Les larves
leptocéphales, ces méconnues
Raymonde LECOMTE-FINIGER,
Caroline MAUNIER & Mehdi KHAFIF (1)
(1) Laboratoire
d’Ichtyoécologie tropicale et méditerranéenne,
UMR 8046 CNRS-EPHE, Université de Perpignan, 52 avenue Paul Alduy,
66860 Perpignan cedex, FRANCE.
Résumé.
- Le bilan des connaissances acquises sur la morphologie, l’anatomie,
la croissance et la métamorphose de la larve leptocéphale
est dressé par les auteurs. L’une des principales caractéristiques
de ces larves est la présence d’une matrice acellulaire gélatineuse.
Une réserve énergétique importante est incluse dans
ces tissus sous forme de glycosaminoglycanes. Celle-ci est principalement
utilisée lors de la métamorphose qui va transformer radicalement
la larve leptocéphale en un individu morphologiquement très
différent. Chez les Anguilliformes, le leptocéphale en forme
de feuille se transforme en civelle cylindrique. La phase larvaire leptocéphale,
d’une assez longue durée par rapport à la phase larvaire
des autres poissons, facilite leur dispersion. Leur signification évolutive
est abordée.
Abstract. - Leptocephali, these unappreciated
larvae.
The authors present a review of the actual state of knowledge on leptocephalus
larvae: morphology, anatomy, growth, metamorphosis. One of the most interesting
characters is the presence of glycosaminoglycans (GAGs) stored in a gelatinous
matrix in the body and used during the non-trophic phase of the metamorphosis.
During the metamorphosis, body shape and internal organs are rapidly transformed.
The leptocephalus larva is one of the most distinctive characters of anguilliform
fishes. It presents a willow-leaf shape. The size of larvae is variable
among the species, ranging from 50 to 100 mm. Morphological characters
are restricted to the total myomeres, anus position, body depth ratio,
dorsal fin position, eye and gut as a simple straight tubular structure.
They are difficult to identify.
The gut is poorly differentiated. Their diet is poorly known and two main
hypothesis are discussed. The nutrition is ensured through epidermal uptake
of dissolved organic matter. Recent studies of the gut content, suggested
that larvae feed on detritical materials as larvacean houses and fecal
pellets. They possess a high level of carbohydrates (GAGs) and lipids
stored in a gelatinous matrix. They present a high water content (90%)
that contributes to their buoyancy and a very low metabolic rate. Ionic
composition is close to the equilibrium with sea water. They present a
long larval stage. When they reach the metamorphosis size they change
into glass eel. Growth stops and body length decreases. They stop feeding.
Shrinkage is related to the breakdown of GAGs in the gelatinous matrix.
Internal tissues and organs are completed during the metamorphosis (muscle,
digest tract, ossification,…). Retina changes in rods and cones.
All these changes suggest a behavioral change from pelagic to benthic
ecology. Artificial rearing and development of larvae had been for a long
time unsuccessful, because the diet was not identified. Recently, larvae
were fed on shark-egg powder for more than 200 days.
The biological significance of this leptocephale stage followed by the
metamorphosis reflects an extreme adaptation that ensures dispersal of
the individuals.
Key words. - Anguilliformes
- Leptocephali - Morphology - Anatomy - Growth - Metamorphosis.
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