Nina photographed this Hiby’s Lamellarid at Koh Bon during a Similan diving expedition, challenging the myth that this velvet-textured gastropod is endemic to the Maldives. These specialist sponge predators have been thriving in Thailand’s waters for decades, wrapping their mantles around prey to digest it externally.
Hiby’s Lamellarid (Coriocella hibyae) is a distinctive marine gastropod belonging to the velvet snail family Lamellariidae. Despite being commonly referred to as the “Maldives Sponge Snail” in some field guides, this species has a much wider distribution throughout the Indo-Pacific region, including waters around Thailand, Indonesia, and beyond. The species name honours marine researcher Hiby, though specific details of the etymology remain unclear in scientific literature.
These gastropods are characterised by their smooth, velvet-textured mantle that can completely envelop a greatly reduced internal shell. Unlike most marine snails, lamellarids are specialist predators with highly specific feeding habits focused on sponges. Their ability to match the colour and texture of their prey makes them masters of camouflage and challenging subjects to locate on the reef, despite being relatively common in suitable habitats.
The expanded mantle gives these molluscs their distinctive glossy appearance, earning them the common name “velvet snail.” When disturbed, they can retract the mantle to reveal a small, delicate shell underneath, though this behaviour is rarely observed in the field. Their cryptic lifestyle and specialised diet make them ecologically important members of reef communities, helping to control sponge populations and contributing to the complex food web dynamics of coral reef ecosystems.
Hiby's Lamellarid inhabits tropical and subtropical waters of the Indo-Pacific region, with confirmed sightings throughout a much broader range than early field guides suggested:
Their range extends throughout the Indo-Pacific, including the Andaman Sea, Gulf of Thailand, Indonesian archipelago, Philippines, and into the western Pacific. Records from Thailand’s Similan diving sites at locations such as Koh Bon and Koh Tachai span over 20 years, challenging earlier assumptions about their limited distribution. They prefer stable water temperatures between 24-29°C (75-84°F) and areas with moderate water movement that supports healthy sponge communities.
Environmental Conditions:
These gastropods are found in areas where their sponge prey is abundant, particularly favouring reefs with diverse sponge assemblages. They require environments with sufficient hiding places during daylight hours, as they are primarily nocturnal. Water quality is important, as both the snails and their sponge prey require clean, well-oxygenated conditions. They are typically solitary but may be found in areas where multiple individuals exploit particularly productive sponge gardens.
Hiby’s Lamellarid demonstrates one of the most specialised feeding strategies amongst marine gastropods, focusing exclusively on sponge consumption:
Hiby’s Lamellarid is a nocturnal predator, spending daylight hours concealed in reef crevices and emerging after dark to hunt for sponges. Their feeding strategy is remarkable and unique amongst gastropods: they locate suitable sponge prey using chemoreception, then position themselves on or near the sponge before extending their mantle to completely envelop it.
Feeding Strategy:
Once the sponge is wrapped within the mantle cavity, the gastropod secretes digestive enzymes that break down sponge tissue externally. This extra-oral digestion allows them to process sponge material, including dealing with defensive compounds and spicules that many sponges produce. The partially digested material is then absorbed through the mantle surface and via the mouth. This process can take several hours to complete, during which time the gastropod remains motionless on its prey.
The mantle’s ability to match the colour and texture of the sponge being consumed provides excellent camouflage during this vulnerable feeding period. After feeding, individuals retreat to shelter before daylight, digesting their meal internally. Their specialised diet means they are typically found only in areas with abundant sponge coverage, and population densities often correlate directly with sponge abundance and diversity.
Like most marine gastropods, females release numerous small eggs encased in gelatinous masses or egg ribbons, which are often attached to substrates or released into the plankton. Males release sperm into the water column, and fertilisation occurs externally. The exact details of spawning behaviour remain poorly documented due to the species’ cryptic nature and nocturnal habits.
Eggs develop into free-swimming veliger larvae within several days, depending on water temperature. The larval stage is planktonic, lasting approximately 2-4 weeks, during which time the developing gastropods feed on phytoplankton and microscopic organisms. Veliger larvae possess a small shell and ciliated velum for swimming and feeding. As they develop, they gradually acquire the characteristics of juvenile lamellarids.
Settlement occurs when larvae locate suitable habitat with appropriate sponge coverage. Newly settled juveniles quickly develop their characteristic mantle expansion and begin their specialised feeding behaviour. The transition from planktonic larva to benthic juvenile involves significant morphological changes, including the reduction of the shell and expansion of the mantle tissue.
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