Everything about the bigfin reef squid of Koh Tao — how they hunt, communicate and change colour, the best dive sites and times to find them on night dives, and how to observe them without causing alarm.
15 May 2025 · updated 27 February 2026 · by Bryan Roseman
In short. Squid encounters in Koh Tao feature the fascinating bigfin reef squid (Sepioteuthis lessoniana), one of the most intelligent invertebrates in the Gulf of Thailand. These remarkable cephalopods display extraordinary colour-changing abilities, complex communication systems and captivating hunting behaviour. Night diving around Koh Tao offers the best opportunities to witness their spectacular light displays and observe their sophisticated social interactions.
The squid around Koh Tao are some of the ocean’s most extraordinary creatures, combining alien intelligence with breathtaking visual displays. These bigfin reef squid, also known as oval squid or glitter squid, showcase abilities that seem almost supernatural — from instantaneous colour transformations to complex social behaviours that researchers are still studying. Understanding when, where and how to encounter these mesmerising cephalopods turns ordinary night dives into unforgettable wildlife experiences that highlight the incredible marine life diversity around Koh Tao.
The squid that Koh Tao divers encounter most frequently are medium-sized cephalopods, typically reaching 30–40 cm in length, though exceptional specimens approaching 50 cm have been documented around the island. Their distinctive wide fins run nearly the entire length of the mantle, creating an oval or diamond-shaped outline when viewed from above — a key identifying feature that distinguishes them from other squid species in Thai waters. If you’re planning to experience these incredible encounters, understanding the best time to dive around Koh Tao significantly increases your chances of memorable squid sightings.
What truly sets these creatures apart is their remarkable skin, covered in specialised cells called chromatophores (for colour), iridophores (for iridescence) and leucophores (for white reflections). This adaptable skin serves as both camouflage and a communication medium, allowing the squid around Koh Tao to blend seamlessly into different environments or broadcast their intentions to others of their kind. These remarkable adaptations are just one example of the diverse marine life behaviours that make Koh Tao a world-class diving destination.
Bigfin reef squid are formidable nocturnal predators that employ coordinated group hunting techniques. They hover motionless before an explosive acceleration, using specialised feeding tentacles to capture prey with remarkable precision.
These intelligent cephalopods use rapid colour changes to communicate with each other, displaying zebra-like patterns during mating and warning colouration when threatened. Each pattern conveys specific information to other squid.
Unlike many marine predators, the squid of Koh Tao frequently hunt in coordinated groups of 4–15 individuals, using visual signals to maintain formation and synchronise movements for greater hunting efficiency.
Peak activity occurs at night, when squid emerge from their daytime hiding spots to hunt. Their colour displays become most vivid in darkness, creating spectacular underwater light shows for night divers.
| Dive site | Depth range | Best time | Squid behaviour commonly observed |
|---|---|---|---|
| Twins | 8–25 m | Night dives | Hunting behaviour in sandy channels |
| Japanese Gardens | 5–12 m | Dusk transitions | Emerging from daytime resting spots |
| White Rock | 12–30 m | Full moon periods | Mating displays and egg-laying |
| Shark Island | 5–18 m | Evening/night | Group feeding on small fish schools |
| Southwest Pinnacle | 15–35 m | Night dives | Undisturbed natural behaviour |
What makes squid encounters in Koh Tao particularly memorable is the clear intelligence behind their behaviour. Like their octopus cousins, squid possess highly developed brains relative to other invertebrates, capable of learning, problem-solving and potentially recognising individual divers who visit their territories regularly.
Their neural organisation features a distributed nervous system with a central brain and additional processing centres in their arms, yet they display unmistakable signs of advanced cognition. Their ability to rapidly assess threats, coordinate hunting in groups, remember locations and engage in complex mating rituals all point to sophisticated information processing beyond simple instinctive responses.
Test your understanding of squid behaviour around Koh Tao (the correct answers are in bold):
Night dives and dusk transitions offer the best chances for squid encounters. They emerge from their daytime hiding spots as darkness falls, displaying their most spectacular colour shows.
Rapid colour changes and pattern displays are their primary form of communication. Each pattern conveys specific information to other squid about intentions, emotions and social status.
Coordinated group hunting with 4–15 individuals is their preferred method. They use visual signals to maintain formation and take turns capturing prey.
4–8 months is their typical lifespan from hatching to natural death. This short life cycle drives their rapid growth and accelerated reproduction.
Despite their remarkable adaptations and intelligence, bigfin reef squid face various challenges in the waters around Koh Tao and throughout their range in the Indo-Pacific region. Their short lifespan makes them particularly vulnerable to environmental pressures.
Commercial fishing targets squid as popular seafood throughout Asia. Their short lifespan makes populations vulnerable to overfishing, with limited recovery time.
Coral reef degradation from climate change, pollution and physical damage affects their hunting grounds and egg attachment sites.
Increasing coastal development and artificial light can disrupt the natural behavioural patterns of these nocturnal creatures.
Plastic pollution poses risks through direct ingestion and damage to the habitats squid depend on for successful reproduction.
Positive conservation factors: their rapid reproduction allows relatively quick population recovery, growing night diving tourism creates economic incentives for protection, and conservation zones around Koh Tao provide sanctuary for breeding populations.
Ready to witness the mesmerising intelligence and colour displays of bigfin reef squid in their natural habitat? La Bombona Diving’s night dives (1,400 ฿) and fun dives (2,000 ฿ for 2 dives) take small groups to the prime squid habitats around Koh Tao, with a marine life briefing before every dive and guides who know where to look. Planning your trip? Check out our guide on how to get to Koh Tao from anywhere in Thailand to make your squid diving adventure seamless.
Bigfin reef squid pose absolutely no danger to divers. Despite their sometimes imposing appearance when displaying dark colouration, they are non-aggressive towards humans and have no means of harming divers. Their beaks are effective for eating small fish but cannot cause significant injury to a person. When approached properly, squid encounters are completely safe and incredibly rewarding.
While both belong to the cephalopod family, bigfin reef squid have torpedo-shaped bodies with triangular fins, while cuttlefish have broader, flattened bodies with undulating fins. Behaviourally, cuttlefish typically hover closer to the reef, while squid occupy the mid-water column. Both display remarkable colour-changing abilities, though cuttlefish are masters of texture change as well.
Koh Tao's squid reproduce year-round, but mating activity increases from February to April and from August to October. Night dives during full moon periods often offer the highest chances of witnessing mating displays, as the extra natural light may play a role in reproductive timing. Mating involves intense colour changes, with males showing zebra-like patterns.
Bigfin reef squid rarely release ink when encountered by divers around Koh Tao. Inking is a high-energy "last resort" defence against an immediate predatory threat. When divers approach properly — slowly, without sudden movements and keeping an appropriate distance — squid typically respond with colour changes rather than inking. Any inking means the squid perceived an immediate threat.
Squid populations around Koh Tao appear generally stable, benefiting from rapid life cycles that allow quick recovery, conservation zones that provide protected breeding areas, and growing night diving tourism that creates economic incentives for protection. However, challenges such as fishing pressure and habitat degradation warrant continued conservation attention.
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