The Mesmerizing Bigfin Reef Squid of Koh Tao: Masters of Color and Light

Main Points

• Koh Tao’s waters are home to the fascinating Bigfin Reef Squid (Sepioteuthis lessoniana), one of the most intelligent invertebrates in the Gulf of Thailand.
• These remarkable cephalopods possess extraordinary color-changing abilities, complex communication systems, and fascinating mating behaviors.
• Night diving offers the best opportunities to witness their spectacular light displays and hunting techniques.
• Conservation challenges facing these short-lived but ecologically important creatures deserve attention from divers and marine enthusiasts.

Introduction to Koh Tao’s Cephalopod Residents

The Bigfin Reef Squid (Sepioteuthis lessoniana), also known as the oval squid or glitter squid, is the most commonly encountered squid species around Koh Tao. They represent the perfect blend of alien intelligence and evolutionary marvel, possessing abilities that seem almost supernatural to human observers—from instantaneous color transformations to complex social behaviors that researchers are still working to fully understand.

The Bigfin Reef Squid: Nature’s Light Show

Physical Characteristics and Identification

Bigfin Reef Squid are medium-sized cephalopods, typically reaching 30-40 cm in length around Koh Tao, though exceptional specimens approaching 50 cm have been documented. Their name derives from the distinctive wide fins that run nearly the entire length of their mantle (body), creating an oval or diamond-shaped outline when viewed from above—a key identifying feature that distinguishes them from other squid species.

Their body structure follows the classic squid design: a tubular mantle housing their internal organs, a distinct head featuring large, expressive eyes, eight arms arranged in a circle around their mouth, and two longer feeding tentacles that can rapidly extend to capture prey. The fins undulate gracefully as they hover, creating the impression of an underwater ballet performer.

What truly sets these creatures apart, however, is their remarkable skin. Covered in specialized cells called chromatophores (for color), iridophores (for iridescence), and leucophores (for white reflections), Bigfin Reef Squid can produce an astonishing array of colors, patterns, and visual effects. This adaptable skin serves as both camouflage and communication medium, allowing them to blend seamlessly into various environments or broadcast their intentions to others of their kind.

Life Cycle and Behavior

Hunting and Feeding Behaviors

Around Koh Tao, Bigfin Reef Squid emerge as formidable nocturnal predators, employing both intelligence and specialized physical adaptations to hunt effectively. They primarily target small fish and crustaceans, using a hunting strategy that combines stealth, speed, and coordination.

When hunting, they often adopt a distinctive posture with arms held together in a cone-like formation while hovering nearly motionless in the water column. This stillness is periodically interrupted by explosive acceleration as they lunge toward prey, unfurling their two specialized feeding tentacles with remarkable precision. These tentacles extend much faster than human reflexes can track, snatching prey and transferring it to their beak-like mouth in fractions of a second.

What makes their hunting behavior particularly fascinating for night divers to observe is their cooperation. Unlike many predators that hunt alone, Bigfin Reef Squid frequently hunt in coordinated groups of 4-15 individuals, using visual signals to maintain formation and synchronize their movements. This cooperative strategy allows them to herd schools of small fish, increasing hunting efficiency as they take turns darting in to capture prey.

Mating and Reproduction

The reproductive behaviors of Bigfin Reef Squid create some of the most dramatic underwater encounters around Koh Tao, particularly during their peak breeding periods. Males engage in elaborate displays to attract females, flashing intense colors and patterns while extending their arms in ritualized postures. Competing males may engage in dramatic “duels” of color, each trying to outshine the other with increasingly vibrant displays.

Like all squid, the Bigfin Reef Squid lives a tragically brief life, typically surviving only 4-8 months from hatching to natural death after breeding. This short lifespan drives their remarkably fast growth and accelerated life cycle, with individuals reaching sexual maturity in as little as 2-3 months.

Where and When to Find Bigfin Reef Squid Around Koh Tao

While Bigfin Reef Squid are present in Koh Tao’s waters year-round, specific locations and conditions significantly increase your chances of encountering these fascinating creatures:

Prime Dive Sites

Twins: This popular dive site’s varied terrain and moderate depths create ideal hunting grounds for Bigfin Reef Squid, particularly along the sandy channels between the two main reef structures. Night dives here regularly yield squid sightings as they emerge to feed.

Japanese Gardens: The shallow depths and mixed habitat of this beginner-friendly site make it excellent for observing squid behavior, especially during dusk dives when they begin to emerge from daytime hiding spots.

White Rock: The protected areas around the base of this large reef formation serve as common locations for egg-laying, making it possible to observe both adult squid and their egg clusters during certain times of year.

Shark Island: The currents around this site attract concentrations of the small fish that squids prey upon, creating a reliable feeding area where groups of hunting squid can often be observed.

Southwest Pinnacle: This deep site on offers excellent opportunities to observe undisturbed squid behavior away from the closer dive sites.

For more information on the top dive sites of Koh Tao, click here.

Timing and Conditions

Night Diving: Undoubtedly the optimal time to observe Bigfin Reef Squid is during night dives, when they are most active and their color displays are most vivid. Dives starting around 7-8 PM typically yield the best encounters.

Dusk/Dawn: The transitional periods between day and night offer unique opportunities to observe squid as they shift from their daytime resting behavior to more active states.

Observation Tips

Successful squid encounters require specific approaches:

Move Slowly: Quick movements startle these sensitive creatures, causing them to flee or display alarm coloration. Gentle, deliberate movements yield longer, more rewarding observations.

Use Red Light: Many night dive guides in Koh Tao equip their divers with red lights or red filters for white lights when specifically looking for squid. Red wavelengths disturb marine life less than white light, allowing closer approaches.

Look for the Eyes: The reflective quality of squid eyes often reveals their presence before their bodies become visible in the darkness. Scan slowly with your light to catch this telltale reflection.

Watch the Sand/Water Column Interface: Bigfin Reef Squid often hunt along the boundary between the sandy bottom and the water column, particularly in areas where the seafloor transitions from reef to sand.

The Intelligence Behind the Light Show

What makes encounters with Bigfin Reef Squid particularly memorable is the clear intelligence behind their behaviors. Like their octopus cousins, squid possess highly developed brains relative to other invertebrates, capable of learning, problem-solving, and potentially even recognizing individual divers who visit their territories regularly.

Their neural organization is fascinatingly alien compared to vertebrates—with a distributed nervous system featuring a central brain and additional processing centers 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 instinctual responses.

Conservation Challenges

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:

Fishing Pressure: As popular seafood items in Thailand and throughout Asia, Bigfin Reef Squid are subject to commercial fishing, including around the Gulf of Thailand. Their short lifespan makes them vulnerable to overfishing, as populations have limited recovery time.

Habitat Degradation: Like many marine species, they depend on healthy reef ecosystems for shelter, hunting grounds, and egg attachment sites. Coral reef degradation from climate change, water pollution, and physical damage impacts their habitat quality.

Light Pollution: As nocturnal creatures highly attuned to light levels, increasing coastal development and the resulting artificial light can disrupt their natural behavioral patterns, particularly around more developed areas of Koh Tao.

Marine Debris: Plastic pollution poses risks through both direct ingestion and damage to the habitats squid depend on for successful reproduction.

Fortunately, several factors work in their favor from a conservation perspective:

Rapid Reproduction: Their fast life cycle and high fecundity (number of offspring) allows populations to recover relatively quickly if fishing pressure is managed sustainably.

Increasing Awareness: The popularity of night diving specifically to observe squid creates economic incentives for their protection, as they become more valuable alive to tourism than as a one-time food source.

Protected Areas: Several conservation zones around Koh Tao where fishing is restricted help provide sanctuary for breeding populations.

Local dive operators increasingly include information about squid conservation in their briefings, encouraging responsible interaction and awareness of these remarkable animals’ ecological importance.

For more information on marine conservation Koh Tao, click here.

Frequently Asked Questions

1. Can Bigfin Reef Squid be dangerous to divers?

Bigfin Reef Squid pose absolutely no danger to divers. Despite their sometimes imposing appearance when they display dark coloration or spread their arms in a defensive posture, they are non-aggressive toward humans and lack any means to harm divers. Their beaks, while effective for consuming small fish and crustaceans, are not capable of causing significant injury to humans. When approached too aggressively, these squid typically respond by either retreating rapidly using their jet propulsion or displaying warning coloration before fleeing. The main concern in squid-human interactions is not human safety but rather the welfare of the squid, as improper approach techniques (such as chasing or touching) can cause significant stress to these sensitive creatures. The most rewarding encounters occur when divers maintain a respectful distance and allow the squid to become comfortable with their presence.

2. What’s the difference between squid and cuttlefish that we might see around Koh Tao?

While both belong to the cephalopod family and share many characteristics, several key differences help divers distinguish between squid and cuttlefish around Koh Tao. Physically, Bigfin Reef Squid have more torpedo-shaped bodies with triangular fins running along much of their mantle length, while cuttlefish have broader, more flattened bodies with undulating fins that extend completely around their mantle. Cuttlefish also possess an internal cuttlebone (providing buoyancy), while squid have a flexible internal gladius or pen. Behaviorally, cuttlefish typically hover closer to the reef or seafloor, while Bigfin Reef Squid more commonly occupy the mid-water column. Both display remarkable color-changing abilities, though cuttlefish are generally considered masters of texture change as well, able to create raised papillae on their skin to mimic surrounding textures. In Koh Tao waters, the most commonly encountered cuttlefish is the Broadclub Cuttlefish (Sepia latimanus), which tends to be more solitary than the often-schooling Bigfin Reef Squid.

3. When is the best time to see squid mating behavior around Koh Tao?

While Bigfin Reef Squid reproduce year-round in tropical waters like those surrounding Koh Tao, local dive guides report increased mating activity during two primary periods: February to April and August to October. These periods seem to correspond with subtle seasonal changes in water conditions around the island. Night dives during the days surrounding the full moon often yield the highest chances of witnessing mating displays, as the increased natural light may play a role in their reproductive timing. The most reliable sites for observing mating behaviors include the sandy areas adjacent to Twins, the deeper sections of White Rock, and occasionally around the artificial structures at the Junkyard dive site. Mating displays typically involve intense color changes, with males showing zebra-like patterns while positioning themselves parallel to receptive females. For the highest chances of witnessing this spectacular behavior, consider booking multiple night dives during your stay, as mating activity can be unpredictable and encounters often come down to fortunate timing.

4. Do squid ink when approached by divers?

Bigfin Reef Squid possess ink sacs and are capable of releasing ink as a defensive mechanism, but they rarely deploy this defense when encountered by divers around Koh Tao. Inking represents a high-energy “last resort” defense typically reserved for immediate predatory threats rather than the more benign presence of human observers. When divers approach properly—slowly, without sudden movements, and maintaining appropriate distance—squid typically respond first with color changes (often darkening) or by repositioning themselves rather than inking. If a squid does ink during a dive, it almost certainly indicates that it perceived an immediate threat, perhaps from being approached too aggressively or cornered. The ink serves dual purposes: creating a visual screen behind which the squid can escape and containing chemicals that can temporarily disorient predators’ sense of smell. After releasing ink, squid typically employ their jet propulsion to rapidly exit the area. Responsible divers should consider any inking event as a sign to modify their approach techniques and increase their distance from these sensitive creatures.

5. Are the squid populations around Koh Tao healthy and sustainable?

The Bigfin Reef Squid populations around Koh Tao appear generally stable, though comprehensive scientific monitoring is limited. These cephalopods benefit from several factors supporting their sustainability: their rapid life cycle allows populations to recover relatively quickly from disturbances, and several conservation zones around the island provide protected breeding areas. However, challenges exist that warrant continued attention. Local fishing pressure, while not specifically targeting squid around dive sites, does impact wider populations in the Gulf of Thailand. Additionally, declining coral reef health affects the ecosystem that supports the small fish squid prey upon. The growing popularity of night diving specifically to observe squid creates positive economic incentives for their conservation, as live squid become valuable tourism assets. Local conservation organizations like New Heaven Reef Conservation Program and Save Koh Tao monitor general marine ecosystem health, which indirectly benefits squid populations by protecting their habitat and food sources. Visitors can support conservation efforts by choosing environmentally responsible dive operators and practicing good buoyancy control to minimize reef damage.