The Intertidal zone – Rocky shores

Geetha Iyer

In my article in the January issue, I wrote about one of the habitats found in the intertidal zones – the sandy beaches. This article is about the rocky shores, another kind of habitat in the same zone. As the name suggests, the habitats in such shores have plenty of rocks. Rocky shores are predominant on the western coast of India. The presence of the Western Ghats has shaped the character of this coast in many ways. Towering above and away from the Arabian Sea, the ghats often extend down to the coast contributing to the formation of the rocky shores and their unique ecosystems. Thus, be it the rocky shores of Karnataka, Kerala or Tamilnadu, the richness of the marine species in these parts have been influenced in part by the evolutionary processes of millions of years in the ghats.

The defining character of the rocky shores, as the name implies, is the presence of solid rocks. While the size and shape of the rocks result largely from the combination of wave action and tides, the bedrock and climatic conditions also contribute, especially to the internal structure of the rocks. Rocky shores are also the result of erosion, caused by multiple factors. All these together have an important role for the presence and survival of diverse organisms.

The tides create changing environmental conditions that range from the rocks offering a terrestrial habitat to a marine ecosystem. The organisms living here are subjected to and therefore are adapted to manage heat stress, desiccation, oxygen depletion and reduced feeding opportunities. The shores are therefore a tough habitat to live on and yet it is seen that rocky shores are one of the biologically richer environments; the best places to observe some of the biological processes one studies in middle to senior secondary classes.

Although both are influenced vastly by tide actions, rocky shores are quite a contrast to sandy beaches. On sandy beaches there is constant movement of the substrate, i.e., the sand (which is loose and porous), whereas the rocky shores offer a substrate that is both hard and stable. The rocks also provide several microhabitats allowing diverse organisms with differing needs to thrive; which makes the habitat so biologically rich. There are shallow or deep crevices, rock pools, puddles, cliffs, platforms and rocky reefs (coral rocks) each sheltering different types of organisms that have adapted to the existing conditions.

Biodiversity of the rocky shores
Invertebrates, algae and some lichens are the most common and abundant inhabitants of the rocky shores. Eels can be found resting in rocky crevices or feeding in rock pools. During a survey at the rocky shores of Kovalam in Trivandrum, Dr Biju Kumar from the University of Kerala recorded the presence of 128 species. These were 30 sea weeds, 11 sponges, 4 coelenterates, 26 molluscs, 7 annelids, 2 sipunculids, 6 isopods, 12 amphipods, 1 anomuran crab, 14 branchyuran crabs, 2 alpheid shrimps, 7 echinoderms, and 6 ascidians. This example which highlights the richness of the rocky shores is only a modest count.

As in any ecosystem, the primary energy producers in this marine area are the algae. Several kinds of seaweeds may be found on the rocky shores of the Indian coast. The highest density of seaweeds in India is found on the coasts of Gujarat and Tamil Nadu. Several species may be seen easily at the marine biosphere reserve extending from Rameswaram to Thoothukudi in Tamil Nadu. Among them are species of Ulva commonly known as sea lettuce, species of Sargassum, the brown alga, Gracilaria sp and, Padina sp commonly called the peacock’s tail.

Molluscs are the most visible on any seashore. Among them gastropods are more numerous than bivalves on rocks. Rock shells, frog shells, chitons, limpets, whelks and periwinkles can be observed during low tides as they come out of their shelter to feed. Periwinkles, limpets and chitons feed on algae. They scrape out the algae from the rocks. Their activity helps maintain the original rock structure and ensure that the algae do not dominate the rocky shores. These are mobile grazers of the marine world and can be compared to the grazers in a forest keeping a check on the plant life. Other herbivorous organisms such as drupe shells and mussels are generally seen on rocks on which green and red algae grow.

Found on rock surfaces are sponges. These are multicellular invertebrates with a cellular level of organization. They vary in shape, size and colour. If you spend some time observing them (with a lens in hand), you will be able to see the tiny pores and large openings through which water circulates in their body.

Rocks that are covered with silt are preferred by murex shells. These are carnivorous molluscs that produce a fluid which anesthetises their prey for an easy catch. Cone shells found here are very poisonous, producing deadly nerve poisons. These poisons are being researched for medical benefits to the nervous system.

There are winter migrants amongst molluscs too. Sea hare is a type of sea snail in the category of slugs that gets its name from the shape of its body which resembles a sitting hare. These may be seen in rocky pools.

Coral rocks are quite rich in diversity and host invertebrates such as polycheate worms, tube worms, sea slugs, bivalves, etc.

I have given more examples of molluscs than other organisms mostly because they are the easiest to observe when introducing students to an intertidal walk. The rocky shores are a habitat for several other organisms. Stone corals may be seen on rocks. Brittle stars, sea cucumber, bryozoans and certain sponges prefer loose rocks. Certain species of hermit crabs also make their homes in rocky shores.

Vertebrate fauna too may be observed. Eels may be found in those parts where the rocks are submerged in water most of the time. Rock pools are nurseries for fish and crustacean species. Mudskippers rest in the crevices of rocks as do sea snakes, that come ashore at night and curl up inside the crevices for a good night’s rest. Gulls and oystercatchers are common around rocky shores. Sea otters and seals breed on rocky shores of islands.

Activity
The intertidal walk activity suggested in the previous article (January issue) holds good for rocky shores too. If you are planning to explore rocky shores then be sure to wear a good pair of shoes preferably with ankle protection to avoid cuts and bruises from barnacles, oysters, etc. Low tide is the best time to explore when many molluscs come out to feed. Some specific activities that can be carried out are:

1. Collect a few samples of the algae found on the rocks, extract the pigments from them and do a chromatography test to find out the pigments present in them.
2. Questions relating to the structure of algae and its significance in adapting to life on rocks could be given. These will help develop skills of observation, critical thinking and encourage students to explore in detail.
3. Observe feeding mechanisms of select organisms. For eg., limpets may appear to be practically dead, but watch those that are on the parts of the rocks under water. You will find them fighting with one another for best feeding spots. Or watch the hermit crab as it moves about or the way it retracts into the shell even if slightly disturbed. Explore to find out which shells are preferred by them and why? This may be a difficult task but can be quite interesting as you are likely to learn a lot more even if you do not get the answer you are seeking.
4. Study the biodiversity in a chosen rock and make a poster. Sketching them where you find them will help develop both drawing and artistic skills.
5. Observe an organism of choice, explore to find how it adapts to live in the conditions observed.
6. Through observation, try to find out the difference between barnacles and limpets. Barnacles are crustaceans whereas limpets are molluscs. Can you justify through observation that the former is a crustacean and the latter a mollusc?
7. At the end of the walk be able to identify a few organisms found on rocky shores and discuss their influence on the rocky habitat.

Importance of rocky shores
They are home to a wide variety of organisms; nursery for fish and crustacean species; provide shelter for many species; provide food for fishes; feeding ground and nesting areas for certain species of birds. Stabilize the inshore sediment.

Threats to the habitat
Marine litter is the biggest problem that these habitats face. Responsible tourism is still not in practice and there seems to be very little happening in the nature of awareness programmes. Tourism can be both beneficial and harmful and currently the latter is a cause for worry. Chemical waste from guest houses, recreational centres can be harmful for many of the organisms that live within narrow ranges of water quality. Ocean acidification is a cause for worry. It will drastically affect the reproduction of several species which may lead to their decline. Climate change leading to frequent storms result in habitats being destroyed, excessive silt deposition and debris changing the quality of the habitats.

Awareness programmes for habitat protection is the urgent need of the day. Availability of easy resources that can educate about the organisms and the habitat is needed. There has to be some regulatory processes put in place to ensure that the tourism industry does not harm this habitat. Those who litter the shores must be fined if all other processes fail. The highest priority lies in reducing pollution through littering.

The author is a consultant for science and environment education. She can be reached at scopsowl@gmail.com.