Morphological Study of the Vertebral Column of the Ponyfish Leiognathus equulus (Family: Leiognathidae) Collected from the Sea of Oman  

Laith A. Jawad1 , L. Al-Hassani2
1. Manukau, Auckland, New Zealand
2. Marine Science and Fisheries Centre, Ministry of Agriculture and Fisheries Wealth, P.O. Box 427, Postal Code 100, Muscat, Oman
Author    Correspondence author
International Journal of Marine Science, 2014, Vol. 4, No. 33   doi: 10.5376/ijms.2014.04.0033
Received: 01 Mar., 2014    Accepted: 02 Apr., 2014    Published: 27 Apr., 2014
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Jawad and Al-Hassani, 2014, Morphological Study of the Vertebral Column of the Ponyfish Leiognathus equulus (Family: Leiognathidae) Collected from the Sea of Oman, International Journal of Marine Science, Vol.4, No.33 (doi: 10.5376/ijms.2014.04.0033)

Abstract

Based on a morphometric study, Leiognathus equulus was divided into five morphologically distinct regions: anterior postcranial, posterior postcranial, anterior middle region, posterior middle region, and ural. The length, height, width and vertebral central width of the successive vertebrae allow showing the characteristic features of the vertebral profiles to be drawn. Differences in vertebral length present in different regions of the vertebral column show regionalization in this structure. These morphological descriptive parameters express a morphotype that seems to have a functional link with the thunniform mode of swimming of L. equulus. Therefore the present work aims to study the biometry of the vertebral column of L. equulus and contribute to other morpho-functional data available for teleost species.

Keywords
Vertebral column; Morphology; Regionalization; Ponyfish; Leiognathus equulus; Sea of Oman
The study of the skeletal characters of this species has not been receiving considerable attention so far. Vertebrates showed variation in the degree of regionalization of the vertebral column. Such differences in the morphology of vertebrae of different regions of vertebral column can be revealed by biometrical studies (Kubo and Asano, 1987, 1990, Desse et al., 1989). The vertebral column of actinopterygian fishes has two basic regions: the pre-anal abdominal region and post-anal caudal region (Grande and Bemis, 1998) with remarked diversity in vertebral form within these regions (Ford, 1937, Pietsch, 1978, Grande and Bemis, 1998, Bemis and Forey, 2001). The abdominal region may include, from anterior to posterior, occipital vertebrae that are incorporated into the skull through ontogeny, middle region vertebrae that are highly modified, and vertebrae that generally bear abdominal ribs (Desse et al., 1989). The ural region includes vertebrae that bear haemal spines and ural vertebrae that bear hypurals. The locomotory function is linked to the structure of the vertebrae in each region of the vertebral column (Ramzu et al., 1992).
Among the important mechanical tasks that the vertebral column gets involved in is the fish locomotion (Learm, 1976, Lindsey, 1978, Weihs, 1989). During the developmental stages, this structure is subjected to different types of biological strains which seem to be expressed by vertebral morphological charactristics (Kubo and Asano, 1987, 1990, Desse et al., 1989). Due to the strong anatomical and functional relationship with the trunco-caudal musculature (Le Danois, 1958, Lindsey, 1978, Vronskii and Nikolaitchouk, 1989) and since the vertebral column is an important structure in locomotion, it is interesting to study its morphology and to make links between this morphology and the locomotion.
The aim of the present work is to study the biometry of the vertebral column of the common ponyfish, L. equulus collected from the Sea of Oman as there is no such information is available in the literature for this species.
1 Materials and Methods
On 31 May 2011 and during the ichthyological survey on the fish fauna in the vicinity of Muscat City, Sea of Oman, 75 specimens of L. equulus ranging in length 65-190 mm SL were obtained and their vertebral column were studied. To prepare dry vertebral column, the method of James (2008) was followed. The fish specimens were boiled to 110˚C to strip the flesh off the bone. After boiling, their vertebral column was brushed in running water. After drying, the vertebrae were separated and numbered then measured with a digital 1/100 caliper (Ted Pella, Inc., Redding, USA). The dry vertebrae were deposited in the ichthyological collection of the marine science and fisheries Centre, Ministry of Agriculture and Fisheries, Oman, Catalogue no. 1245. Four vertebral measurements were selected: vertebral length (VL), the distance along the left mid-ventral line. It is considered among the factors controlling the degree of the body’s flexion (Ramzu et al., 1992); vertebral height (VH) the maximum vertical distance of the anterior side of the vertebrae; vertebral width (VW), the maximum horizontal length across the anterior surface of vertebra; and central length of vertebra (VCW). The presence of bony crests might affect significantly the mechanical constrains exerted by the intermediary of muscles (Ramzu and Meunier, 1999). The data are presented as average values (Table 1). To indication any statistical significant difference is present in the vertebral measurements, t-test was used to show such difference.
From these four measurements, it is possible to establish a vertebral profile which reflects the variation of these parameters along the vertebral axis (Desse et al., 1989, Ramzu, 1994, Kacem et al., 1998, Ramzu and Meunier, 1999, Nowroozi, 2012). To avoid individual variation and to facilitate future comparisons with other samples, even other species, each vertebral measurement was converted into a vertebral index Vi (Ramzu and Meunier, 1999):
Vi =P/SL
Where, P is the vertebral parameters (VL, VH, VW and LC) and SL the standard length. Profiles of the vertebral column were drawn by plotting VL, VH, VW and LC against the ordinal number of the vertebrae.
The number of abdominal and caudal vertebrae was counted and the mean value is calculated for each vertebra then species means were calculated for abdominal vertebral number (AVN) and caudal vertebral number (CVN). The thoracic vertebrae were defined as those that were cranial to vertebrae with separated haemal arches. The caudal region was defined as the region from the first fused haemal arch posterior to the last centrum including the ural centrum. The mean vertebral aspect ratio (AR= centrum height/ centrum width) for each region was calculated for each individual. The means were then calculated for abdominal aspect ratio (AAR) and caudal aspect ratio (CAR). Osteological terminology mainly follows Chapleau (1988), Ramzu and Meunier (1999), and Nowroozi (2012).
2 Results
All the twenty three specimens of L. equulusanalysed here for gross morphology all had 23 vertebral centra from cranial to caudal excluding the urostyle: 10 abdominal vertebrae, 13 caudal vertebrae, and the Urostyle (Figure 1). It is possible to divide the vertebral column of L. equulus into five regions: anterior postcranial region (V1), posterior postcranial region (V2-V5), anterior middle region (V6-V10), posterior middle region (V11-V18) and ural region (V19-V23). The choice to separate the vertebral column in five regions is supported by differences in the length and the height of vertebra (t ≥ 1.98; p ≤ 0.05).


Figure 1 Vertebral column of Leiognathus equulus showing regionalization (Magnification = X1)

Vertebra no. 11 marks the boarder between thoracic and caudal vertebrae. The 10 vertebrae of the anterior region, define the abdominal region or truncal, delimited by the presence of the gut, the two haemal arches, remain separated and the haemal spine is absent. The caudal vertebrae belong to the tail; their haemal arches are fused from the 11th vertebra and prolonged by a haemal spine. This latter spine is at its maximum length at V12 and decreased in length posteriorly. The anterior surface of the haemal arch is characterized with presence of wide trough-like cavity which getting narrower posteriorly. Haemal spine appears at V11. There is no intraspecific variation in the shape of vertebrae is noticed.
The centrum is made of two halves of sphere joined by their curved sides (Amphicoelic vertebrae), whose chordal cavities connected by a thin hole. There is pair of anterior processes and a pair of posterior processes protrudes from the dorsal aspects of each centrum. The anterior and posterior processes are close to each other and lie 1–2 mm from one another with external ligament bounding them together. Neural spines and arches project from the dorsal aspect of each vertebral centrum.
The first 3 vertebral centra differ qualitatively from the remaining abdominal vertebrae. The adjacent centra are joined together through the dorsal anterior processes of one vertebra which overlap to a greater degree with the dorsal posterior processes of the other vertebra, e.g. the dorsal anterior processes of centrum of the 2nd vertebra overlap substantially with the proximal portion of the dorsal posterior processes of the first centrum near the base of the neural spine. This overlap of processes is present for the first 4 vertebra. In addition, these four vertebrae have parapophysis are dorsally located relative to the parapophysis found on the abdominal rib-bearing vertebrae and located at the base of the neural spine. These ribs do not extend down to surround the viscera.
Vertebrae 2–10 have a typical abdominal vertebral morphology with well-developed anterior and posterior dorsal processes but gradually become less developed posteriorly to V11. In addition, the two portions of each centrum, dorsal and ventral, are connected to each other by significant of bony material. In having both haemal arches and short ventrally located parapophyseal projections that articulate with ribs that extend ventrally around the viscera, the vertebrae V2-V5 are considered to have a transitional form. Finally, vertebrae 11–23 have a typical caudal vertebral morphology with reduced dorsal processes relative to the first 3 vertebrae, an absence of ribs and parapophyseal articulations, and presence of haemal arches and spines. In the transitional and caudal regions, a prominent lateral ridge is clear along the midline of each centrum.
The vertebral column of L. equulus showed characteristic regionalization (Table 1). Firstly, the anterior postcranial region, immediately at the back of the skull, ensures articulation with it and is composed of three vertebrae that present relatively important variations of their parameters. The first vertebra is a small vertebra with fine neural spine. There are no anterior zygapophyses in the first vertebra as this vertebra supports the skull; instead, it has two facets for the skull to rest on. In V2 & V3, the anterior and the posterior zygapophyses are overlapped to greater degree. The development of the lateral parapophysis decreased posteriorly. Parapophyses of V1-V3 are more dorsally located relative to those of the abdominal vertebrae.


Table 1 Average values (M) (mm) of vertebral length (VL), vertebral height (VH), vertebral width (VW) and vertebral central width (VCW) of the vertebral column of Leiognathus equulus(SD = standard deviation)

Secondly, the middle region appears to be made up of two morphological entities: the anterior middle region (V6-V10) and the posterior middle region (V11-V18). Here the length of the trunk vertebrae increases and continue in their increase until the 19th vertebra. The trend of the width of the vertebrae showed was the same shown by that of the vertebral length until the 19th vertebra, and then shows steady decrease until the 23st vertebra.
Middle region vertebrae have well developed anterior and poeterior zygapophyses. In addition, these vertebrae have ribs that extend ventrally toward viscera. At V10, the lateral bony ridge that separates the dorsal and ventral portions of each vertebra is present.
Thirdly, the ural region includes vertebrae V18-V23. It corresponds to the caudal peduncle and is characterized by a fall of the four vertebral measurements studied.
The profiles corresponding to the four parameters measured on all the vertebrae were the same in all the specimens studied, indicating a similar trend. The morphometric analysis shows that the vertebral axis of the common ponyfish, L. equulus has complex division. Five regions can be characterized along the vertebral axis according to the changes of the four parameters measured from one vertebra to another.
The vertebral profile given by the variation of the vertebral length along the axis shows one maximum at V19 (34.7) and two minima at V23 (18.54). There is a great increase between V4 & V19 then the value drops down between V19 & V21.
The variation in the vertebral height showed the following profile: The lowest height value is observed at V1 & V23 while the maximum value is observed at V3. There is an increase between V1 & V3 followed by steady decrease at V4 then the values showed slight variation until V18 after that it drops dramatically between V19 & V23.
There are two minima at V1 and V23 and one maximum at V3 showed by the profile of the vertebral width. In this profile, there is a dramatic increase in the value between V1 & V3 then it decreases between V4 & V5. An observed fluctuation in the values is observed between V6 & V10 followed by a slight increase between V11 & V19 then the values showed a decrease between V20 & V23.
In the profile of the central width of the vertebrae, there are two minima at V1 and V23 and one maximum at V13. There is a slight increase between V1 & V3followed by slight variation between V4 & V9. The values showed gradual increase combined with slight variation between V10 & V15 then a slight decrease is evident between V16 & V20. The value at V20-V22 looks very similar, but it drops dramatically between V21 & V23.
The total number of vertebrae of L. equulus is 23.Out of this number; there are 10 vertebrae as abdominal vertebrae and 13 as caudal vertebrae. The vertebral aspect ratios for the abdominal and caudal regions are 7.48 and 7.31 respectively.
3 Discussion
The vertebral column of L. equulus is composed from five regions. This is evident through the biometric study of four vertebral dimensions: 1) anterior postcranial; 2) posterior postcranial region; 3) anterior middle region; 4) posterior middle region and 5) ural region. The 3rd and 4th regions are characterized by variation in vertebral parameters; in regions 1, 2 &5, these variations are characterized by a slight increased or decreased cline of the four parameters.
The post-cranial region, immediately at the back of the head, insures the articulation with the skull. The first three vertebrae V1-V3 are morphologically similar resulting into specific parameters of vertebral length, height, width and vertebral central width. For L. equulus, the first vertebra plays as anterior ventral concavity which is articulated with the basioccipital. This first post-cranial vertebra is designed to articulate with the posterior region of the skull, forming with the next vertebra a link between the two main elements of the axial skeleton, which is a function that requires a unique some morphological feature (Videler, 1993). As the vertebral biometric parameters increased for the six vertebrae beyond V4-V9, these vertebrae could be considered like transition vertebrae (Ramzu and Meunier, 1999).
The posterior middle region (V1-V18) includes the limit between the truncal and caudal regions (V11) which corresponds to the haemal arch closing. It is therefore composed of caudal vertebrae and forms morphological units. In these regions the increase is regular until a maximum value before decreasing progressively.
The ural region comprises of five vertebrae (V18-V23). It corresponds to the tail and is characterized by a decrease of the three values of the analyzed parameters.
As in other teleost fishes (Ramzu et al., 1992), the substitution of classical anatomical trunco- and caudal region by more than two regions as in the case of L. equulus, is probably linked to the mechanical constraints of swimming. Moreover, the antero-posterior development of the three morphological parameters studied with the variations of the postcranial and ural regions on the one hand, and the maximum of the middle regions on the other, favors this hypothesis. The common ponyfish is known to present a thunniform mode of swimming (Breder, 1926, Lindsey, 1978, Webb, 1978) in which the vast majority of movement is concentrated in the very rear of the body and tail. Thunniform swimmers in which the vast majority of movement is concentrated in the very rear of the body and tail. The fact that the maximum of the length of the vertebra (VL) occurs around the vertebrae V17-V19; this can be the structure response of these vertebrae to the local presence of maximal mechanical constraints.
Regarding the fourth region, its specific parametrical variation might express the major role performed by the caudal vertebrae in the motor process of swimming. The caudal skeleton responds to the alternate contraction of the intrinsic muscles on sides of this region, thus torsion of the caudal peduncle is created, when they move suddenly toward a prey or run away from a predator. This result backs those of Bainbridge (1963) on different species of fishes.
The regionalization in the vertebral column of L. equulus could be developed through the difference in length of vertebrae in different regions of the vertebral column which in turn is due to different mechanisms that regulate growth of vertebrae in each region (Fjelldal et al., 2005).
The similarity in the value of the aspect ratio of both abdominal and caudal region obtained in this study may indicate that the changes in the vertebral length in abdominal and caudal regions are closely linked (Ward and Brainerd, 2007).
The morphometric analysis of the vertebral column has revealed a significant difference between the variation of the vertebral length, height, width and vertebral central width studied taken on each vertebra of L. equulus. Therefore, characterization of the vertebrae is possible on the bases of these parameters.
Acknowledgements
We would also like to thank the Ministry of Fisheries Wealth, Marine Science and Fisheries Centre, Ministry of Fisheries Wealth and the directorate of Agriculture and Fisheries Developmental Fund for giving us the opportunity to work on the fish samples within the qualitative and quantitative distribution of marine organisms in Sultanate of Oman and to provide the appropriate financial support.
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