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«Preliminary study on the ecomorphological signification of the sound-producing complex in Carapidae Eric Parmentier * & Michel Chardon & Pierre ...»

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Topics in Functional and Ecological Vertebrate Morphology, pp. 139-151.

P. Aerts, K. D’Août, A. Herrel & R. Van Damme, Eds.

© Shaker Publishing 2002, ISBN 90-423-0204-6

Preliminary study on the ecomorphological signification

of the sound-producing complex in Carapidae

Eric Parmentier * & Michel Chardon & Pierre Vandewalle

Laboratoire de Morphologie fonctionnelle et évolutive, Université de Liège, Belgium

Abstract

Carapidae can be classified in four ecological groups : pelagic, dermersal, commensal and parasitic.

Carapidae display otophysic structures associated with the anterior part of the swim bladder and highly modified labyrinths, which suggest particular acoustic performances. The commensal and parasitic species have the best developed sound-producing features and also the thickest sagitta within the largest otic cavity, and surrounded by the thinnest cranial wall. However, these features do not necessarily imply a direct relation between the sound emission and reception in a given species but suggest a selective pressure lying in the habitat use of the species. The structures involved in sound-production and hearing are seemingly adapted to match the loss of energy of the sonic vibrations when travelling through the host tissues.

Key words: ecomorphology, sound apparatus, ear, Carapidae.

Introduction The aim of ecomorphological studies is to reveal and understand possible relationships between organism morphology and its way of life (Norton et al. 1995).

Ecomorpholical studies on fishes often focus on the relation between one morphological trait and one ecological feature : for example, buccal apparatus morphology and diet (Clifton and Motta, 1998; Kotrschal, 1989), digestive tract length and diet (Veregina, 1991), body shape and fin position, and habitat (Webb, 1988 ; Belwood and Wainwright, 2001). Ideally, the study of the relations between the ecological characteristics of a species and the particularities of one of its morphological systems requires firstly a complete knowledge of all the biological roles the morphological system fulfills in the natural environment of the studied species. In other words, the mechanical performance of the system interacting with the environment should thus be examined. Secondly, a comparison with related species living in different ecological conditions is essential to allow an evaluation of the potential advantages of the morphological character complex in a given environment. Moreover, an organism does not consist of one single morphological complex, but of many such complexes, interacting to improve the fitness of the individual in given ecological conditions (i.e. adaptation to the many components of the ecological niche; Goldschmid and Kotrschal, 1989). The morphology of a species consequently appears as an assembly of functional characters interacting with environmental factors.

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In carapid fishes (Paracanthopterygii: Ophidiiformes), two ecomorphological studies, each focusing on a different functional complex, have been performed. The first one aimed to understand the relations between diet and cephalic morphology in a comparitive study (Parmentier et al., 2000 ;

Parmentier and Vandewalle, in press). The second one focussed on the links between internal ear (size and shape of sagittae, labyrinths and otic region of the skull) and habitat use in different species (Parmentier et al., 2001).

Carapidae display spectacular specializations of the anterior vertebrae, swim bladder and associated muscles which are generally interpreted as a sound-producing device (Courtenay and Mc Kittrick, 1970 ; Markle and Olney, 1990 ; Parmentier et al., 2000). Additionally, highly modified labyrinths are present which suggest an improved acoustic performance (Parmentier et al., 2001).

The present study is a comparison of the sound-emitting and sound receptor apparatuses in different species of Carapidae, including representatives of the entire spectrum of habitat use in this family. The sound-producing structures in the Carapidae are described in detail, and used to investigate pontential ecomorphological relationships between habitat use and the morphology of the sound producing and receiving apparatusses.

Materials and methods

10 specimens of Carapus boraborensis (TL: 13 to 30 cm), 5 specimens Carapus homei (TL: 8 to 17 cm) and 3 specimens Encheliophis gracilis (TL: 16 to 24 cm) were collected by the authors in Opunohu bay, Moorea, French Polynesia. These specimens were found in three holothurians: Bohadschia argus, Thelenota ananas and Thelenota anax. Eight adults of Onuxodon fowleri (TL: 57-90 mm) were collected in Hansa Bay (Bismarck sea) in Papua New Guinea. They were found in specimens of Pinctada margaritiferae (Lamellibranch). Three specimens of Echiodon drummondi (TL: 20 cm) were collected from the North Sea. All fishes were fixed in formalin and stored in 70% ethanol for further analyses.

Specimens of other species were gifts or loans: Snyderidia canina n°9669 (TL: 17 cm, University of

Kyoto,) Pyramodon lindas I. 25804-018 (TL: 23 cm, Australian Museum), Pyramodon punctatus:

I.29744001 (TL: 240 mm, Australian Museum), Echiodon cryomargarites: NMNZ P25406 (TL: 215 mm, National Museum of New Zealand), Echiodon exsilium SIO 65-292 (TL: 118 mm, Scripps Institution of Oceanography, California), Echiodon rendhali: IB.4353 (TL: 106 mm, Australian Museum).





The fishes were dissected and examined using Wild M10 binocular equipped with a camera lucida and camera (Leica Camera AG).

Results

Ecology Parmentier and Vandewalle (in press) consider the seven genera of Carapidae as belonging to four ecological categories (EC). The genera Snyderidia and Pyramodon are pelagic (EC1 -Trott, 1970).

Echiodon and Eurypleuron are demersal (EC2 - Fries et al., 1893 ; Trott, 1970 ; Williams, 1984 ;

Nielsen et al., 1999). Onuxodon and the members of the tribe Carapini (Carapus and Encheliophis) are able to penetrate into different invertebrates such as sea cucumbers (Smith, 1964 ; Smith and Tyler, 1969 ; Shen and Yeh, 1987 ; Jangoux, 1990), sea stars (Meyer-Rochow, 1977, 1979), bivalve molluscs (Tyler, 1970 ; Machida, 1989 ; Castro-Aguirre et al., 1996 ; Paredes-Rios and Balart, 1999) and ascidians (Weber, 1913). Species of the genera Onuxodon and Carapus seem to be commensals (EC3), and those of the genus Encheliophis are parasites (EC4 - Parmentier et al., 2000).

Ecomorphology of the sound-producing apparatus in Carapidae 141 Data about the habitat use and lifestyle of Echiodon drummondi (demersal; North Sea), Carapus boraborensis, Carapus homei, Carapus mourlani (commensal; Madagascar, Moorea), Carapus acus (commensal; Calvi, Corsica) and Encheliophis gracilis (parasitic; Madagascar, Moorea) were confirmed by recent personal observations.

Figure 1. Lateral view of the first vertebrae with the associated epipleural ribs and swim bladder plate in five species of Carapidae.

Sound-production apparatus In all the Carapidae, the 'bauplan' of the presumed sonic apparatus displays common characteristics.

The first two vertebrae display epipleural ribs that are movable in all directions (Fig.1) and linked to the swim bladder by a ligament inserting at their distal end (Fig.2). The third vertebra bears paired, broad, ossified plates resulting from the fusion of the third epipleural rib with an ossified area of the swim bladder wall : the "swim bladder plates" (Fig.1). The parapophyses of the following vertebrae bear no or indistinct epipleural ribs (Markle and Olney, 1990). The almost 142 Parmentier et al.

cylindrical swim bladder may be divided into three regions (Fig.2); the anterior regions (1 and 2) are dilated anteriorly and the posterior region (3) is strongly compressed against the abdominal vertebrae. The wall of the second region is situated just under the swim bladder plate and is thinner;

it is the "swim bladder fenestra" (name coined by Howes, 1992, for ophiidiforms). Paired "primary sonic muscles" (coined by Courtenay and Mc Kittrick, 1970) run from the anterior end of the swim bladder (region 1), in front of the swim bladder fenestra, up to the upper wall of the orbit. Two pairs of secondary sonic muscles join the first two epipleural ribs with the epiotics.

Figure 2. At the same scale, dorsal views of the anterior part of the swim bladder in Echiodon cryomargarites (A), Encheliophis gracilis (B), Carapus boraborensis (C) and Snyderidia canina (D).

The total lengths of the swim bladder are respectively 35mm, 30mm, 20,5mm and 13,75mm. The swim bladder regions are shown in Carapus boraborensis.

Intergeneric specializations concern the shape and size of the epipleural ribs, the swim bladder plate and fenestra, and the lateral elements of the 4th vertebra.

1. In the three studied Pyramodontinae (Pyramodon lindas, Pyramodon punctatus and Snyderidia canina), the 3rd epipleural ribs are the broadest and the thickest ones. In S. canina, the dorsal ossification of the swim bladder forms one single plate fused anteriorly to the 3rd epipleural ribs and with the ventral part of the 3rd vertebra. In Pyramodon lindas and Pyramodon puncatus, the dorsal ossification is also fused to the ventral part of the 4th vertebra. In the latter three species, the 4th centrum bears parapophyses surmounting the swim bladder plate.

2. In the genus Echiodon, the swim bladder plate is concave and principally composed of a thin anterior ossification of the swim bladder wall followed by the epipleural ribs of the 3rd vertebra (Fig.1). Specific differences are as follows : Echiodon cryomargarites and Echiodon drummondi possess epipleural ribs on the first three vertebrae. Echiodon rendhali and Echiodon exsilium bear 4 and 5 pairs of epipleural rib respectively. In Echiodon exsilium and Echiodon drummondi, the ligaments of the first two epipleural rib pairs insert laterally onto the swim bladder, in front of the small swim bladder fenestra. In Echiodon cryomargarites, the ligaments of the epipleural ribs 1 and 2 fuse with each other at the level of an antero-lateral fibrous area (Fig. 2A) (Markle and Olney, 1990). In Echiodon rendhali, the ligaments of the first epipleural ribs are well separated from the ligaments of the second epipleural ribs; the latter ligament inserting laterally on the swim bladder. The ligaments of the two first pairs of epipleural ribs reach the same insertion in the middle of the anterior face of the swim bladder.

Ecomorphology of the sound-producing apparatus in Carapidae 143

3. In Carapini, the swim bladder plate is proportionally the broadest and it is characterized by its continuous convexity. It is very difficult to observe a border between the 3rd epipleural rib and the swim bladder wall ossification. The 4th vertebra bears a pair of short epipleural ribs above the swim bladder plate to which they are attached by connective tissue fibres. In the parasitic species Encheliophis gracilis, the swim bladder plate is the most developed and reachs the 6th vertebra (Fig.

1). In all the Carapini, the ligaments of the epipleural ribs 1 and 2 insert on the left and the right in front of the swim bladder fenestra (Fig. 2B).

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4. The sonic apparatus of Onuxodon margaritiferae and Onuxodon parvibrachium was described by Courtenay and Mc Kittrick (1970). It is very similar in Onuxodon forwleri as examined in the present study (Fig. 3). The swim bladder is a more or less regular cylinder and the swim bladder fenestra constitutes its anterior face. The swim bladder plate is wedge-shaped. The distal parts of the swim bladder plates lean against the swim bladder fenestra. The anterior part of left and right wings forming the swim bladder plate are bent anteriorly (Fig.4 : Me SBP) and delimit a longitudinal corridor closed posteriorly by the swim bladder fenestra (Fig. 4A, C). That latter corridor is filled by the so-called "rocker bone" (name coined by Courtenay and Mc Kittrick, 1970), which is probably not a bone, since it does not stain in an alizarin solution; its exact nature remains to be determined. The posterior face of the rocker bone displays a knob imbedded in the tissue of the swim bladder fenestra tunica. The primary sonic muscles insert by means of a long tendon onto the knob, and not on the swim bladder wall. The ligaments of the first epipleural ribs attach on the left and right faces of the rocker bone, under the knob and in front of the primary sonic muscles. The position of the ligaments provides a perpendicular axis of support for the rocker bone. The two ligaments of the second epipleural ribs are thicker than in the other species in the present study and 144 Parmentier et al.

insert on the lateral faces of the swim bladder plate. The swim bladder wall is generally thinner than in the other Carapidae studied.

In all Carapidae but Onuxodon, rhythmic contraction of the sonic muscles seems to result in a forward movement of the part of the swim bladder anterior to the fenestra and hence, tension and vibrations in the thin area. The secondary sonic muscles could modulate the swim bladder volume via the ligaments of the epipleural ribs 1 and 2. In Onuxodon, contraction of the sonic muscles moves the rocker bone forward. This causes the latter to swing around the vertical bissector of the ligaments of the first epipleural ribs and moves the knob and the swim bladder fenestra forward.

Sound could potentially result from vibration of the swim bladder thin area of the fenestra when the knob moves forward.

Figure 4. Onuxodon fowleri.

A. Ventral view of the sound producing complex (without the muscles); B. front view of the swim bladder plate and the swim bladder fenestra; C.

Schematic frontal view according to the dotted line in the figure 2.

Sound reception organ Part of the following results were recently published by Parmentier et al. (2001). The internal ear in Carapidae (Fig.5) is fundamentally organized as in generalized teleosts (e.g. Platt and Popper, 1981; Popper, 1982), but differences in shape and proportions of the parts are considerable.



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