It's Cephalopod Week 2021 and we're squidding with the Squids!

Cephalopod Week (which takes place annually around the last 2 weeks of June) champions octopuses, squids, cuttlefishes and all their kin! This year, the week runs between June 18-25, 2021. Among the cephalopod species, the octopuses are most well-known for its intelligence! If you missed out on learning about these cool animals, check out this article written by Megan Ng for the last International Octopus Day on October 8th. Our budding marine enthusiast is back with another lovely piece on the octopuses’ lesser-known relatives – the squids. Let’s dive in!

About the author: Megan is currently a final year Life Sciences major with a minor in Aquatic Ecology and Forensic Science at the National University of Singapore. Her passion for the ocean started when she was a young girl, as she was always very curious about nature especially marine life. Megan is a strong believer about conservation and tries to find ways to help our planet. She is writing for Celebrating Singapore Shores to spread the word about our marine life and conservation to the public through popular science writing!

See the Sharp, Spunky and Stunning Squids of Singapore! 

Diagram of squid anatomy hand drawn by Megan Ng. 

Source and information credited to Ms Meg Sodano. 

Cephalopods are molluscs?

The Octopus, Nautilus, Cuttlefish and Squid are all members of the Class Cephalopods. But, did you know that they are all also classified in the Phylum Mollusca? 

Though the common molluscs members may seem vastly different, they share similar characteristics with cephalopods too! For example, like molluscs, squids have a mantle cavity and a radula – which is an organ that acts as a tongue. This special organ is covered in sharp teeth to help cut food into smaller pieces and to help with swallowing. In squids, the radula is present in the beak, which plays a huge role in ensuring that food is cut into small pieces before being swallowed into their narrow oesophagus which goes through their doughnut-shaped brain!

 

Illustration of the beak of a Giant Squid with the radula (yellow);

Credited to Ms Sally Parker at the Smithsonian Institution.  

General illustration of mollusc anatomy with all parts, including the radula which is lined with teeth, being labelled. This illustration is taken from Enyita (2015).

However, there are some important differences between cephalopods and other members of molluscs too! For instance, squids have 3 hearts, while their gastropod cousins only have 1! Moreover, squids have a closed circulatory system - which means that their blood only flows from organ to organ through blood vessels, like us. But for other gastropods like snails, they have an open circulatory system so blood can flow freely between their organs! 

Unlike gastropods such as snails, squids do not have the characteristic external shell and instead, they have evolved to have an internal shell known as the pen. The pen is usually transparent and is composed of chitin and other proteins to allow the organs of the squid to anchor and protect them! The quill, which is a part of the pen, forms the backbone of the squid as well.  

Squids are Not So “Blur” After All?

Like their octopus cousins, these squids have chromatophores too! They mainly change their colour to match and blend with their surroundings. They can also use these chromatophores to talk to each other. Have you ever wondered why squids can look different at different angles? This is because they, like octopuses, have a special group of chromatophores known as iridophores! These cells reflect light differently depending on the angle that they are being looked at, allowing the squids to blend in with their surroundings in a blink of an eye. Interestingly, they themselves are also unable to see colour changes! Our eyes have photoreceptor cells known as rods that produce black and white colours in our brain, and cones that produce normal colours such as red and blue. Squids lack these unique cone cells and thus cannot see colour! 

Some other squids like the Bobtail squid from the family Sepiolidae don’t just use chromatophores. They also use bioluminescence to produce their own light, allowing them to glow in the dark. This is possible through using bioluminescent bacteria which are stored in a light organ within the squid. Not only that, they can also use bioluminescence for defence! By glowing in the dark, this hides the Bobtail squid from predators which dwell deeper in the ocean and look up to find prey. Bioluminescence then allows them to blend in with the top layers of the ocean above, evading capture! Other squids like the Firefly squid (Watasenia scintillans) can also use these colourful lights to attract mates! 

Video of the Hawaiian Bobtail squid (Euprymna scolopes) using its bioluminescence!

Aside from camouflage, squids have a variety of weapons that they use to hunt prey and defend themselves! Squids can also squirt a cloud of ink to allow them to escape predators, similar to their octopus cousins. This is actually the reason how the local phrase “blur sotong” came about – from their ink “blurring” and confusing their predators –not because they are slow! 

A Bigfin Reef squid (Sepioteuthis lessoniana) squirts out ink that looks like the squid itself. 

Photo credit to Ms Ria Tan.

The suckers that line the arms of the squid are essential for hunting, as they powerfully latch onto prey and deliver them to their beak. Each squid has 8 arms and 2 tentacles, making a total of 10 deadly weapons. Some squids even have hooks on their arms to sink into their targets! These tentacles are also important for allowing the squid to swim quickly, both towards their prey and away from predators.

Other than their limbs, some species like the Pygmy squid (Idiosepius species) also have a special gland that secretes glue, allowing them to stick themselves to the underside of blades of seagrass to await their prey. Once the prey is caught in their nifty arms, these small squids quickly use their sharp beak to bite its nerve cord, killing it instantly. Although these squids are tiny and cute, they are truly not to be underestimated!

 

A Pygmy squid sighted at Pulau Sekudu with a tiny shrimp caught. 

Photo credit to Mr Loh Kok Sheng.

Another Pygmy squid sighted at Tanah Merah. The squid has a shrimp caught in its tentacles too. Photo credit to Ms Ria Tan.

Squid of All Trades – the Clever Acrobats of the Sea

Diagram of squid flight hand drawn by Megan Ng. 

Information sourced from Oceanswell on Twitter. 

Some squid, like the Japanese flying squid, can even “fly” rapidly through the air! They do so by taking up water into their mantle and then forcefully expelling it out of their siphon allowing them to propel themselves in any direction! They can also use membrane structures hidden in between their tentacles, allowing them to form a structure that helps them to soar through the air temporarily, before quickly diving back into the ocean to flee from predators. 

Video on how the Japanese flying squids soar through the air, above the ocean! 

Squids are also incredibly smart! Did you know that the giant squid axon was reported to be as thick as 0.5 millimetres? It is nearly 25 times larger than a human’s axon measuring only 20 micrometres! In order to swim, there is a complex network of 6 giant neurons controlling jet propulsion and mantle contraction – which is all needed just for swimming. By using their smarts, they also control the patterns of their chromatophores to communicate amongst themselves! Nerve impulses from the brain are sent to the chromatophores, causing the chromatophores to become bigger and smaller rapidly - allowing them to quickly change their appearance and talk to one another. This quick thinking also allows them to hide and flee from predators in the nick of time!

Video on all the defences that the squid has in its arsenal! All of these defences point back to the squid using its smarts to beat their predators and hunt prey.

They also have complex courting rituals where the males dance and change colours to woo the females! If he’s successful, the male squid then inserts a specialised arm, called a hectocotylus, into the female’s mantle and her oviduct. The hectocotylus, or “mating arm” then releases sperm and fertilises the female’s eggs. The female then lays her offspring in egg sacs! 

Video from The Blue Earth by BBC Earth about the interesting mating rituals of squids and how they reproduce! The squids use colour to make themselves more attractive and also dance to court females (you can start from 0:51) :-)

In contrast, some species of squid such as Octopoteuthis deletron do not discriminate, and attach packets of sperm to any squid that they meet – male or female! This gives them a higher chance of their offspring surviving in case the male squid encounters cannibalistic members. In another species, Gonatus onyx females can store their offspring in specialised egg sacs, protecting their offspring from predators and parasites until they are born. Pretty cool right?

Squid Sightings in Singapore

Yes, we have squids in our Singapore waters! They can be commonly seen either at the shallower areas like our seagrass fields or even deeper in the ocean. Some species include the Bigfin reef squid with its captivatingly beautiful colours. Interestingly, this species can also be cannibalistic, as the smaller squid often school together to protect themselves from being eaten by larger members of the same species! 

 

The Bigfin Reef squid in a school (top) and alone (bottom) captured at Tanah Merah. 

Photos credit to Ms Ria Tan. 

Other species that have been sighted on Singapore shores are the slender Indian squid (Uroteuthis duvaucelii) as well as the aforementioned cute Pygmy squid which only grows until about 1cm.

A pencil squid (Loliginidae species) on the top and an Indian Squid on the bottom captured at Tanah Merah. Photos credit to Ms Ria Tan. 

 

A Pygmy squid becoming thin when approached. Photo credit to Mr Ron Yeo.

Another species that is commonly seen are the Bobtail squids! These small squids are often mistaken for cuttlefish but are actually squids. They have a uniquely rounded mantle, together with smaller round fins on both sides – making this squid look like an underwater Dumbo!

 

 

A Bobtail squid captured at Beting Bronok; Photo credit to Mr Loh Kok Sheng. 

Another Bobtail squid captured at Tanah Merah. Here, you can see more clearly the smaller rounded fins on the side of its mantle. Photo credit to Mr James Koh. 

In Singapore, the Indian squid and Bigfin reef squid are commonly sold as food and have been caught using several methods such as trawling and even traps using eggs to lure the squids. Luckily, these squids are not on the IUCN Red List for Endangered Species. But, more data is still needed to properly assess how safe our squid friends are. 

Needless to say, squids play an important role, not just in the food chain, but in our local biodiversity as well. Let’s do our part to protect them from overfishing, making sure to preserve and enhance our biodiversity here in our Little Red Dot!

References used:

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