Photo StoryPublished : Jun 09, 2021Updated : Sep 24, 2023
As exclusive inhabitants of the marine realm, squids have evolved incredible diversity dating back to the Jurassic period. Their unique lifestyles and reproductive strategies make them a success in deep and shallow waters
Text by: Mahima Jaini
As exclusive inhabitants of the marine realm, squids have evolved incredible diversity dating back to the Jurassic period. Their unique lifestyles and reproductive strategies make them a success in deep and shallow waters
Squids have occupied the world’s oceans for their 300-million-year evolutionary history. While other molluscs, like snails and mussels, have adapted to terrestrial and freshwater ecosystems, the smartest of them — cephalopods — have remained in the seas. Cephalopods include nautiluses, squids, cuttlefishes, and octopuses. They are the molluscan epitome of intelligence, speed, and camouflage. And their evolutionary advances enabled most of them to discard the external shell.
Amongst cephalopods, squids are the most diverse, ranging in size from less than a thumbnail to the length of a badminton court. “Squid” describes numerous animals all of which have soft bodies, eight arms and two tentacles. True squids (orders Oegopsida and Myposida) are characterised by a pen-shaped internal shell (gladius). While others, like the ram’s horn squid, bobtail, and pygmy squid, either have specialised internal shells or no shells at all. There are over 300 species of squid, and their habitat preferences range from the tropics to polar latitudes, from the intertidal zone to the twilight depths (200 to 1,000 m).
“Live fast, die young” is the squid manifesto. Their lifespans range from a few months to a few years, and most species die after reproducing. Male squids have specialised arms to transfer their sperm to females. Squids lay eggs in gelatinous, well-protected masses. Temperature influences growth; warmer temperatures speed up development, yolk consumption and often result in smaller-sized offspring.
Most squid babies, after hatching, live near the surface waters, amongst the plankton. Far from the ocean floor, they are well protected from the benthic predators (seafloor dwellers). The hatchlings drift with the water currents, feeding on zooplankton for few weeks or months before they are old enough to settle into reef, sea grass or mid-water environments. Unlike most other marine larvae, squid babies resemble the adults and have their parents’ abilities to change skin colour and shoot ink.
True squids have a pen-shaped, chitinous (hard, protein) internal shell (left). The mysterious deep-sea ram’s horn squid (Spirula spirula) has a loosely coiled, chambered internal shell (right). Such 1.5-cm-wide Spirula shells are found along the southern coast of Little Andaman Island. These cephalopods can live in waters as deep as 800 metres, so it is no surprise that their remains wash up on this island surrounded by 1,000-m-deep seas on three of its four sides. Photos: Dan Bagur/Shutterstock (left), Maple Ferryman/Shutterstock (right) Cover photo: GRB-George Berninger Jr., CC BY-SA 3.0
Squids have soft bodies, eight arms, two tentacles, and speedy colour-changing abilities. Squid skin is like a symphony orchestra; their pigment cells (chromatophores) rapidly change size and colour and the visual effect is enhanced by light reflecting cells (iridocytes) and in some species light producing organs (photophores), as well. Furthermore, they also shoot melanin (dark pigment) containing ink to confuse predators and aide in escapes. True squids have slender bodies and circular eye pupils (reef squid, Sepioteuthis sp. in picture). Prized as exports, squids comprise 48 per cent of India’s molluscan fisheries catch. Photo: Philippe Bourjon, CC BY-SA 3.0
True squids like calamari (the ones we like to eat) live in both deep and shallow seas. Oegopsids are the oceanic, deep-sea kind, considered “open-eyed” as they lack any optical membrane. Myposids, on the other hand, are near-shore, shallow-water squids with “closed-eyes” covered by a transparent membrane or cornea. Oceanic squids typically release eggs that drift in the water (planktonic), often in a single, large tubular or elliptical gelatinous mass (left). These egg masses are usually 1-4 m long and can contain 10-100,000 eggs. Shallow water squids lay eggs in capsules generally attached together in large clumps called “sea mops” and secured onto the seafloor. Each capsule of the sea mop may contain a few to hundreds of eggs (right). Photos: Jao Cuyos/Shutterstock (left), BlueTrend/Shutterstock (right)
Some tropical shallow-water squids are capable of mating multiple times during their short adult lives, laying large clusters of sea mops along coastlines, often within the intertidal zone. Females lay the fertilised eggs in capsules and carefully secure each to algae or rocks using special compounds in the outer membrane that harden on contact with seawater. Good bacteria cover the capsules and protect the jelly coat from fungal infections and subsequent predation by fish and invertebrates. In some species, females also cover the capsules in a chemical (called microseminoprotein) that increases male-to-male aggression and guarding behaviour when touched by the male squid. Squid embryos develop by utilising the attached yolk sac, which looks a lot like a pacifier in a baby’s mouth. Photo: Sarang Naik
Squid hatchlings look a lot like the adults, comprising the same body plan and thus are often called “paralarvae”. Here we can see one squid baby hatching and empty egg sheaths nearby. Upon hatching, squid paralarvae dash for surface waters to feed on the abundant zooplankton and grow before settling back on the seafloor. Photo: Sarang Naik
This newly hatched pencil squid (loliginid; likely a Sepioteuthis sp.) can swim, although in jerks, using the contraction of its mantle muscles and by jettisoning water out of its siphon. At this stage, its fins are underdeveloped and don’t aid in locomotion as in the adults. Bigfin reef squids (Sepioteuthis lessoniana) produce hundreds of offspring, but each egg capsule only contains up to 13 eggs, arranged one above the other. Their eggs take about three weeks to hatch. The paralarvae rapidly grow from 0.5 cm to 10 cm, attaining sexual maturity within four months. After that, their lifestyle revolves around building energy reserves and reproducing. They can live up to 5-6 months in the wild and 11 months in captivity. Photo: Tasneem Khan
Identifying juvenile squid species is challenging. We found this little critter (a few mm in length) attached to a seagrass blade. Upon closer inspection, we realised it might be an adult pygmy squid (order Idiosepiida). Pygmy squids tend to live amongst seagrasses and have an adhesive disc for attachment. They lack internal shells, the largest species rarely get longer than 2 cm, and their life span is 2-5 months. Their mating strategies are similar to other squids. Fertilised eggs are laid on the seagrass and protected by multiple jelly coats; 50-200 eggs hatch into one-mm-long paralarvae after developing for 1-4 weeks. Life in the plankton is a gamble, and only a few make it back to repopulate the seagrass beds. Photo: Umeed Mistry While much is unknown about these fascinating creatures, scientists have shown how a warming climate threatens to accelerate their growth, shortens their lifespans, and reduces both their size and age at maturity. Such unprecedented changes to population structure endanger squid ecologies and the fisheries that depend on them.
is a marine biologist, broadly interested in reproductive ecology, marine larvae and population connectivity, forever fascinated by marine invertebrates
