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A group worthy of interest for the team: the Pleuronectiformes


Two pleuronectiformes or flatfishes

a Klein's sole a topknot
A Klein's sole (Synapturichthys kleinii, Soleidae).
Image: G. Dallavalle.
A topknot (Zeugopterus punctatus, Scophthalmidae).
Image: B. Chanet.


The study of the evolution of flatfishes, Order Pleuronectiformes, is one of the favourite subjects of several members of the team. This work is in keeping with the history of the Muséum national d'Histoire naturelle (MNHN) and the diversity of approaches and results on this taxonomic group.



Paul Chabanaud (1876-1959), In Dolfus (1960).The first scientific works dedicated to the study of the evolution of this group at the MNHN are those of Paul Chabanaud (1876-1959) when he worked at the laboratory of Fisheries and Colonial Productions of animal origin (laboratoire des Pêches et Productions Coloniales d’origine animale) from 1926 to 1959, in the same building where most of the members of the team work today.

He was the world expert of Pleuronectiformes at that time. He published alone about 170 studies about the systematics, geographical distribution, anatomy and palaeontology of flatfishes. His articles and books are essential to anyone interested in the systematics and the Alpha-taxonomy of these animals. However his style, his vocabulary, his constant practice of defining new terms which he was the only one to use and to describe again already named structures... makes his works a painful journey for a XXIth century reader, even a native French-speaker.

An illustration of Chabanaud's style:

« Le complexe uroptérygiophore est épaxonalement diplospondylique et hypaxonalement triplospondylique.»

Chabanaud (1938).

Translation:

[The skeleton supporting the caudal fin is dorsally organised around two vertebrae, while it is ventrally organised around three vertebrae].



Faced with that problem, two members of the team wrote a bilingual glossary (glossaire) of the terms used by Paul Chabanaud in order to have the works of this author read, understood and studied, especially by English-speaking people.

Paul Chabanaud's style had locked the study of flatishes systematics until Martine Desoutter in 1980 began to work on the systematics of these acanthomorphs.



Meanwhile, a paleontologist of the Museum, Jacques Blot (1933-1988), suggested a scenario about the origin of flatfishes: they were the result of a lateral rotation of an animal destabilized by a very high and thin body. Through this explanation, this author made flatfishes descend from batfishes (Ephippidae) or Monodactylidae. This hypothesis has been totally squashed by the results of molecular studies (Li et al., 2009 [1]), where ephippids and monodactylids appear as only distant relatives of flatfishes.



Since 1992, Bruno Chanet has been working on these animals within the MNHN, by studying fossils and determining their position in the pleuronectiform tree on the sole basis of synapomorphies.

Microchirus abropteryx, Soleidae
Microchirus abropteryx, Soleidae. Specimen from the Upper Miocene (5-6 million years ) from Oran (Algeria)
and conserved in the Laboratoire de Paléontologie du MNHN, ref. MNHN-1246 (Chanet, 1997 [2]).
Image: B. Chanet.
 


Later, in collaboration with Belgian and Canadian teams, the members of the team worked on:

  • alpha-systematics
Urohyal de Solea aegyptiaca (soléidés)
Urohyal of Solea aegyptiaca, Soleidae (Vachon et al., 2008 [3]).
Such an urohyal is only present in the species belonging to the genus Solea (Vachon et al., 2008 [3]).
  • ontogeny (study of development)
Caudal endoskeleton of a young turbot
Caudal endoskeleton of a young turbot (Scophthalmus maximus, Scophthalmidae)
after 14 days of development (cartilaginous elements are here stained in blue by Alcyan Blue)
(Chanet et al., 2001 [4]).
Image: F. Wagemans (University of Liège, Belgium).

 
  • comparative anatomy
Intermuscular ligaments of a sand sole
Intermuscular ligaments (shown here by black arrows) in the dorso-abdominal region
of a sand sole (Pegusa lascaris, Soleidae) (Chanet et al., 2004 [5]).
Image: B. Chanet.
 
  • interrelationships
 Arbre de relations de parenté des scophthalmidés
Tree depicting the interrelationships of scophthalmid species (Chanet, 2003 [6]).

More recently, studies based on molecular data with some other members of the team provided elements about the systematic position of flatfishes within acanthomorphs. Works dedicated to cytogenetics of flatfishes are scarce because of the low caryotypic polymorphism within the group and the very small size of the chromosomes of flatfishes (which makes them hard to study).


But, why flatfishes ?

Some flatfishes have an economic importance in fisheries - such as the common sole (Solea solea, Soleidae), the brill (Scophthalmus rhombus, Scophthalmidae) or the plaice (Pleuronectes platessa, Pleuronectidae) - or in aquaculture - such as the turbot (Scophthalmus maximus, Scophthalmidae). They are important in benthic ecosystems and they are ... odd. They look like some portraits painted by Pablo Picasso. Their external asymmetry when adult is a unique feature within the vertebrates !

An adult plaice of Pleuronectidae
An adult plaice (Pleuronectes platessa, Pleuronectidae), front view.
The animal lies here on its left side, with the twoo eyes on its .... right side.
Image: J. Chanet.
 

When adult, common soles or plaice (see former and following images) have two eyes on one of their lateral sides. The eyes are on the right side in these two species, the left one in the topknot (see at the top of this page) or the turbot. They have then a blind side, without eyes, and without coloration, on which they lie on the bottom of the sea (see below):

Blind side of a plaice
Image: J. Chanet

and an ocular side, with the two eyes, coloured and exposed by the animal (see below):

Ocular side of a plaice
Image: J. Chanet.

All the previous three images were taken on the same animal.
 

The more stricking is that this particularity appears as the young flatfish grows. Before its metamorphosis, the larva is symmetrical and lives in the water column. During the metamophosis, one of the two eyes migrates onto the head to occupy its permanent position on the other side ( look at the phenomena in films : here and here).

The ocular migration in a flatfish
The ocular migration in a flatfish (modified from Chanet et Chapleau 2004 [7]).
The pelagic larva becomes benthic and lives on the substrate at the end of the metamorphosis.

These animals interest human populations since a long time ... paintings and carved objects with flatfishes have been discovered in several Magdalenian caves of Spain and South-West of France (Citerne et Chanet, 2005 [8]).

a 20,000 years old representation of flatfish
A 20,000 years old representation of flatfish (Pileta Cave, Andalucia, Spain).

More than 700 species, divided in 13 families are included in the group:

 Arbre des relations de parenté entre familles de poissons plats
Interrelationships tree of flatfish families (Chanet et al., 2004 [5]).
The polarity of the asymetry (eyes on right or left side in adults) is a useful feature for identification, but is irrelevant to understand the evolution of the group.
 

After two centuries of morpho-anatomic studies, recent molecular analyses (Li et al., 2009 [9]), show that pleuronectiform fishes belong to the clade of  Carangimorphes with:

  • jacks (Carangidae): Trachurus trachurus
  • dolphinfishes (Coryphenidae): dolphinfishes
  • barracudas (Sphyrenidae): Sphyraena barracuda
  • archerfishes (Toxotidae): archerfishes
  • swordfishes (Xiphiidae): Xiphias gladius

among few others...

what would it mean ?

First; our classification of the group needs to be reconsidered.

Second; the ocular asymmetry would have appeared twice; in a lineage conducting to psettodids and in an other conducting to all the other flatfishes (=Pleuronectoidei).

It is an old question, already proposed by several authors , likePaul Chabanaud (1876-1959), In Dolfus (1960). Paul Chabanaud , once again! Nevertheless,these ancient views were weakly supported and belonged to a "world of ideas". They were not based on a rigourous analysis of the distribution of characters among species.

In the first half of the XXth century, some authors considered the problem in asking the question "how becaming a flatfish ?" to try to understand the origin of these animals. By this way, each tiny biological similarity was interpretated as a clue, or as a proof of relationship. You can observe that in aquariums wrasses Balland wrass (Labrus bergylta)(Labrus bergylta, labridés) can lay or swim on one side, that was enough to affirm and publish that labrids were the ancestors of flatfishes!

It stayed a long time in the literature ... while a simple visit in an aquarium would have shown that very diverse teleosts (from wrasses to triggerfishes) act this way! Moreover, let's not forget that living beings are classified ont he basis on what they have and not on the basis of what they do ! As long as different animals can dothe same thing with different structures (ex. a sparrow and a mosquito can fly, but each with different structures!).

It is the analysis of the distribution of structures that will enlighten the problem of the orgin of flatfishes. This question is one of the problems that the team of Acanthoweb try to decipher.

All that can be viewed as a marginal question, but solving this kind of items helps to clarify some parts of the Tree of Life and provide elements to understand better one of the most peculiar character existing among acanthomorph species.


References