Friday, 19 April 2013

Evolution trumps ecological influence in deep-sea fish

AN INVESTIGATION by UWA scientists into factors which affect eye size in certain deep-sea fish has found that eye size varies based on the individual fish’s evolutionary status, rather than ecological pressures.

Researchers at the UWA Oceans Institute studied how types of Lanternfish (Myctophidae) have visually adapted to navigating the ocean’s mesopelagic zone (200m-1000m) where sunlight diminishes exponentially with depth.

The mesopelagic region or Twilight Zone has the greatest density of bioluminescent emissions, or light emitted by the animals themselves, which they use to interact with prey and predators.

They examined 237 members of the Lanternfish family spanning 61 different species, with eye and lens diameters ranging from 0.6mm to 11.1mm, and were collected from research cruises in different parts of the world including in the Coral Sea.

UWA PhD student Fanny de Busserolles says while in the field she had noticed significant differences in eye size compared to body size between different Lanternfish species and wanted to investigate the difference.

“Knowing that a larger eye will be more sensitive we thought there might be a connection between the amount of light present in the environment [sunlight or bioluminescence] and the relative eye size of a species,” she says.

“We hypothesised that species with a deeper distribution in the mesopelagic zone and/or a reduction of bioluminescent emissions will have smaller eyes, and that ecological factors rather than the evolutionary history of the family will drive the evolution of their visual system.”

The study found there was great diversity in eye size among Lanternfish at all taxonomic levels including the subfamily, tribes and genus.

But contrary to their expectations the team did not find a link between the eye size and the depth/luminous tissue patterns on the fish identified.

“A strong phylogenetic signal was observed for the eye size variable suggesting that this characteristic may be mainly influenced by the evolutionary history of the family instead of the ecological factors as we expected,” Ms Busserolles says.

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