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What is morphology?

Morphology is the study of the shape and external structures of organisms. Researchers observe, describe and analyse these on different species in order to evaluate the importance and meaning of the variations of shape within a species for taxonomic studies, speciation and adaptation. Sometimes, this variation can be linked to biological traits or to the influence of the environment on the organisms.

turbot (flatfish) Porte-écuelle
A turbot
(Scophthalmus maximus).
A cling fish
(Lepadogaster lepadogaster).
(Pictures: G. Dallavalle)

Many acanthomorphes possess a flattened body, like for instance flatfishes (turbot, Scophthalmidae) or cling fishes (Gobesiocidae). Others have a fusiform body, like mackerels, Scombridae, or mahimahi (also called dolphinfishes), Corphaenidae... The first live on the bottom of the sea, while the others are fast swimmers capable of tremendous accelerations. The body shapes provide information on the biology of these species. These informations are also of interest for palaeontologists, who cannot see how the species they see in fossil form behaved in their environment.

mackerel dolphinfish
A mackerel
(Scomber scombrus).
A dolphinfish
(Coryphaena equiselis).

Wrasses are acanthomorph fishes from a family common on European seashore: the Labridae. They present a high variability in several external features: number of rays and spines on the fins, colour of the body...

Ballan wrasse (Labrus bergylta)One of the activities of the morphologist is to take into consideration the whole polymorphism of morphological features to define species accurately. In Ballan wrasses (Labrus bergylta), between 10 to 12 rays and 18 to 21 spines can be present on the dorsal fin dependending on the individual. Are these differences relevant? Could they justify describing another species?

In some wrasses, the colour of the body can be a real problem for both morphologists and taxonomists, because it depends on the age (juveniles often present a pattern of coloration different from the adults) and the sex. Depending on the species, males and females exhibit different colours cuckow wrasse (Labrus mixtus)and can even display them successively as, in several species, these animals change their gender as they grow... In some species (Ballan wrasse), individuals are first male and then female. Others, like the cuckoo wrasse (Labrus mixtus), are first female and then male.

The morphologist studies these phenomena to define and identify species. He studies the phenotypic plasticity of the features of the organisms.

On the basis of morphological (colour, shape of the scales, presence of a membrane between fins...) and morphometric (vertebra and fin-ray numbers) data, two species of similar and sympatric soles have been identified in the Mediterranean Sea: the common sole (Solea solea) and the Egyptian sole (Solea aegyptiaca). This distinction has been confirmed by both biological data and several molecular studies.

Sometimes the variations first identified by morphologists help to understand genetic evolutionary phenomena.

In the stickleback, Gasterosteus aculeatus , sticklebackindividuals living in open waters present developed pelvic spines (indicated by a * on the picture) whereas bottom-dwelling individuals do not possess such spines. The presence of these elements is interpreted as a defence against open waters predators catching sticklebacks by their belly. A genetic study of the genes implicated in the expression of the character “pelvic spines” shows that they are switched off and not expressed in bottom-dwelling sticklebacks. Without morphological studies to begin with, this mode of genetic inactivation could not have been elucidated.

Studying the morphology of organisms is a first step for studies on the biology of species. It helps to understand their biological traits, their history, their reproduction and the genetic controls involved in apparition and evolution of features.