Cave-dwelling invertebrates

These are the hardest animals on the reef list to see, and everything depends on buoyancy and patience. Move slowly, stay off the floor, and use a frog kick — one downward fin stroke lifts a silt cloud that hides the animals and ruins the passage for the divers behind you. Use a narrow, low-powered beam and sweep it slowly across the silt, wall cracks and the shimmering layer of the halocline rather than lighting the whole passage; transparent bodies show up best when the light rakes across them from the side. Look for movement, not shape: a Typhlatya shrimp is a flicker of a fingernail-sized shadow with impossibly long antennae. For photography, a snooted macro strobe and manual focus work far better than a wide beam, and a single frame is usually all you will get. Never touch, collect or corral these animals — they are fragile and extremely slow to recover. Follow cave-diving rules without exception: cavern zone only unless you hold a full cave certification, always in the daylight zone with a continuous guideline, and with a qualified local guide. Use only biodegradable, mineral-free sun protection, or none at all, because everything on your skin ends up in the aquifer that these species and the region's drinking water share.

Cenote diving on the Riviera Maya runs year-round, and the cave animals themselves show no seasonal cycle — water temperature stays near 24–25 °C every month. The practical difference is visibility and crowding. November to April is the dry season, with the clearest water, the sharpest halocline and the most reliable conditions; this is the best window for macro work in the caves. June to October brings heavy rain and hurricane season: runoff can push tannic surface water into the shallow sections and reduce visibility in some systems for days, although deeper passages usually stay clear. Early morning dives are best in every month, because the caverns are quieter and the silt has had all night to settle.

Anchialine and freshwater cave systems of the Yucatán Peninsula in Mexico — the cenotes and the flooded limestone passages that connect them, including Sac Actun, Ox Bel Ha and Dos Ojos. The community is layered: small Typhlatya shrimps and amphipods dominate the freshwater layer in the first metres of the cave, larger Creaseria morleyi shrimps patrol the floor and walls deeper inside, and remipedes and specialised isopods live in the salt water below the halocline, often between 10 and 30 metres. They favour total darkness, stable temperatures around 24–25 °C and undisturbed silt floors, and they disappear quickly where surface water, light or pollution intrudes. Related stygobitic faunas occur in comparable limestone aquifers in Cuba, the Bahamas and the Canary Islands.

Detritivores and, in the case of the remipedes, active predators. Because no sunlight reaches the caves, the food web depends entirely on imported and chemically produced energy: plant debris and soil washed in through cenote openings, bat guano from the air-filled chambers, fine organic particles suspended in the water, and the bacterial mats that grow along the halocline where fresh and salt water meet. Shrimps and isopods graze this material from the silt and from rock surfaces; amphipods feed on decaying matter. The remipede Xibalbanus tulanensis is the exception — it injects venom into other crustaceans and feeds on the liquefied tissue, the only known venomous crustacean in the world. Food is scarce and unpredictable, so most of these animals can survive long periods of starvation.