Scanning electron microscopic observations on the head morphology of seven different leptocephali belonging to six eel families (Anguilliformes)

Helgoland Marine Research, Feb 1993

Scanning electron microscopy observation was carried out on head morphology, number and construction of teeth, nostril position and the presence of neuromasts of seven species of leptocephali from six families of the Anguilliformes. The number and form of the teeth vary within the different families, from 76 teeth inGnathophis sp. (Congridae) to 32 teeth inAnarchias yoshiae (Muraenidae). In most of the genera the number of teeth on upper and lower jaws is almost equal, except inGnathophis sp.,Anguilla anguilla andKaupichthys hyoproroides. The unusual size and form of the leptocephali's teeth led us to speculate on the feeding behaviour of these larvae. Tentatively we suggest that the tooth formation of the leptocephali may act as a filtering system, rather than as a predatory apparatus, as its vicious appearance might suggest. Given the largely passive nature of a filtering system, the larvae would be able to concentrate on their migration. Of all the investigated leptocephali, onlyNemichthys scolopaceus shows one olfactory opening on each side of the head, whilst all the other species show two openings. The data obtained from these investigations suggest that teeth form as well as form and position of the nostrils are characteristics of the genera, and possibly also of the species.

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Scanning electron microscopic observations on the head morphology of seven different leptocephali belonging to six eel families (Anguilliformes)

HELGOL.~NDER MEERESUNTERSUCHUNGEN Helgol~nder Meeresunters. 0 9 Biologische Anstalt Helgoland , Hamburg 1 i Jacob Blaustein Institute for Desert Research, Ben-Gufion University of the Negev; Sede Boker - 84993, Israel 2 University of British Columbia, Department of Animal Sciences, Faculty of Agricultural Sciences; Vancouver , B. C., V6T 2A2 , Canada 2 3Institut fiir Zoologie (Morphologie & Zeflbiologie) der Universit~t Dfisseldorf; D-W-4000 Dfisseldorf , Federal Republic of Germany Scanning electron microscopy observation was carried out on h e a d morphology, n u m b e r and construction of teeth, nostril position and the presence of neuromasts of seven species of leptocephah from six families of the Anguilliformes. The n u m b e r and form of the teeth vary within the different families, from 76 teeth in Gnathophis sp. (Congridae) to 32 teeth in Anarchias yoshiae (Muraenidae). In most of the genera the n u m b e r of teeth on upper and lower jaws is almost equal, except in Gnathophis sp., Anguflla anguilla and Kaupichthys hyoproroides. The unusual size and form of the leptocephali's teeth led us to speculate on the feeding behaviour of these larvae. Tentatively we suggest that the tooth formation of the leptocephali may act as a filtering system, rather than as a predatory apparatus, as its vicious appearance might suggest. Given the largely passive nature of a filtering system, the larvae would be able to concentrate on their migration. Of all the investigated leptocephali, only Nemichthys scolopaceus shows one olfactory opening on each side of the head, whilst all the other species show two openings. The data obtained from these investigations suggest that teeth form as well as form and position of the nostrils are characteristics of the genera, and possibly also of the species. - I N T R O D U C T I O N " Present address for all correspondence c a u g h t in the o c e a n d u r i n g y e a r s of investigation, none h a v e shown signs of food items in the digestive tract of l e p t o c e p h a l i (Kracht & Tesch, 1981; Kracht, 1982; Pfeiler, 1986; Brusl~, 1989) . S u g g e s t i o n s have b e e n m a d e on the food a n d f e e d i n g h a b i t s of l e p t o c e p hali. Hulet (1978), Pfeiler (1986) and, more recently, M o c h i o k a & I w a m i z u (Paper p r e s e n t e d at "Eifac/FAO W o r k i n g Party on Eels", in Porto, 1989) a n d W e s t e r b e r g (1990); the latter h y p o t h e s i z e d that d i s c h a r g e d a p p e n d i c u l a r i a houses are t h e m a i n food of leptocephali. However, this question still r e m a i n s u n a n s w e r e d . Our p r e s e n t a t i o n is an a d d i t i o n a l step t o w a r d s solving the m y s t e r y of l e p t o c e p h a l i food a n d f e e d i n g . MATERIAL AND M E T H O D S Two l e p t o c e p h a l i of A. anguilla, m e a s u r i n g 65 m m a n d 73 m m total length, w e r e e x a m i n e d . T h e y w e r e c a u g h t on 25 O c t o b e r 1991, using IKMT, in the Bay of Biscay a n d off the I b e r i a n c o n t i n e n t a l shelf, at a d e p t h of 1500-2000 m. T h e h e a d w a s s h o r t a n d v e r y stocky s h o w i n g l a r g e e y e s with a d i a m e t e r of 1.1 m m (Fig. 1). The e y e i n d e x w a s 2.8. In Figs 1-2. Anguflla anguilla (Anguillidae). 1: Head with nostrils. Note the front pair of teeth in the upper jaw (25:1). 2: Higher magnification of the round teeth of the upper jaw (95:1) Figs 3-4. Kaupichthyshyoproroides (Xenocongridae). 3: Head with ingestion and egestion openings extremely far apart (23:1). 4: Front part of the head with round teeth (55:1) A-C: neuromasts; E = eye; arrowhead = ingestion opening; arrow = egestion opening A K. hyoproroides leptocephalus measuring 51 m m in total length was studied. The head was conical and elongated, exhibiting round, large eyes with a diameter of 0.78 m m (Fig. 3). The e y e i n d e x was 4.1. The m o u t h of K. hyoproroides h a d an u n u s u a l shape; while at the posterior part the jaws could a p p r o a c h e a c h other; at the m o r e anterior part t h e y r e m a i n e d apart. In the anterior part of both jaws the t r i a n g u l a r t e e t h , some of w h i c h w e r e interlocking, p r o t r u d e d (Fig. 3). In the posterior part of the mouth, only the u p p e r j a w was p r o v i d e d with n u m e r o u s small, c y h n d r o - c o n i c a l teeth (Figs 3, 4). In total, sixtytwo teeth w e r e counted, but only 16 occurred in the l o w e r jaw, all the o t h e r t e e t h w e r e in the u p p e r jaw. Each side of the h e a d e x h i b i t e d two nostrils w h i c h w e r e s i t u a t e d close to the j a w e d g e , directly a b o v e the hps of the u p p e r j a w (Fig. 4). The i n g e s t i o n a n d e g e s t i o n o p e n i n g s w e r e p o s i t i o n e d e x t r e m e l y far from e a c h other. The a n t e r i o r o p e n i n g w a s l o c a t e d almost on the tip of the snout, s h o w i n g a skinfold, w h e r e a s the e (...truncated)


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S. Appelbaum, R. Riehl. Scanning electron microscopic observations on the head morphology of seven different leptocephali belonging to six eel families (Anguilliformes), Helgoland Marine Research, 1993, pp. 113-124, Volume 47, Issue 1, DOI: 10.1007/BF02366187