Grass Cay

Grass Cay is a small uninhabited cay off the north coast of St. John in the US Virgin Islands, its name reflecting the sea grass beds that extend in the shallow waters around its base — one of the Caribbean's most valuable but often overlooked marine habitats. The dive site encompasses both the sea grass environment and the rocky reef sections that characterise the cay's submerged base, providing a varied and interesting dive for beginner divers who take the time to appreciate the ecological value of these different habitat types. Sea grass meadows are among the most productive environments in the Caribbean marine ecosystem, despite their deceptively simple appearance. The dense stands of Thalassia testudinum — turtle grass — create an underwater forest at a small scale, the blade-like leaves hosting a community of attached organisms and providing the cover that juvenile fish and invertebrates require for their vulnerable early life stages. Green sea turtles are closely associated with grass beds, and Grass Cay lives up to its name by hosting regular turtle visitations — animals that can often be observed grazing methodically through the grass, their movements slow and purposeful as they crop the vegetation. The rocky sections around the cay base transition from the grass beds in gradients that reflect the light availability and substrate composition at different depths and exposures. Coral patch reefs develop where rocky substrate provides attachment points, and these patches accumulate the fish diversity that the surrounding grass beds cannot directly support. The transition zones between grass and coral are the richest areas of the dive — the edge effect that ecology predicts wherever two different habitats meet is visible here in the concentrated fish activity at the grass-to-coral boundary. Queen conch — now rare on heavily fished Caribbean reefs — are present in the grass beds at Grass Cay, their large shells partly buried in the sediment as they feed on algae and detritus. Seahorses have been reported from the sea grass areas, clinging to blades with their prehensile tails in their characteristic upright posture. And the nurse sharks that are ubiquitous in undisturbed Virgin Islands habitat are reliably present in the shaded areas beneath the reef structures adjacent to the grass beds. Grass Cay is a site that rewards the patient, observational approach to Caribbean diving.