While scuba diving is classified as a sport, it’s also an activity that brings out the marine biologists hiding deep inside even the least scientific among us! Almost all divers enjoy watching marine life, and many also enjoy photographing and filming fish and other creatures they encounter during dives. You might know that understanding the animals you meet underwater will help you to enjoy your encounters more; part of that understanding comes from greater knowledge about the marine food chain and the way it works.
The Best Things Come in Small Packages
When we pause to take a closer look at the marine food chain, it becomes evident that all life in the ocean depends on tiny clumps of cells known as plankton. These microscopic plants and animals are found in all oceans, though they exist in varying numbers that increase and decrease depending on external factors. All plankton are referred to as “producers,” since they essentially give rise to the marine food chain. All the creatures that rely on them – and on the life forms that feed on them – are known as consumers.
The plant constituents in plankton are known as phytoplankton. As they consume energy from the sun and then convert it into a form which can be readily consumed by animals, they are also referred to as producers. These producers, by virtue of their inherent ability to break down solar energy, are also referred to as autotrophs – life forms which are capable of synthesizing their own food using natural, organic, and solar resources.
The animal constituents in plankton are known as zooplankton. Many zooplankton are actually larvae which, when given proper conditions, grow into larger marine animals; others are microscopic animals which will never grow larger.
A number of marine animals large and small consume plankton as their main food source. For example, tiny fishes and other organisms that feed solely on plankton are known as plankton pickers. Other animals, including baleen whales, manta rays, and whale sharks – the biggest of all fish in the sea – consume vast amounts of plankton. While these larger animals are at the upper edge of the food chain by virtue of their size, smaller fish are subject to predation. Despite their size variations, all of these animals are known as primary consumers, since they prey directly upon the producers.
As the food chain continues, we see secondary consumers – animals that eat the primary consumers – come onto the scene. The animals which in turn prey on the secondary consumers are called tertiary consumers, and so on; however, all consumers are also referred to as heterotrophs, as they are incapable of synthesizing their own nutrients.
Following the Food Chain
There are so many players involved in the marine food chain that it is often referred to as a food web; in fact, most marine life forms are inextricably linked with several others so that if a single strand in the web breaks, the entire life cycle is changed.
Since we’re looking mostly at a web rather than at a straight-edged food chain, we’ll take a look at some basic types of individuals.
Herbivores are animals that consume plants. They are most prevalent in shallow water, since shallow areas are the best places for plants to grow. Marine herbivores come in a stunning array of sizes and shapes; colorful wrasses that nibble at algae are often herbivores, and the manatees you’ll find grazing on beds of seagrass are herbivores too.
Carnivores are animals that consume other animals and nothing else. Most sharks are carnivorous; so are barracuda, groupers, and many reef fish.
Still others are omnivores; these animals consume an interesting blend of plants and animals. Many sea turtles are omnivorous – they like lots of seagrass, but they’re also fond of sponges, which look much like plants but which are also part of the animal kingdom. Whale sharks and other plankton eaters are omnivorous as the plankton they consume includes both plants and animals.
Others are opportunistic feeders; they don’t typically hunt other animals. Instead, they eat whatever they can find. Many crustaceans are opportunistic feeders, and at first glance this might seem unimportant, but they do the vital job of keeping the oceans clean.
Among carnivores and omnivores, we find a number of predatory species. Many are surprisingly small, while others are large and are seemingly menacing. Depending on what type of food is available, predators hunt and consume a wide range of other animals, though there are some that do restrict their hunting activities to a few select species. Most, though, will hunt and kill almost indiscriminately when favored prey animals are not available. This often extends to the young of the same species, and in some cases, even their own young. Some sharks have such strong predatory instincts that they’ll eat their own litter mates while in utero!
Among predators, there are different levels, with some smaller fish being at the bottom of the predatory food chain, and with mid-level predators making up the majority. Some mid-level predators include creatures such as:
- Dolphins and porpoises
- Barracudas and other predatory fish such as groupers
- Pinnipeds, including seals, sea lions, and walruses
- Predatory seabirds, including penguins, puffins, and pelicans
These large mid-level predators have only a few natural enemies. These enemies go by a special name – apex predators. Apex predators are at the very top of the food chain or food web; they are the only creatures physically capable of hunting and killing other large animals. Marine animals which are considered to be apex predators include:
- Carnivorous sharks
- Orcas
- Polar bears
There are very few apex predators in the oceans, and their numbers are rapidly declining due to a number of factors including habitat loss, climate change, and human interference. While this might seem like a lucky break for the animals they prey on, the fact is that their role in the food web is a vital one – they prey mostly on the weakest animals, including diseased individuals that could spread illness to other members of their species. Many of these animals also act as opportunistic feeders, consuming the carcasses of larger animals that have died of natural causes, which in turn helps to keep the oceans clean.
In the web of life, every creature has its place. As divers, we are fortunate to witness this web in action; and we are honor bound to share our knowledge with others in the hope that more people will come to gain knowledge of and respect for the delicate balance that exists within our planet’s oceans. By learning all you can and by sharing what you know, you do your own part to make the web stronger – and you make the oceans better for all who depend on them, including humanity.