Chemosynthesis: Life Finds A Way

Animals have to eat in order to stay alive. The majority of life on Earth is based on a food chain that revolves around sunlight, whereby plants use energy from the Sun to make food via photosynthesis. But what about environments, like the deep ocean, that are completely dark and devoid of sunlight? In these areas, marine snow, which consists of dead plants, animals, plankton, and other material that sinks down from the upper reaches of the ocean to the seafloor, are the primary sources of food. But some animals have found a different way to survive: through a process called chemosynthesis.

Chemosynthesis occurs when bacteria and other organisms use chemical energy from sulfides, methane, and other inorganic compounds to produce food. Some of these bacteria are free-living colonies that are eaten by other animals, but many other chemosynthetic bacteria fuel life by forming symbiotic relationships with larger animals, living directly in the tissues of these animals. Together, these bacteria form the basis of the food chain for life at places on the seafloor around hydrothermal vents, cold seeps, whale carcasses, and even sunken ships.

The animals that make up these chemosynthetic communities are truly unique — not found anywhere else on Earth. They are key components of the ocean ecosystem, yet our knowledge of them is relatively new, brought to light by ocean exploration when humans first observed a vent on the deep ocean floor in 1977 and found a thriving community where there was no light. Since then, chemosynthetic communities have been found in ocean basins all over the world.

No one had ever though to look for them, but these communities were there all along. And now we know that together photosynthesis and chemosynthesis fuel all life on Earth.

Sea Cucumbers: Not Your Garden-Variety Ocean Animal

Despite their common name, sea cucumbers, also known as holothurians, are not something you’d ever find in your garden. Instead, these squishy invertebrates are found throughout the world’s ocean basins, from shallows to great depths. They belong to the phylum Echinodermata, which also includes sea stars and urchins.

Sea cucumbers are particularly plentiful in the deep ocean and have developed some interesting adaptations to survive conditions at depth.

Sea cucumbers lack lungs or gas-filled spaces that would make them more susceptible to the intense pressures of the deep ocean. Instead, sea cucumbers breathe as water is pumped through two “respiratory trees” located on each side of their digestive tracts.

Speaking of digestion, as food is scarce in the deep ocean, sea cucumbers feed on nutrients attached to sediments on the seafloor. They use their tube feet to “eat” seafloor mud, extracting what little nutrition may have persisted after slowly sinking from the productive upper ocean, and then expelling undigested mud. For this reason, sea cucumbers are sometimes called the “vacuum cleaners of the sea.”

Not all sea cucumbers spend their time slowly ambling over the seafloor, eating mud. Some are able to lift themselves off the seafloor using a large fin-like structure. They then swim through the water column with strong, rhythmic flexing motions until they reach neutral buoyancy and drift along until their next feeding trip on the bottom. Some sea cucumbers never touch the seafloor, instead spending their entire lives swimming in the water column!

The Invisibility of Being Red

If you’ve ever looked beneath the surface of the ocean or other bodies of water, you may have noticed that water transforms light and that colors appear different. This is because water absorbs colors with longer wavelengths such as red at shallower depths than those with shorter wavelengths such as blue, which penetrates to a much deeper depth. Red light, with the longest wavelength in the visible spectrum, is quickly absorbed from water as depth increases and effectively never reaches deep-ocean depths. This provides animals in the deep ocean with an interesting form of camouflage: being red.

The wavelength of a color must be present in the surrounding environment in order to be seen. So, in an environment like the deep ocean where there are no red light wavelengths, a red animal appears to be lacking color and is essentially invisible to animals around it.

When scientists exploring the deep ocean shine the artificial lights of an instrument like a remotely operated vehicle on animals, we bring in all wavelengths of the rainbow and are often treated to a bright array of colors, many of which are vibrant shades of red.

The Armored Searobin: A Fish Out For A Stroll

For our next dose of deep-sea weird, meet the armored searobin! Found only in the deep ocean, these unusual fish in the family Peristediidae can actually “walk” across the seafloor.

Like their shallow-water relatives, armored searobins have fan-like fins (pectoral fins) with stiffened rays on each side of their bodies behind their heads. The first few rays are free from the membranes of the rest of the fin and are very thick. The fish uses these thickened, stiff fin rays to "walk" along the bottom of the ocean. So rather than swimming, like other fishes, the usual form of locomotion for armored searobins is walking!

Armored searobins also have four rows of bony plates along their bodies. Each plate has a thick, curved, short spine — hence the fish’s common (armored) name. They also have horn-like projections on each side of their snouts and branched barbels that look like whiskers on the bottom of their heads, in front of their mouths. These branched barbels have taste buds and are used to sense food on the bottom.

Siphonophores: In This Together

Siphonophores are a colonial type of hydrozoan. As invertebrates belonging to the phylum Cnidaria, they are related to corals, sea anemones, and jellyfish. There are approximately 175 described species of siphonophores, and they are all marine, found in ocean basins around the world. The familiar Portuguese man o’ war is the only species of siphonophore that lives in surface waters. As the rest are found at greater depths, they are animals that are often encountered while exploring the deep ocean.

When looking at a siphonophore, you may think that these marine invertebrates are single organisms, but a siphonophore is actually a colony of many individual hydrozoans (called zooids) working together. Each individual zooid is specialized for different functions such as swimming, feeding, reproduction, or defense. They are similar to other solitary animals, but the individuals are all attached to each other instead of living independently. Siphonophores are considered to be some of the most complex colonial animals.

Siphonophores are delicate, gelatinous animals that break easily and thus are difficult to collect and bring back to the surface for scientists to study, making our understanding of them somewhat limited. But we do know that they come in a fascinating variety of shapes, sizes, and colors!

Scientists estimate there may be between 700,000 and 1 million species of organisms living in the ocean (excluding most microorganisms, of which there are millions). By volume, the deep ocean is the largest living space on Earth, so it’s not surprising that it’s home to such a large number of organisms. What may be more surprising is that scientists estimate that roughly two-thirds of the species in the ocean, possibly more, have yet to be discovered or officially described. Each year, almost 2,000 new species are accepted by the scientific community — but that still leaves a lot left to discover! And with each new mission to explore the deep, we make new discoveries that increase our understanding of one of the largest, but least understood, living spaces on our planet. And those discoveries aren’t just limited to bizarre and wild marine life.