Clinal phenotypic variation

Coris julis - a common but interesting species

I started working with this species during my final project for my BSc/MSc and it later became the focal species for my PhD work at the University of Catania (Italy).

Coris julis is a fish commonly found in coastal areas of the Mediterranean Sea and Eastern Atlantic Ocean. It belongs to the family of wrasses (Labridae) and shows a number of interesting characteristics:

  • As typical for wrasses, it is a protogynous hermaphrodite, meaning that normally these fish are female when younger and then later change sex becoming male.
  •  The change in sex is accompanied by change in colour (from a primary/initial colouration through a transition phase leading to a secondary/terminal colouration), as well as in behaviour (terminal males are more aggressive and territorial)
  • The change in sex can be triggered by social factors (lack or paucity of terminal males)
  • Some individuals (primary males) are males but retain the female (primary) colouration so they don’t have to compete with larger terminal males
  • It has a long planktonic larval phase

These characteristics make it an ideal species to study a number of phenomena, including phenotypic variation in geographic space when the potential for genetic connectivity is high.

My contribution

Before my PhD, not much was known about the phenotypic variation of this species across its wide distribution, and very little was known about its genetic variation.

I, then, worked on this species using a various tools, including analyses of neutral genetic variation, as well as advanced geometric morphometrics. The latter include some of the earliest uses of spatially-explicit statistical methods with geometric morphometric data. Even today, only a handful of studies have leveraged these complex but powerful statistical tools in geometric morphometrics.

Main findings to date

  • Coris julis shows an interesting pattern of neutral genetic variation with a discontinuity between Mediterranean Sea and Atlantic Ocean characterised by an intermediate situation in the Alboran Sea. This is unusual, as normally marine species show a discontinuity either at the Gibraltrar Strait or at the Almeria-Oran front.
  • Within the Mediterranean Sea gene flow is extremely high and the whole basin may represent a panmictic unit. Incidentally, this would justify the use of this species as bioindicator for the whole Mediterranean Sea.
  • Contrasting to the neutral genetic homogeneity across the whole Mediterranean, this species shows marked clinal phenotypic variation (gradual change across geographic space)
  • Surprisingly, the main directions of phenotypic change in geographic space (clines) are markedly different between primary (females) and secondary (males) fish. Primary fish are also more variable across geographic space compared to secondary fish
  • Even though fish acquire a more “masculine" body shape as they become larger (and therefore primary and secondary individuals normally exhibit different shapes), there is no sharp change in body shape at the sex/colour change.

Selected papers

Coris julis in its natural environment. Top: initial/primary coloration (usually female). Bottom: terminal/secondary coloration (male). Photo credits: top, Stefano Guerrieri; bottom, Carmelo Fruciano.

Contrasting phenotypic clines in Mediterranean Coris julis. Arrows depict the main directions of the clines for primary (A, solid arrow) and secondary (B, dashed arrow) individuals.