Cephalic Morphology of Ariosoma Gilberti (Bathymyrinae: Congridae)

Iran. J. Ichthyol. (March 2014), 1(1): 39–50 Received: January 01, 2014 © 2014 Iranian Society of Ichthyology Accepted: March 01, 2014 E-ISSN: 2383-0964 ; P-ISSN: 2383-1561 doi: doi: Cephalic Morphology of Ariosoma gilberti (Bathymyrinae: Congridae) Soheil EAGDERI1*, Dominique ADRIAENS2 1Department of Fisheries, Faculty of Natural Resources, Karaj Campus, University of Tehran, Karaj, Iran. 2Evolutionary Morphology of Vertebrates Laboratory, Department of Biology, Ghent University, K.L. Ledeganckstraat 35, 9000 Ghent, Belgium Email: [email protected] Abstract: The head bones and relevant muscles are important elements composing the feeding and respiration apparatus and can be considered as highly important structures for giving rise to evolutionary specializations. The eels of the Anguilloidei is a diverse group and structural diversity in their cranial musculoskeletal system is thus to be expected. Ariosoma gilberti is a member of Bathymyrinae (Congridae) with a hydraulic based prey transport and closely related to muraenid family with mechanical prey transport system. Since there is no information about the cephalic morphology of A. gilbert, hence this study was conducted to provide detailed description of its cranial osteology and myology. The immobile maxillary with large pointed teeth, an anterior moderate mouth gape, large jaw closing muscles and large lateral and ventral bony elements, providing more room for the orobranchial cavity of A. gilberti suggesting the presence of a hydraulic and suction-based mechanism. Keywords: Anguilliform, Osteology, Myology, Cranial morphology, Adaptation. Introduction each year (Smith & Kanazawa 1977; Castle & The head musculoskeletal system and its integration McCosker 1999; Chen & Mok 2001; Smith and in the functioning of the skull can be considered as a Karmovskaya 2003; Smith 2004), and more are highly important pattern for giving rise to expected to be recognized as various habitats are evolutionary specializations by means of natural examined more closely, especially those of the deep selection. The head of fishes is quite complex, with seas. The diversity of eels is astounding and over 30 individual bony elements that can be moved structural diversity in their cranial musculoskeletal by their associated muscles. Throughout evolution, system is thus to be expected. The study of such a quite some variation originated in the cranial diversity will provide information on how their musculoskeletal system of fishes. Judging from that, structures have changed through adaptive (or non- it can be expected that many combinations of adaptive) evolution and this may contribute to a muscles and bones are possible to perform a certain better understanding of Anguilliformes classification function, such as those for mouth opening and and evolution. closing. The head bones and relevant muscles are The Congridae is one of the largest and most important elements composing the feeding and divers anguilliform family (Smith 1989). This family respiration apparatus. comprises three subfamilies (Heterocongrinae, The eels of the Anguilloidei are a suborder Bathymyrinae and Congrinae) with 32 genera and comprising fifteen families, thereby forming the roughly 160 species (Nelson 2006). The Congridae richest suborder within the Elopomorpha (Nelson are found worldwide in tropical and subtropical 2006). This suborder still needs to be fully explored. latitudes and occur in the Atlantic, Pacific and Indian Many new species of Anguilliformes are described Oceans. Ariosoma gilberti is a member of 39 Eagderi & Adriaens- Cephalic Morphology of Ariosoma gilberti Bathymyrinae with a hydraulic based prey transport Adriaens et al. (1997) terminology. Terminology of system and form a monophyletic clade with cranial skeletal elements follows Nelson (1966), Muraenidae with a mechanical prey transport system Böhlke (1989) and Rojo (1991). The epiotic of (Mehta & Winwright 2007), based on mitochondrial teleosts is termed "epioccipital", thereby following 12S ribosomal RNA sequences (Obermiller & Patterson (1975). The terminology of scarf joint Pfeiler 2003). Hence, comparison of Bathymyrinae follows Hildebrand (1995). and Muraenidae belonging to a monophyletic group with a single evolutionary origin of an eel-like body, Results provides an opportunity to better understand the Cranial osteology: The neurocranium forms one evolutionary changes in the cranial musculoskeletal unit, tapering from the otic region towards the system, of which, some are the adaptations to anterior end of the frontal bone and then continuing different modes of life. Since there is no information as a narrow bar towards the spatulate-like snout (Fig. about the cephalic morphology of A. gilbert, we 1a). The ethmoid region comprises the fused provide a detailed description of its cranial myology premaxillaries, ethmoid and vomeral bones, as the and osteology and attempt to compare it with those premaxillo-ethmovomeral complex and nasals. The of muraenids published in Mehta & Winwright nasal bone is described with respect to the lateral line (2007) and Eagderi (2010). The cranial osteology system (see below). The premaxillo-ethmovomeral and myology of Gymnothorax prasinus and complex bears a lateral process, which is directed Anarchias allardicei (Muraeninae and anteroventrally with a small horizontal process at its Uropterygiinae: Muraenidae) were described in end (Figs. 1 and 2a) and the olfactory rosette is details by Eagderi (2010). The results of this study located anterior to this process. The anteroventral can help to better understand the morphological face of the premaxillo-ethmovomeral complex is changes of the head that have occurred during dentigerous with anterior, longer and recurved evolution toward mechanical based prey transport caniniform teeth (Fig. 1b). system. The orbital region comprises the two small lacrimal bones, four infraorbital bones, frontal Material and methods bones, parasphenoid, basisphenoids and For the anatomical description, two specimens of pterosphenoids. Posteriorly, the frontal bones are Ariosoma gilberti (UF 230612, 133 mm TL and UF connected with the parietal bones by an extensive 10803, 182 mm TL), obtained from the Florida scarf joint (Fig. 1a). Also, left and right frontal bones Museum of Natural History, were examined. The show a minor scarf joint (Fig. 1a). The two specimens were cleared and stained with ventroposterior rim of the frontal bone encloses the Alizarin red S and Alcian blue according to the anterior portion of the temporal canal, which enters protocol of Hanken & Wassersug (1981) for this bony canal at the connection between the frontal osteological examinations. Dissections with muscle and pterosphenoid (Fig. 2a). The frontal forms the fiber staining were performed according to Bock & posterodorsal wall of the large orbit, and the foramen Shear (1972). Specimens were studied using a opticum lies between the ventral portions of the stereoscopic microscope (Olympus SZX-7) frontal bones. Anteriorly, the frontal bears a small equipped with a camera lucida. lateral process over the orbit with a pore on its The musculature terminology follows posterior part. The basisphenoid forms the Winterbottom (1974), De Schepper et al. (2005) and posteroventral wall of the orbit and bears a small Mehta & Wainwright (2007). The circumorbital fossa on its anteroventral face. The dorsal part of this bones of the cephalic lateral line system follow bone runs into the optic foramen and overlaps 40 Eagderi & Adriaens- Cephalic Morphology of Ariosoma gilberti Fig.1. Neurocranium of Ariosoma gilberti. (a) Dorsal view and (b) ventral view. af susp A, anterior suspensorial articulation facet; af susp p, posterior suspensorial articulation facet; fr-Opt, foramen opticum;fr-Tri.fac, foramen trigemino-facialis; o-BOc, basioccipital bone; o-BSph, basisphenoid; o-Epi, epioccipital bone; o-ExOc, exoccipital bone; o-F, frontal bone; o-Par, parietal bone; o-PMx-Etv, premaxillo-ethmovomeral complex; o-Pro, prootic bone; o- PSph, parasphenoid; o-Pt, pterotic bone; o-PtSph, pterosphenoid; o-Sph, sphenotic bone. ventrally with the parasphenoid. The pterosphenoid which opens at the posterodorsal face of the pterotic is surrounded by the frontal, basisphenoid, (Figs. 3a). Posteriorly, this bone connects to the parasphenoid, sphenotic and prootic bones. The exoccipital bone and contributes to form the pterosphenoid possesses a pore on its lateral face posterior wall of the neurocranium. The posterior (Fig. 2a). The parasphenoid is bifurcated at its two articulatory facet for the hyomandibula is formed by ends and forms the ventral element of the orbit. At the pterotic bone (Fig. 1b). The large prootic forms the midpoint, the parasphenoid bone bears distinct the anterior part of the otic bullae, with the foramen alar processes as well as a ventral keel (Fig. 1b).The trigemino-facialis at the anterior edge of the bulla otic region is comprised of the sphenotics, pterotic, and two pores anterior to this foramen (Fig. 1b). The prootics, parietals and epioccipitals. The sphenotic is prootic possesses a long dorsal projection on its connected to the prootic bone by a bony strut of the anterodorsal corner that runs under the posterior prootic. The sphenotic contributes to form the portion of the frontal. The anterior part of the parietal anterior suspensorial articulatory facet, together with is covered by the frontal bone forming a relatively the prootic bone (Figs. 1b and 2a). The sphenotic extended scarf joint. bone bears

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