Two species of highly similar deep-sea octopuses are hard to tell apart — unless you look closely at their “warts,” a new study finds.
Octopuses in the Graneledone genus are pink and pebbly, with trademark bumps on the skin of their mantles — the bulbous body part resembling a head. Taxonomists have traditionally used the number of warts to differentiate between the species Graneledone pacifica, which lives in the Pacific Ocean, and Graneledone verrucosa, an inhabitant of the Atlantic Ocean. But with limited access to specimens, these warty distinctions didn’t always hold up across larger numbers of octopuses, the study authors wrote.
This new investigation, in which scientists analyzed 72 octopuses, is the first to comprehensively examine dozens of G. pacifica and G. verrucosa specimens to determine what about these warts really distinguishes the two octopus species — and the scientists conducted their analysis one wart at a time.
Physical features that are unique to a certain animal species can take many forms: the size, shape and number of teeth, distinctive colors or patterns in fur, scales or feathers, the color of an iris, the shape of a thorax or the sweep of a fin, to name just a few. Biologists also listen for vocalizations that no other animal produces, and peer at animals’ DNA to tell species apart.
But deep-sea octopus species can be particularly tricky to distinguish, the study’s lead author Janet Voight, an associate curator of invertebrates at The Field Museum of Natural History (FMNH) in Chicago, told Live Science.
As with any deep-sea creature, observing and collecting octopuses is challenging, so there are simply fewer individual animals to study and compare, Voight said. Specimens in museum collections — and most of the octopuses in the study were FMNH specimens — can be centuries old, or might have been collected and preserved before DNA analysis was feasible, making it impossible to extract genetic material from their tissues, she said.
“In deep-sea invertebrates, you don’t have song, or color, or behavior. You have a specimen preserved for — in some cases — decades,” Voight said.