Squids can swim upside down and fast backwards to get away from it all. These cephalopods range from pygmy squids of 10-16 millimetres to giant squids of more than 10 meters. As a member of the „Savvy softies: Octopus, cuttlefish and squid“ they are highly intelligent invertebrates.
Squid can often be observed to swim upside down. This squid was photographed at night in Amed (Bali, Indonesia). In many locations, divers can spot different species such as reef squid forming small groups hanging out or hunting close to the surface during the day as well as at night.
Squids have elongated tubular bodies and short compact heads. Two of the 10 arms have developed into long slender tentacles with expanded ends and four rows of suckers with toothed, horny rings. The body of most squids is strengthened by a feathery-shaped internal shell composed of a horny material.Britannica: Squid
As part of the superorder Decapodiformes, deca meaning 10 and poda meaning feet in Greek, they have ten arms (see full classification at the end of the article). While the long tentacles are used to grab their prey, the arms hold it in place allowing the strong beak to tear off chunks which are swallowed as a whole.
Squid play an important role in the open water food web where they are hunted by sharks, bigger oceanic fish species such as tuna or dolphinfish, sea birds, seals and cetaceans, particularly sperm whales, – and humans, of course. When prepared for human consumption, they are often called calamari.
How these magical creatures change colouration as well as examples for their outstanding intelligence are described in „Savvy softies„. Squids don’t only hang upside down, they also go fast backwards when trying to escape. Some species create such strong jet propulsion that they can shoot out of the water just like flying fish.
When relaxed squid can slowly move by a gentle undulation of the muscular lateral fins as shown on the picture above, allowing forward motion. While jet propulsion is achieved by contracting the muscular wall of the mantle, the combination of both allows the fast backwards escape of these creatures.
Many squid have a fluid-filled receptacle, equivalent to the swim bladder of a fish, in the coelom or connective tissue. This reservoir acts as a chemical buoyancy chamber, with the heavy metallic cations typical of seawater replaced by low molecular-weight ammonium ions, a product of excretion. The small difference in density provides a small contribution to buoyancy per unit volume, so the mechanism requires a large buoyancy chamber to be effective. Since the chamber is filled with liquid, it has the advantage over a swim bladder of not changing significantly in volume with pressure. Glass squids in the family Cranchiidae for example, have an enormous transparent coelom containing ammonium ions and occupying about two-thirds the volume of the animal, allowing it to float at the required depth. About half of the 28 families of squid use this mechanism to solve their buoyancy issues.Wikipedia: Squid – Buoyancy
Let’s have a look at a family member that can be often found on the seafloor in tropical waters. In addition to the camouflage used by all cephalopods, it has a special trick up its tiny body.
Small, but special: Bobtail squids
Hummingbird bobtail squids (Euprymna berryi), also known as Berry’s bobtail squids, grow only up to 3 cm (male) and 5 cm (female). During the day they stay buried in the ground, at night they hunt. Bobtail squids possess a special light organ, where bioluminescent bacteria lives in a symbiotic relationship. Fed by sugar and amino acids, they hide the silhouette of the squid when viewed from below by matching the amount of light hitting the top of the mantle. Counter-illumination is one of the methods of camouflage used in the animal kingdom.
In the case of this little guy (maybe 2.5 cm), it didn’t keep those pesky divers away. Hence it buried itself again – incredibly cute to watch. Please, never stress them by forcing them back out again. Respect nature – below and above the surface!
Scientific classification: Squids
Squids: Upside down and fast backwards
Hummingbird bobtail squid
Species: Euprymna berryi
More about scientific classifications on „Let’s talk scientific! Pictures and classifications of marine life„.