A large body size or the ability to raft on flotsam across an ocean may help tropical fish spread to new homes.

Reef fish species that occur on both sides of known oceanographic dispersal barriers have special features that have enabled them to cross these
obstacles. An international team of scientists have analysed 985 tropical fish species across two major Atlantic marine barriers and identified
various success predictors, such as adult size and/or living in a variety of habitats.

Researchers have previously assumed that major variations in larval biology, such as the duration of the larval phase, were the main determinants
of species dispersal. However, a new study suggests that the traits in adult fish may play an important role.

“In our analyses, variation in larval-development mode, which is highly correlated with the time larvae spend as plankton, wasn’t a significant
factor and adult traits have explained better the dispersal potential for those fishes,” said Macquarie University PhD student Osmar Luiz, lead
author of the research published in Proceedings of the Royal Society: B.

Luiz said that the methods used by the fish may be dependent on the nature of the oceanographic barrier. Fish that are able to live amongst the
flotsam in the ocean stand a better chance of crossing the mid-Atlantic barrier- the large body of water between the Africa and American
continents.

Other reef species that can also survive in sandy or muddy habitats, may live within coastal barriers such as the Amazon-Orinoco Plume. This
ability will allow them to eventually cross the barrier through a ‘stepping stone’ effect.

“Other features, like possessing a large body-size, may help species after crossing the barrier by enhancing their chance to establish a resident
population in the new place, and indeed large size was detected as an important trait for both barriers,” said Luiz.

The researchers have also been looking at how these traits may help fish travel from the Great Barrier Reef to the Sydney area. Luiz said that at
least 50 species from the Great Barrier Reef have been spotted in Sydney Harbour the last years.

The tropical species usually arrive when temperatures increase in summer and die when they decrease in winter. “However, because the sea’s
lower temperature limits have been increasing due to climate warming, a few species were detected surviving through winter.”

Climate change is predicted to influence species dispersal, including that of reef species. Luiz said more tropical fish may appear in the Sydney
area if the current warming trend continues.