Secrets of Survival: Struggle and Strife Between the Tides
Photo StoryPublished : May 10, 2021Updated : Nov 14, 2024
When the tide recedes along the coast of the Andaman and Nicobar Islands, an array of marine creatures step out of their hideouts for some time in the sun
Text by: Swapnali Gole
When the tide recedes along the coast of the Andaman and Nicobar Islands, an array of marine creatures step out of their hideouts for some time in the sun
As the tide is on the ebb and the ocean pulls out, a hidden, little-known world starts emerging on the exposed seafloor. Contrary to the dead and unglamorous first impression of this site, the place is teeming with life. All it takes is for you to stoop down a little and look under rocks.
The intertidal zone, which is the area concealed between two tidal zones, is one of the most dynamic marine regimes, exposed to never-ending tidal shifts. Dwellers of these unique ecosystems epitomise the phrase “survival of the fittest”. With growing competition, either the fittest ones survive while the weak perish or they learn to coexist.
The other aspect of life in the tidal zone is symbiosis, where organisms have mutually beneficial relationships. Vibrantly coloured carpet anemones for instance are known to host multiple such relationships with symbionts (partners) who are immune to the deadly sting of their hosts. The potential stinging cells called nematocysts or cnidae which are in the tentacles can be fatal to other invertebrates who try to mess with the host or its symbionts.
Receding tides expose lifeforms barely known to terrestrial beings. Brittle stars dare to come out of their hideouts. Sea cucumbers like lollyfish and black Tarzan dive into the mini tide pools formed by the retreating water. Their branched tentacles actively feed on benthic bottoms (sea beds); coiled, neatly piled up excreta, or pellets of rejected sand particles, serve as proof of their massive feeding routine.
Hermaphrodites like this blue button jellyfish (Porpita porpita) are commonly found afloat in the mini pools of intertidal shores. They are carried in by the high tide and get stuck. This jellyfish is not a true jellyfish but a colonial hydrozoan polyp that passively drifts in the sea and feeds on micro plankton. The circular disc is called a “float”, to which multiple colonies of individuals are attached; each possesses stinging cells called nematocysts. Photo: Dhritiman Mukherjee
Sea cucumbers, like this black-spotted (Pearsonothuria graeffei), are the nutrient recyclers of marine habitats. These deposit feeders ingest sediments to feed on micro-particles, break them down, and help in nutrient recycling. There are around 1,200 species of sea cucumbers found globally, out of which many are exploited commercially. The “beche-de-mer” industry has been harvesting sea cucumbers to meet the huge demand for dried skins of the animal, used as a delicacy, leading to its local extinction in many places. Photo: Dhritiman Mukherjee
Sea slugs are not marine flatworms, though they may look like that. They are a group of gastropods called nudibranchs. With no shell, uncovered gills, a pair of sensory rhinophores (horn-like receptors) and in some cases cephalic tentacles (tentacles on the head), they rule intertidal and subtidal habitats. Seen here are the fairly common white-margin sea slug or caramel nudibranch (Glossodoris rufomarginata) (1) and Cerberilla annulata (2). Unlike sea slugs, flatworms don’t have ruffles. They have oval flat bodies, two sensory organs, and a pair of primitive eye spots that are sensitive to light. Flatworms (3) have been spotted in the intertidal areas of the Nicobar Islands. Photos: Sagar Rajpurkar (1,2), Samuel John (3)
The famous “Nemo” fish (Amphiprion sp.) (1), spotted porcelain crab (Neopetrolisthes maculatus) (2) and five-spotted glass shrimp (Periclimenes brevicarpalis) (3) each form what is called a “symbiont complex” on carpet anemones (green in images) in the intertidal zones of the Andamans. These creatures (symbionts) choose anemones to live in. They reciprocate by actively defending the host anemones against intruders and providing their excreta as nutrients (which the symbiotic alga inside the host anemone photosynthesises). Photos: Dhritiman Mukherjee
Seagrass meadows support a diversity of associated macro-fauna and large marine herbivores like dugongs and green sea turtles. Indian waters are productive for seagrass growth and support 15 of around 60 species found globally. Being marine plants, seagrasses are one of the major primary producers and a crucial carbon sink. Seagrasses are being lost globally at a rate of 110 sq km per year since 1980. They are ecologically sensitive habitats that are protected under Category I of India’s Coastal Regulation Zone notification (2011). Photo: Dhritiman Mukherjee
In a parallel intertidal world, a stroll on Radhanagar beach in Havelock (now called Swaraj Dweep), reveals sun-shaped patterns on the sand. These designs are made by marine artists like the tiny sand bubbler crabs. Sand bubblers are filter feeders, they eat micro-nutrients from the sand brought in by the high tide. As the tide goes out, crabs crawl out of their burrows and perform “surface deposition feeding”. Essentially, they eat microscopic particles from sediments, then form small pellets of the undigested sand, and throw them in a radially symmetrical pattern, with their burrow hole being the centre of this artwork. Photos: Vladimir Volodin/Shutterstock (1), Shreeram MV (2)
Navigating through the estuarine channels of the Mahatma Gandhi Marine National Park in Wandoor the breathing roots of mangroves are visible. With the onset of low tide, shortly after the slack time of one hour, rhizophora plants start breathing through the emerging prop roots. Mangroves are halophiles (salt lovers) which exhibit some remarkable adaptive strategies, allowing them not just to survive in the constantly changing environment, but to propagate. A propagule (seed) germinates and falls off its mother plant, drifting with the currents or finding its fate in soil nearby. Photo: Dhritiman Mukherjee
About the contributor
Swapnali Gole
works on dugongs in the Andaman Islands with the Wildlife Institute of India. She believes in communicating research to the local masses, which is crucial for the success of conservation projects.
