The pear-shaped bobtail and bottleneck squids have an "invisibility cloak" like Harry Potter, thanks to its symbiotic relationship with bioluminescent bacteria living in the mantle of its special light organ.
According to Monterey Bay Aquarium, the bacteria hide the silhouette of the squids when they leave the safety of the seafloor to hunt in the night. They do this by matching the amount of light hitting the top of its mantle, making it virtually invisible in the moonlit waters. In a way, the squids would also provide the bacteria with sugar and amino acid solution, which is their food.
But baby squids are not born with these bacteria, and they have to secret a mucus around their light organs to capture the bacteria. Less than a day from being born, these hatchlings have also become invisible like the adults.
Now, researchers are curious about their evolution and diversification time, which might explain the evolutionary relationships of these squid species with one another.
Major Biogeographic Events Might Have Shaped the Evolution of Bobtail Squids
Bobtail and bottletail squids are cephalopods related to true squids, octopuses, and cuttlefish renowned for their intelligence and complex behaviors. Since they are easy to reproduce and raise in large numbers even in laboratory settings, they make useful model animals for research, according to Science Daily.
The study, "Phylogenomics illuminates the evolution of bobtail and bottletail squid (order Sepiolida)," published in Nature Communications Biology, discussed how bobtail and bottletail squids share a common ancestor and how major biogeographic events could have changed their evolutionary relationship and the diversification timing.
Lead author Dr. Gustavo Sanchez from Hiroshima University said that out of 68 known species, the bobtail squid is a very diverse family that is divided into three subfamilies, which are further split into two geographic lineages. But in 2019, he and his research team described a new species of bobtail squid that highlights both the diversity present and the mysteries surrounding the squid.
The science news outlet reported that the bobtail and bottletail squids make up the Sepiolida, which are small squids that are only between 1cm to 8cm. They live in a range of marine habitats around the world, like in the shallow waters to the open ocean.
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Dr. Sanchez and colleagues from Okinawa Institute of Science and Technology Graduate University's (OIST) and the National University of Ireland Galway collected 32 species of bobtail and bottletail squids from different oceans and used genome skimming to sequence their whole genomes to estimate their evolutionary relationships and their diversification timing.
They found that the bobtail and bottletail squids might have split into different families about 66 million years ago, which aligns with the time when there is rapid diversification of modern marine fishes after ancient groups died out due to mass extinction that killed the dinosaurs.
Bobtail squids went on to split into three subfamilies, known as Sepiolinae, Rossinae, and Heteroteuthinae. The Sepiolinae further split into two tribes which are now found in the Indo-Pacific Ocean, Mediterranean Sea, and the Atlantic Ocean. the split also coincided with the closure of the Tethys Sea around 50 million years ago.
Evolution of Light Organ of Bobtail Squids
Another interesting part of their study showed the evolution of the light organ of bobtail squid that allows the production of luminescence, an essential survival mechanism for the species to keep hidden from predators at night.
Science Daily reported that ancestors of the Sepiolinae subfamily likely possessed a bilobed light organ that stores the bacteria that creates the luminescence effect, which is then passed on to many species that exist today. Two groups have lost this characteristic which is found in the Indo-Pacific and Mediterranean-Atlantic tribes.
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