Life in the oceans-Planktons
The Ocean – Part 2
Geetha Iyer
In this second part of the article on oceans we look at life in the oceans and how they survive the various forces, described in the first part (Teacher Plus, July 2021). When you look at the sea from the beach it appears to be just water. But the sea is anything but a uniform column of water. The ocean is a biome with several regions or zones. Each of these zones have their own unique characteristics as well as organisms adapted to the conditions in the zones.
The term ‘habitat’ is seldom used in ocean parlance; it’s always regions or zones. But these regions can be compared with the habitats that I have been writing for non-oceanic biomes so far. There are several ways in which these zones can be studied. The terms one comes across in a science or biology class are pelagic, demersal, benthic, etc. Life in oceans is governed, among others, by two very crucial factors – the presence or absence of light and the pressure exerted by the water column at various depths. An easier way to learn about the creatures in the oceans is to know about them with reference to these two factors. This is not to infer that these are the only two factors that play a role in an organism’s life. You are already familiar from the previous article about the forces that rule the lives of organisms in this biome.
The zone in the sea where light is able to penetrate is called the sunlit zone. Hence you will find organisms that are able to photosynthesize and provide nutrients to those living in this zone as well as to those deeper below. This is the zone where you find seaweeds. There are several different kinds of organisms to be found here. Far too many to fit into a small article such as this. Hence I am focusing on one that is often talked about but not in its entirety.
The planktons1. Dr Tony Michaels of the NOAA (National Oceanic and Atmospheric Administration) calls them drifters. According to him, any organism in the sea that can’t move very far compared to the currents, are planktons. Quite a different view from the traditional textbook definition that says they are micro-organisms found in the waters. The definition suits phytoplanktons but not all planktons in general. Planktons are not transported by the currents, stay put in the water columns wherever they are and at best move around for a hundred meters or so. They are found not only in the sunlit zones but there are groups of these that live deep inside the sea and come up at night to feed and then return to the depths. The size spectrum of the planktons is huge.
There are the viruses. Surprised? But true. There are the tiniest of the planktons measuring in fractions of a micrometer. There are planktons that are bacteria and then there are those that belong to the Kingdom Archaea. Planktons that are archaeans are found in cold ocean conditions such as in polar seas or some thermophiles in hot geysers. These planktons too are quite small and cannot be seen with the naked eye. Then there are microalgae, protozoans, etc. It is estimated that approximately 1029 types of microbes live in the sea (weighing one trillion (1,000,000,000,000) tons), which by weight comprise an estimated 90 per cent of life in the ocean. The primary source of nutrients that flow through the ocean’s food webs comes from the photosynthetic activities of the phytoplanktons living in the sunlit zones of the ocean. These microbes are not only crucial to maintain the food chains, but are essential to drive many of the important chemical reactions that will enable elements like carbon and nitrogen to move through the oceans.
Says Dr Michaels, “If you take a teaspoon of sea water (about 5ml) and count all of the little living things in just an average teaspoon of seawater, you have about 100,000,000 viruses; 10,000,000 bacteria; 5,000 microalgae; 3,000 single cell animals – protozoans.” A diversity that far out matches what we have on land.
Calculate for fun: Solve this: If there are six billion human beings in India, calculate how much of sea water would be necessary to equal the number of people on earth?
Now, if you recall that the surface of the ocean covers about 360 million square kilometers at an average depth of about 4,000 meters, can you visualize the vast number of organisms that live in oceans? We are still discovering them and like the forests we destroy, these oceans are also being pilfered in many ways that will lead to the disappearance of the diversity without our knowledge of several of them.
Now for the grazers of the sea. These are the first line feeders of the nutrients produced by the phytoplanktons. Some of them may even consume the phytoplanktons. From about a depth of 200m to about 1000m, penetration of light declines gradually and the water pressure increases. This is called the twilight zone. Beyond this depth it is all dark, cold and water pressure is the greatest. Recent explorations have shown this dark region to be populated by creatures weird in form and function. Food is scarce in the twilight zone and so organisms living here must “capture” their food. Among several invertebrates found in this region, a variety of crustaceans are part of the zooplankton community.
Among the first line feeding grazers found in the twilight zone, the most common are the copepods and euphasiids. A well-known example of euphasiids are the krills. Krill is the favourite food of whales, seals, penguins, squids and fish. Because the source of food for copepods and euphasiids are located near the surface of water where their predators roam around looking for them, some of these euphasiids stay at a depth of 300m, in the dark part of the sea and come up only at night to feed. Since their predators hunt by sight, coming up to the surface of the water to feed at night is an adaptation for survival.
Jelly fish are examples of the predatory zooplanktons living in the twilight zones. There are two types of jellies (as they are commonly called) – those belonging to Phylum Ctenophora and others belonging to the Phylum Cnidaria. The cnidarian are considered to be true jelly fish. The ctenophores that swim with the help of cilia are an interesting bunch. The cilia reflect light thereby giving the animal a rainbow-like ‘wow’ look. These are fierce predators and will gobble up fish larva and copepods for food. Mnemiopsis an efficient predatory ctenophore was responsible in 1989 for the collapse of the million dollar anchovy fishery, as it had eaten up every single anchovy fish larvae. Since the adult fish lives only for a couple of years this was a devastating loss. From sticky tentacles, to fleshy lobes or simply a grab and gulp down, the ctenophores are successful predators of the midocean.
There are three groups of cnidarians – the jellyfish, anemone and corals. (To know more about corals please see Teacher Plus January 2011 issue.) Jelly fish too are active predators.
The magnificent sea anemone is a cnidarian that has two feeding methods. It has a symbiotic relationship with the dinoflagallates that reside within it and produce food by photosynthesis. It is also a predator. It uses its tentacles with the nematocysts present in them to stun, immobilize and capture prey such as smaller invertebrates and fish larvae.
Salps are planktonic tunicates that look like jelly fish but are not connected to them physiologically. They are tunicates – urochordates with a notochord (forerunner to our vertebral column) hence more related to humans than to jellies. They can be seen floating on the surface of oceans either singly or as a chain. They reproduce both sexually and asexually through budding. They graze on phytoplanktons. Whenever there is a plankton bloom Salps quickly bud off clones. These clones graze and grow at a rate faster than any other multicellular organisms. Often, their feeding decimates the plankton population. They also poop a lot that sinks to the bottom of the ocean. If the phytoplankton population is dense then Salps are unable to take advantage of them. They clog and drop down dead. Through these two processes, Salps play an important role in carbon cycling. As the dead Salps and their faecal matter sink, they carry a lot of carbon to the bottom. Thus their abundance or absence has a significant effect on the carbon cycle in the ocean.
The oceans are home to diverse living things. From the microscopic plankton to the large blue whale, it is one of the large creatures that often catches our attention; but the small and the invisible that play crucial roles in ecosystems are rarely discussed. The marine diversity is unimaginably vast and I have just grazed the surface of an important group namely planktons to give you an idea of diversity. The sea floor fauna which we have only recently begun to investigate and understand are quite a different story altogether. One I shall share as I continue to write about habitats and ecosystems.
Understanding the life cycles, habits, habitats and inter-relationships of marine life will contribute to our understanding of the planet as a whole as well as the processes needed to conserve. Hardly 3 per cent of the ocean is protected. Ocean resilience is important for combating climate change. Toxic spills, oxygen-depleted dead zones, marine debris, increasing ocean temperatures, over fishing, and shoreline development are daily threats to marine life. The oceans need more protection if we are to safeguard marine biodiversity. Over the past 100 years, scientists have seen sea levels rise steadily. Indonesia might disappear, parts of several coastal towns and cities in India will sink. According to data put out by NOAA, “Sea levels will continue rising even if the climate has stabilized, because the ocean reacts slowly to changes. 10,000 years ago the ocean level was about 110 m lower than it is now. If all the world’s ice melted, the oceans would rise 66 m.”
Diversity of life in oceans
An educated guess estimates the oceans to hold more than 1 million multicellular species.
According to World Register of Marine Species (WoRMS) there are currently at least 236,878 named marine species (1/25/2021).
Since 2000, on an average, 1650 new marine species have been named each year.
Scientists estimate that of the one million species of animals that live in the ocean, 95 per cent are invertebrates.
Mitch Sogin, of the Marine Biological Laboratory in Woods Hole, Massachusetts, and a leader of the International Census of Marine Microbes is of the view that a liter of seawater may have 30,000 to 40,000 types of microbes.
Sogin says, “So if we take all 1,200 samples (from the microbial wing of the ocean census), we very conservatively estimate that they contain one-half million kinds of microbes…we have only sampled 1,200 liters, which is 1 in 1018 parts of the total ocean.”
References
- All about planktons: http://www.biologyeducation.net/news-and-articles/plankton/
- Classroom activity: https://www.noaa.gov/education/resource-collections/marine-life/aquatic-food-webs
- More about life in oceans: https://whyfiles.org/2010/life-in-the-oceans/index.html
- Gallery of ocean creatures: https://oceanexplorer.noaa.gov/image-gallery/welcome.html
The author is a consultant for science and environment education. She can be reached at scopsowl@gmail.com.
