SCOTTISH researchers have uncovered evidence that the biggest fish ever to have swum the seas grew to 16.5 metres in length.

Studying the skeletal remains of the prehistoric created that lived about 165 million years ago, researchers from National Museums Scotland and the Universities of Glasgow and Edinburgh deduced the Leedsichthys could grow to a hefty size over its average 38-year lifespan.

Archaeologists have until now struggled to establish information about the bony, plankton-eating fish because the remains haven't been well-preserved.

The experts believe their findings reveal an important missing piece in the evolutionary story of fish, mammals and ocean ecosystems.

Professor Jeff Liston of National Museums Scotland led the team of experts from the University of Glasgow's SCENE (Scottish Centre for Ecology and the Natural Environment), the Grant Institute of the University of Edinburgh and the Royal Tyrrell Museum of Alberta, Canada.

He said: "The giant plankton-feeders we know to live in today's oceans are among the largest living vertebrate animals alive. The Leedsichthys was the first animal known to occupy this role.

The team also collaborated with the Royal Tyrrell Museum of Alberta, Canada.

Professor Liston said: "What we have demonstrated here is that a small adult Leedsichthys of eight or nine metres could reach that length within around 20 years, whereas after 38 years it would be around 16.5 metres long - possibly even outgrowing today's massive whale sharks.

"This fish was a pioneer for the ecological niche filled today by mammals, like blue whales, and cartilaginous fish, such as manta rays, basking sharks and whale sharks.

"The existence of these large suspension-feeding fish at this time is highly significant, as it would seem to be clear evidence of a major change in plankton populations in Earth's oceans of Jurassic Earth."

The results are published in Mesozoic Fishes 5: Global Diversity and Evolution - Proceedings of the International Meeting.