Ancient Fish Had Sophisticated Electrical Detectors

CT scan of a sensory organ in a 400 million-year-old lungfish skull. Blue areas detected pressure changes in the water, while red areas detected electric fields. King et al/Palaeontology

Around 400 million years ago, when even having a spine was a fairly newfangled development, some fish had evolved electroreception organs, giving them the capacity to detect electric fields as sharks do today.

It is unsurprisingly difficult to study the senses of long-dead animals, but Benedict King and Professor John Long of Flinders University, Australia, have made remarkable progress. The astonishing Gogo Formation preserved fish from the Devonian Era so perfectly they have been described as “swimming in stone”. By applying micro-CT scanners to reveal every tiny bump and pit on Gogo fish, King and Long have shown that some of these fishes had organs so similar to those seen in their descendants that the function must have been very similar. Meanwhile other fish from the same era have an unusual organ whose use we can only suspect.

The studies, along with those from another site dating to a similar time on the other side of Australia, have been published in Palaeontology.

“To date, we do not know the origin of electroreception, which is a major sensory system in living vertebrates, but this paper represents a groundbreaking study giving the first detailed overview of electrosensory systems in ancient fish,” Long said in a statement.

The Young's apparatus in two ancient fish. Scale bars represent 5 mm (0.2 inches) (A) and 20 mm (0.8 inches) (B). King et al/Palaeontology

Long explained to IFLScience that Devonian lungfish have pores on their snouts very similar to their modern descendants. While some of these pores resemble those used today to detect movements and pressure gradients in the water, others, Long said; “match the location and morphology of electroreceptors in a similar group of living fish, so we're pretty confident.” Similarly, organs in early ray-finned fish, which are now easily the most common fish category, resemble those found in some of their descendants.

As interested as the authors are in discovering the earliest examples of these sophisticated detectors, they're also intrigued by a new organ they found in some placoderms from the same sites. Placoderms were heavily armored bony fish that once dominated the seas but died out 360 million years ago. King and Long found some had pits on their cheeks that appear to have had a sensory role. They called this “Young's apparatus”, but its exact nature cannot be confirmed. Long told IFLScience he and his colleagues suspect it was also an electroreceptor, but as yet nothing can be confirmed.

As appealing as we might find the idea of an additional sense, there must have been a considerable price to pay in energy or brain space because most fish gave it up, along with most land animals. (As always the platypus is an exception, along with some amphibians.) Long noted electroreceptors are really only used in niche environments today, such as when hunting for prey buried in sand.

The capacity to detect electric fields has been lost many times in evolutionary history. King et al. Palaeontology

 

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