Dive Deeper

Coral reefs are ecosystems that are considered the rainforest of the sea due to the incredible diversity of their inhabitants. While coral reefs cover less than 1 percent of the area of the ocean, they are home to fully 25 percent of marine species. Coral reefs are large geological structures made from the calcium carbonate (CaCO3 ) skeleton of marine cnidarian animals called corals. Individual coral animals are small anemonelike polyps, but they form colonies of hundreds or thousands of individuals. Reef-building corals contain symbiotic algal partners called zooxanthellae. Zooxanthellae are photosynthetic and provide corals with energy in the form of glucose. In return, corals provide zooxanthellae with a protected environment and the compounds needed to photosynthesize. Coral reefs are distributed in tropical oceans because they require shallow, warm, clear, nutrient-poor water to grow.

Seaweed communities exist in shallow, clear waters where light and nutrient conditions are optimal. Thus, seaweed communities are common in temperate and polar regions and are highly productive ecosystems. Kelp forests, such as those formed by the giant kelp, Macrocystis pyrifera, off the Pacific Coast of North America, are among the most well-known, fastest growing, and largest seaweed communities.

The space where freshwater from rivers enters the sea is called an estuary. Estuaries are typically semi-enclosed with some level of mixing of freshwater and saltwater. Estuaries are also commonly known as lagoons or bays. Estuarine habitats are very productive and serve as nurseries for many marine species. These habitats are divided into four major groups based upon their origins.

The vast open ocean is often referred to as the pelagic zone. In the open ocean, many organisms spend their life suspended in water. Food webs in the open ocean are supported by photosynthesis from primary producers, which serve as a major food source for consumers. Both phytoplankton and zooplankton are common inhabitants of the open ocean. Other common inhabitants include small crustaceans, such as krill and copepods. These small crustaceans serve as major food sources for fishes, seabirds, and whales. Gelatinous, transparent animals called salps are also common in the open ocean. Salps feed on phytoplankton through the process of water filtration.

The deep sea is dark and cold and lacking in photosynthesis. Without photosynthesis, many organisms in the deep sea depend on food that sinks down from surface layers. As a result, much of the deep sea is sparsely populated with organisms. Organisms that do occupy this zone are often unusual, alien-like creatures, reminiscent of those from science fiction movies.

The deep sea is constantly dark and cold, and salinity remains relatively uniform. Due to the weight of the water from above, the deep sea is highly pressurized. Most animals of the deep-sea are drab gray, off-white, or black. A few deep-sea fishes and shrimps are bright red, which is a mechanism of camouflage because red light does not penetrate deep water, so the organisms appear black. Animals in the deep sea commonly use bioluminescence for prey attraction, communication, and courtship displays. Because of bioluminescence, some animals of the deep sea do have small, functional eyes, though many are blind.

Seaweeds and plants are multicellular autotrophs, producing their own food through photosynthesis. In coastal areas, seaweeds and plants play an important role and provide many ecosystem services. Seaweeds are multicellular algae commonly found on rocky shores and in other shallow, nearshore marine environments. Sometimes seaweeds are referred to as macrophytes or macroalgae. Large seaweeds are rare in warm, nutrient-poor tropical waters but are common in cold, nutrient-rich temperate waters. Seaweeds are eukaryotic organisms that lack the specialized structures and reproductive mechanisms found in most land plants.

Angiosperms, or flowering plants, are dominant on land but are less abundant in the ocean. Angiosperms are vascular plants that use conductive vessels to transport oxygen, water, and nutrients. Flowering plants have true leaves, stems, and roots. Only a few angiosperms have re-colonized the ocean and live in shallow coastal waters.

Mangroves are trees and shrubs that are adapted to live along tropical and subtropical shores, but like cordgrasses, they do not tolerate total submergence.

The majority of multicellular species on Earth are animals, and of these, more than 97 percent lack a backbone. Animals without a backbone, or row of bones called vertebrae, are called invertebrates. Animals with a backbone are called vertebrates.

Sponges, which belong to the phylum Porifera, are filter-feeding animals made up of aggregations of specialized cells. Sponges lack true tissues and organs. All sponges are sessile, meaning they are fixed in one place. Sponges feed by pumping water through their bodies to strain food particles from water. Almost all sponges are marine, with a few species being found in freshwater habitats. Sponges are incredibly diverse, with a wide variety of shapes, sizes, and colors. They can be found in both deep and shallow marine habitats from the poles to the tropics

Cnidarians (phylum Cnidaria) and Ctenophores (phylum Ctenophora) are common multicellular, heterotrophic, gelatinous animals in marine ecosystems. Cnidarians include jellyfishes or jellies, sea anemones, and corals. Cnidarians have true tissues and exhibit radial symmetry, with body parts arranged around a central axis, like a pie. Although they have no head, Cnidarians have a mouth on one end, which is surrounded by tentacles. All cnidarians are predatory organisms that capture prey items with specialized stinging structures called nematocysts, which are found within specialized cells in the tentacles. There are two basic body forms in cnidarians. The polyp form is sac-like with the mouth and tentacles pointed upward, as in the sessile, adult stage of anemones. The medusa is a jellyfish-like, upside-down polyp that is adapted for swimming. In a medusa, the mouth is oriented toward the substrate. Some cnidarians, like corals and sea anemones, are only polyps; some have both a polyp and medusa stage; and others are only medusae.

Worms are the simplest animals that exhibit bilateral symmetry, which is the arrangement of body parts in such a way that the left and right halves are mirror images. There are many different groups of worms that live in marine environments. We will briefly discuss a few of the major and most common groups of worms here. Flatworms (phylum Platyhelminthes) are called flatworms due to their flattened body shape. These are some of the simplest animals with tissues and organs as well as a central nervous system, where information is stored and processed. Additionally, flatworms have an aggregation of nerve cells in the head, which functions like a simple brain.

Clams, snails, and octopuses are members of phylum Mollusca. Molluscs exhibit incredible diversity and are found in a variety of marine habitats ranging from intertidal communities to deep-sea hydrothermal vents. Many molluscs are consumed by humans across the globe as a source of protein. Overall, they have a soft body enclosed in a calcium carbonate shell. Mollusc bodies are covered by a mantle, or thin layer of tissue that is responsible for secreting the shell. , molluscs have a muscular foot. A radula is a structure unique to molluscs that is formed by a ribbon of small teeth on the ventral surface used for feeding. All molluscs have this basic body plan, but it is often incredibly modified in different species. For example, the shell is absent or is a rudiment in nudibranchs, octopuses, and squids. Additionally, in some molluscs, the radula is modified or absent.

The majority of arthropods are insects; however, arthropods are also dominant in marine habitats. The most diverse group of arthropods in marine habitats are crustaceans, a group that includes crabs, lobsters, shrimp, barnacles, and many other small shelled animals. Horseshoe crabs and pycnogonids are marine arthropods that are not crustaceans. The body of an arthropod is segmented and bilaterally symmetrical, with jointed appendages. Arthropods have an exoskeleton—a tough nonliving external skeleton. Exoskeletons provide support and protection but limit growth because the animal must molt or shed the exoskeleton to grow.

Echinoderms (phylum Echinodermata) are spiny skinned animals and include sea stars, sea urchins, sea cucumbers, feather stars, and sea lilies. Echinoderms are secondarily radially symmetrical, which means that their planktonic larvae are bilaterally symmetrical, and only adults develop radial symmetry. More specifically, adult echinoderms have five-point radial symmetry, giving them a star-like appearance. Most echinoderms have an interior skeleton made of calcium carbonate. Echinoderms are exclusively marine and contain a system of water-filled canals that are used for locomotion as well as for food, waste, and oxygen transport. Echinoderms are capable of regeneration, meaning they can regrow lost or damaged body parts. Some echinoderms can regenerate lost arms, while in some species a lost arm can grow into an entire new individual

The phylum Chordata is divided into three major groups: tunicates, cephalochordates, and vertebrates. Tunicates and cephalochordates lack a backbone, while members of the third group, known as vertebrates, have a backbone. All chordates, including humans, have four major characteristics that they all share at some point in their lives. Specifically, they have a single hollow nerve cord that runs along the dorsal length of the body; pharyngeal gill slits—small openings along the anterior part of the gut; a flexible rod called a notochord, which is used for support and lies between the nerve cord and the gut; and a post-anal tail (a tail that extends beyond the anus).

Vertebrates have a vertebral column or spine, which we call a backbone. The vertebrae protect the nerve cord, or spinal cord, which terminates in a brain that is protected by a skull. Vertebrates originated in the ocean approximately 500 million years ago, and their ancestral forms were fish-like. Approximately 350 million years ago, vertebrates from the ocean came onto land. The earliest land vertebrates came from a lineage called Tetrapod, meaning "four-footed." Reptiles, or air-breathing vertebrates that have scales or bony plates as protective covering, evolved from amphibians. Birds and mammals evolved from different groups of extinct reptiles. More recently, many reptiles, birds, and mammals, returned to the ocean.

The most ancestral fishes currently alive on Earth are the jawless fishes (Agnatha). Jawless fishes feed by suction with a round muscular mouth because they lack jaws. The bodies of jawless fishes are cylindrical and elongated and lack paired fins and scales. Hagfishes and lampreys are the only extant members of jawless fishes. Hagfishes are slimy, deep-water. fishes that feed mostly on dead or dying prey items. Lampreys are primarily freshwater fishes that breed in rivers and lakes but move to the ocean as adults. They are parasitic, attaching to other fishes or invertebrates.

Sharks, skates, rays, and ratfishes are cartilaginous fishes (Chondrichthyes). Cartilaginous fishes have a skeleton made of cartilage, a tissue more flexible than bone. They have movable jaws with well-developed teeth, paired fins, and rough scales called placoid scales. Sharks are mainly fast-swimming predators with a fusiform, or torpedo-shaped, body. Their tail, or caudal fin, is well developed and makes them powerful swimmers. Sharks also often have two dorsal fins on their dorsal (upper) surface. Their paired pectoral fins are broad and wide and are used for steering. Sharks have five to seven gill slits behind their head on each side of their body. Sharks vary in size from the tiny spined pygmy sharks (Squaliolus laticaudus) to the giant whale shark (Rhincodon typus), the largest fish in the sea. Although they are found throughout the oceans from shallow to deeper waters, most sharks are concentrated in the tropical seas.

The majority of fishes are bony fishes (Osteichthyes) and have a skeleton made at least partially of bone. Bony fish live in marine habitats, from tide pools to the abyss, and are an extremely diverse group, consisting of the largest class of extant vertebrates on Earth. They have highly modified fins and show great diversity in body shape, size, color, feeding habits, and reproductive patterns. Bony fish use coloration and body shape for a locomotive advantage or for camouflage. The jaws of bony fishes are protrusible, meaning they project outward from the mouth. Many bony fishes have a swim bladder, which is a gas-filled balloon-like sac about the stomach that aids in buoyancy. Bony fish are a major protein source for humans across the globe.

Marine reptiles include lizards, snakes, turtles, and crocodiles that live in the ocean. Marine reptiles first appeared 300 million years ago and thrived in the ocean in the Age of Reptiles. Now, only a few reptiles still roam the sea. Marine reptiles have skin covered in scales and salt glands to concentrate and excrete excess salts. The majority of marine reptiles must still return to land to lay eggs.

Birds, in general, evolved from fast-running dinosaurs approximately 160 million years ago. Only a small portion of birds on Earth are considered seabirds. Seabirds are birds that spend a significant portion of their lives at sea and feed mainly on marine organisms. The majority of seabirds live in the southern hemisphere. Many seabirds, such as albatrosses, have webbed feet, salt glands in their head to help excrete excess salts, and breed in large colonies. Most seabirds are predators of fishes, squids, and bottom invertebrates.

Mammals evolved from now-extinct reptiles approximately 200 million years ago. About 65 million years ago, mammals underwent a rapid diversification when dinosaurs were no longer dominant. All mammals, with only a few exceptions, produce live offspring instead of laying eggs. Newborn mammals are fed by the mother’s mammary glands, and the mother invests a lot of time and energy into parental care. At least five different groups of land mammals have independently and successfully invaded the ocean. The ocean is the only uniting feature of the different groups of marine mammals.