The bigfin reef squid — Sepioteuthis lessoniana — is not the most visually dramatic animal on a coral reef. It won't stop a dive in its tracks the way a whale shark does. But for divers who pay attention to the reef at a finer level, it represents something genuinely remarkable: one of the most neurologically sophisticated invertebrates on the planet, exhibiting behaviour that challenges how we define intelligence.
It's also one of the more reliably observable species on reefs around Koh Tao, active both day and night, and behaving in ways that become fascinating once you know what to look for.
Taxonomy and Distribution
S. lessoniana belongs to the family Loliginidae and is distributed throughout the Indo-Pacific, from the Red Sea and East African coast through to Hawaii and Japan. The species was formally described in the early 19th century by René Lesson, the French naturalist who participated in Louis Duperrey's global scientific voyage. It's one of the largest loliginid squids, reaching mantle lengths of up to 38 centimetres in mature individuals, though reef-dwelling populations tend toward the smaller end.
In Thailand's Gulf waters, bigfin reef squid are year-round residents rather than seasonal visitors, making them reliable targets for divers across all seasons.
Chromatic Communication
What makes S. lessoniana exceptional is its skin. Packed with millions of chromatophores — pigment-containing cells controlled by direct muscle contraction rather than hormonal signalling — the squid can produce and alter complex chromatic patterns in milliseconds. These patterns serve multiple functions: camouflage against reef surfaces, predator warning displays, and intraspecific communication during courtship and competition.
Peer-reviewed research published in the Journal of Experimental Marine Biology and Ecology has documented that bigfin reef squid can simultaneously display different patterns on each side of their body. A male courting a female may display mating colours on his female-facing side while displaying territorial 'male warning' patterns to a competing male on the opposite side — a level of asymmetric communication that has few parallels in the animal kingdom.
Behaviour That Rewards Observation
Bigfin reef squid typically hover in loose aggregations at the edge of reef structures, particularly in areas where sand meets hard coral. They're most active at dawn and dusk, when feeding runs and social interactions peak, but they can be observed throughout the day.
Watch for:
Hovering with fins rippling in a continuous undulation — this is normal cruising behaviour, not agitation
Flash patterns across the mantle that indicate feeding readiness or proximity to prey
Males presenting laterally to each other in standoff postures — competition without contact
Egg masses deposited in coral rubble or sandy substrate, occasionally visible to attentive divers
Reproductive Biology
S. lessoniana has a compressed lifespan of approximately 4 to 6 months in tropical reef conditions. Adults die shortly after spawning, and the entire population in a given area may turn over multiple times per year. Females deposit clusters of white, torpedo-shaped eggs in sheltered reef cavities, typically in groups. Males compete intensively for fertilisation access throughout the process.
The brevity of the lifespan relative to the complexity of the behaviours the species displays is one of the things that makes it a subject of ongoing biological interest. How much learning occurs within four to six months? How much is pre-programmed? These questions don't have complete answers yet.
How to Observe Squid Responsibly
Squid are highly sensitive to rapid movement and strong light. Approach slowly, from a lateral position rather than directly ahead. Torchlight during night dives should be kept diffuse — pointing at the squid rather than spotlighting it directly. Chasing or cornering squid will cause them to jet ink and flee, which benefits neither party.
Many divers find that staying still and letting squid approach on their terms produces far better encounters than active pursuit.
FAQs
Are bigfin reef squid dangerous?
No. They are not aggressive toward divers and have no venom relevant to human contact. Their ink is a predator deterrent and causes no harm to humans.
When are squid most active on Koh Tao reefs?
Bigfin reef squid are active day and night but tend to be most accessible and behaviourally interesting at dusk and on night dives, when feeding and social interactions are most visible.
How long do bigfin reef squid live?
Approximately four to six months under tropical reef conditions, significantly shorter than related species in cooler water. The entire lifespan — hatching, growth, reproduction, and death — occurs within a single breeding season.
Are they the same as giant squid?
No. Giant squid (Architeuthis dux) are deep-ocean species that can exceed 10 metres in length. Bigfin reef squid are shallow-water reef residents reaching a maximum of around 38 centimetres. The two species are not closely related beyond both being cephalopods.
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