The Pacific leaping blenny is a remarkable land dwelling fish. Males of the species have impressive head crests, red coloured dorsal fins and ‘dance’ to attract their mates. But males vary in how big their head crests are and how bright their dorsal fins are - why? Our research found that how ‘showy’ males were depended on how much competition and predation there was. Differences between populations in how they display their features is thought to be an important factor in the evolution of new species.

The Pacific leaping blenny (Alticus arnoldorum) can be found living in the rocky intertidal area around the Micronesian Island of Guam. Males can be identified easily as they have a head crest, used in addition to the dorsal fin for sexual displays. When attempting to attract females, males ‘dance’, bobbing their head, shaking their body, and erecting the dorsal fin to flash its bright red colouration that is easy to spot against the grey background of the rocks.

To investigate the reasons why these flashy signals differed between populations, Terry Ord from the University of New South Wales, Georgina Cooke from the Australian Museum Research Institute and I traveled to Guam.

At five different field sites we spent many hours catching blennies and photographing them before releasing them so that I could take measurements of their body length, fin area, intensity of red colouration on the dorsal fin and the size of male head crests. We also took a small tissue sample and extracted the DNA so that we could examine how related each of the populations of these unusual land fish were to each other.

This was a particularly challenging task as the blennies are well equipped to leap about on land and out of nets. During these missions it was always best to prepare for the unexpected, a lesson I learnt the hard way after following blennies under rock ledges, thinking I had them cornered, only to be met with the gleaming red eyes of rock crabs centimeters from my face on more than one occasion.

In addition to measuring the variation in appearance between populations, we also looked at possible reasons for these differences in blenny - were blennies more showy when males outnumbered the females, or did they tone themselves down if predators such as birds were abundant?

Many hours were spent counting blennies hopping in and around the rock platform. To ensure that I didn't frighten the blennies or change their behavior, I would have to sit perfectly still for 10 minutes before counting began. It was during these times that the rock pools came alive with eels hunting, and carefully camouflaged frog and scorpion fish finding new hiding places.

To look at predation, we made blenny models out of plasticine. Models were left out for several days in the rocky habitat and then we examined them for predation attempts (bite marks!) by birds, crabs and lizards. We also looked at how wary they were of potential predators by measuring how close I could get to a blenny before it would leap away (provided I didn't trip over on approach, which was more easily said than done).

We found that in blenny populations where competition between males for females was more intense, males had larger head crests, and in populations that experienced higher levels of predation, males had duller dorsal fins.

Whilst elaborate and exaggerated colours, dances and ornaments might be a hit with the ladies, they are also easily spotted from afar by hungry predators. It therefore makes sense that when there is a lot of competition, male blennies throw caution to the wind when attracting females. However, in places crawling with predators, it might pay to be a little more understated in their advances.

Most excitingly, we found that while blenny populations could look really different from each other, they were very similar genetically. This means that physical differences are either a recent occurrence or individuals are able to respond quickly to the changing nature of their environment.

Courtney Morgans
Masters Student, Evolution and Ecology Research Centre, University of New South Wales.

More information:

Morgans, C.L., Cooke, G.M, & Ord, T.J. (2014) How populations differentiate despite gene flow: sexual and natural selection drive phenotypic divergence within a land fish, the Pacific leaping blenny. BMC Evolutionary Biology 2014, 14:97 doi:10.1186/1471-2148-14-97.

This research is a collaboration between the University of New South Wales and the Australian Museum Research Institute.