiform (seen in triggerfish and
their relatives, some cichlids, centriscids, and flatfish). Finally, tetraodontiform locomotion is
defined as oscillations of short-based dorsal and anal fins (seen in pufferfish and ostraciids).
Median fin propulsion is associated with relatively slow, but precise, locomotion in complex
habitats such as coral reefs and obstacle-strewn stream or sea beds (Lindsey, 1978, Videler,
1993). Most fishes that use median fin propulsion generate low-frequency waves (E2 Hz) of
high amplitude in the fins, allowing them to swim with high hydrodynamic efficiency (defined
as useful power output divided by power input to move the fin; Blake, 1980). The family
Syngnathidae, comprising the seahorses and pipefish, is an exception members of this group
which undulate their dorsal fins at very high frequencies (440 Hz in seahorses(Breder and
Edgerton,1942,Blake, 1976) with low amplitude, leading to reduced efficiency and very slow
swimming speeds (Blake,1980). Because the swimming speeds of seahorses and pipefish are
slow, one may expect that the musculature powering dorsal fin movements would be relatively
weak. However, the dorsal fin muscle is required to contract against substantial resistance as it
beats the fin back and forth through the viscous medium of water. Furthermore, the muscle
must contract at a high frequency while performing positive work against the environment.
Indeed, the oscillation is so rapid
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MATERIAL AND METHODS
Sampling and configurations. Specimens of Hippocampus kuda were obtained from by catch
in Tuticorin coastal environment during summer 2009. One adult male was selected according
to its good maturity status and lack of pathological or sub pathological alterations. One
individual has been used because of the explorative purpose of this study. The specimen has
been cleaned by immersion in 2-3 % KOH solution for few hours. Digestion of skin and soft
flesh parts has exposed the skeleton and the characteristic bony plates that cover syngnathids
body. (Fig.1)
Head
Pectoral fin
Eye
Dorsal fin
Snout
Brood
pouch
Tail fin
Fig.1: Morphology of Hippocampus kuda (male)
RESULTS:
The exposed skeletal parts of seahorses were arranged in order with respect to the vertebral
status (Fig.2 &3). The dorsal and cervical vertebrae are clearly distinguished.
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Fig.2. Skeletal morphology of H.kuda
Fig.3.Cephalic part of H.kuda
Cervical vertebrae:
This group is heterogeneous, with the three elements showing different shapes. The
first is mainly characterized by a compression of the posterior area, backward bending of the
entire structure and protruding of the neuro-caudal edge. The second vertebra shows the
enlargement of the neuro-cephalic quarter, and reduction of the neuro-caudal one. The third
cervical vertebra is the less diversified, showing just a minor anterior enlargement of the upper
neural edge.
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Abdominal vertebrae.
Compared to the consensus average shapes, the abdominal structures are characterized by a
slightly shorter and higher neural area that is, a vertical stretching of the upper
part.(Fig.4&5).This pattern is more evidenced in the pre-dorsal segments than in the postdorsal ones.
Dorsal vertebrae:
The two dorsal elements show the vertical stretching of the posterior body, with
consequent bending of the whole structure becoming relatively shorter. The supradorsal
vertebra is conversely characterized by a marked development of the neuro-cephalic quarter,
and reduction of the neuro-caudal area. The pattern is similar to that expressed in the 3rd
vertebra, just more stressed and exaggerated. Furthermore, in the 10th element the enlargement
of the neuro-cephalic area is more vertically directed. The dorsal fin ray is wedged (Fig.6)
3
Fig.4. 12th Abdominal vertebrae
4
Fig.5. 19th Abdominal vertebrae
5
Fig.6. Dorsal Fin ray
DISCUSSION:
Outstanding position of the seahorses within the framework of the teleostean variability
provides a very good opportunity to analyze a discrete and well delineated evolutionary casestudy. H.kuda displays a unique structural arrangement of the axial skeleton compared to the
basic organization of teleosts, with changes in the relationships of the metameric elements
(Rauther,1925, Hale, 1996). The tail apomorphy in seahorses differ from species to species and
this diversity brings about their slight differences in their locomotion .The vertebral series in H.
kuda shows variations both in size and shape. Concerning size, there is a general decrease from
the cephalic to the caudal extremes. Nevertheless, changes are not always gradual a reduction
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is evident mostly after the dorsal elements, and a homogeneous decrease in size is only
displayed in the caudal set. The structural configuration of the vertebral series accounts for the
vertical locomotion and posture, while the caudal morphology supports the prehensile
properties of the tail. This study is aimed at characterizing the vertebral series in H. kuda
testing the patterns of shape variation through a morphometric approach. Using morphometrics
it is possible to recognize this taxon. These groups can be conventionally defined according to
their position and role along the vertebral sequence. Seahorse body posture and dorso-ventral
tail bending ability probably opened to a series of new ecological opportunities that have been
revealed successful and possibly involved in the high speciation rate of the genus.
ACKNOWLEDGEMENT:
The authors are thankful to the Ministry of Earth Science; Government of India for providing
financial supports for our research work and as well the director, CAS in Marine Biology,
Annamalai University for rendering a great support.
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FIGURE LEGENDS:
Figure 1
- Morphology of Hippocampus kuda (male)
Figure.2
- Skeletal morphology of H.kuda
Figure.3
- Cephalic part of H.kuda
Figure.4
- 12th Abdominal vertebrae
Figure.5
- 19th Abdominal vertebrae
Figure.6
- Dorsal Fin ray
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