Krill in the context of "Crustacean"

Penguins are a group of flightless, semi-aquatic, sea birds which live almost exclusively in the Southern Hemisphere. Only one species, the Galápagos penguin, lives at, and slightly north of, the equator. Highly adapted for life in the ocean water, penguins have countershaded dark and white plumage and flippers for swimming. Most penguins feed on krill, fish, squid and other forms of sea life which they catch with their bills and swallow whole while swimming. A penguin has a spiny tongue and powerful jaws to grip slippery prey.

They spend about half of their lives on land and the other half in the sea. The largest living species is the emperor penguin (Aptenodytes forsteri): on average, adults are about 1.1 m (3 ft 7 in) tall and weigh 35 kg (77 lb). The smallest penguin species is the little blue penguin (Eudyptula minor), also known as the fairy penguin, which stands around 30–33 cm (12–13 in) tall and weighs 1.2–1.3 kg (2.6–2.9 lb). Today, larger penguins generally inhabit colder regions, and smaller penguins inhabit regions with temperate or tropical climates. Some prehistoric penguin species were enormous: as tall or heavy as an adult human. There was a great diversity of species in subantarctic regions, and at least one giant species in a region around 2,000 km south of the equator 35 mya, during the Late Eocene, a climate decidedly warmer than today.

The humpback whale (Megaptera novaeangliae) is a species of baleen whale. It is a rorqual (a member of the family Balaenopteridae) and is the only species in the genus Megaptera. Adults range in length from 14–17 m (46–56 ft) and weigh up to 40 metric tons (44 short tons). The humpback has a distinctive body shape, with long pectoral fins and tubercles on its head. It is known for breaching and other distinctive surface behaviors, making it popular with whale watchers. Males produce a complex song that typically lasts from 4 to 33 minutes.

Found in oceans and seas around the world, humpback whales typically migrate between feeding areas towards the poles and breeding areas near the equator. Their diet consists mostly of krill and small fish, and they usually use bubbles to catch prey. They are polygynandrous breeders, with both sexes having multiple partners. Males will follow females and fight off rivals. Mothers give birth to calves in shallower water. Orcas are the main natural predators of humpback whales. The bodies of humpbacks host barnacles and whale lice.

Nekton or necton (from the Ancient Greek: νηκτόν, romanized: nekton, lit. 'to swim') is any aquatic organism that can actively and persistently propel itself through a water column (i.e. swim) without touching the bottom. Nekton generally have powerful tails and appendages (e.g. fins, pleopods, flippers or jets) that make them strong enough swimmers to counter ocean currents, and have mechanisms for sufficient lift and/or buoyancy to prevent sinking. Examples of extant nekton include most fish (especially pelagic fish like tuna and sharks), marine mammals (cetaceans, sirenia and pinnipeds) and reptiles (specifically sea turtles), penguins, coleoid cephalopods (squids and cuttlefish) and several species of decapod crustaceans (specifically prawns, shrimp and krill).

The term was proposed by German biologist Ernst Haeckel to differentiate between the active swimmers in a body of water, and the plankton that are passively carried along by the current. As a guideline, nektonic organisms have a high Reynolds number (greater than 1000) and planktonic organisms a low one (less than 10). Some organisms begin their life cycle as planktonic eggs and larvae, and transition to nektonic juveniles and adults later in life. This may make distinction difficult when attempting to classify certain plankton-to-nekton species as one or the other. For this reason, some biologists avoid using this term.

Filter feeders are aquatic animals that acquire nutrients by feeding on organic matter, food particles or smaller organisms (bacteria, microalgae and zooplanktons) suspended in water, typically by having the water pass over or through a specialized filtering organ that sieves out and/or traps solids. Filter feeders can play an important role in condensing biomass and removing excess nutrients (such as nitrogen and phosphate) from the local waterbody, and are therefore considered water-cleaning ecosystem engineers. They are also important in bioaccumulation and, as a result, as indicator organisms.

Filter feeders can be sessile, planktonic, nektonic or even neustonic (in the case of the buoy barnacle) depending on the species and the niches they have evolved to occupy. Extant species that rely on such method of feeding encompass numerous phyla, including poriferans (sponges), cnidarians (jellyfish, sea pens and corals), arthropods (krill, mysids and barnacles), molluscs (bivalves, such as clams, scallops and oysters), echinoderms (sea lilies) and chordates (lancelets, sea squirts and salps, as well as many marine vertebrates such as most species of forage fish, American paddlefish, silver and bighead carps, baleen whales, manta ray and three species of sharks—the whale shark, basking shark and megamouth shark). Some water birds such as flamingos and certain duck species, though predominantly terrestrial, are also filter feeders when foraging.

Holoplankton are organisms that are planktic (they live in the water column and cannot swim against a current) for their entire life cycle. Holoplankton can be contrasted with meroplankton, which are planktic organisms that spend part of their life cycle in the benthic zone. Examples of holoplankton include some diatoms, radiolarians, some dinoflagellates, foraminifera, amphipods, krill, copepods, and salps, as well as some gastropod mollusk species. Holoplankton dwell in the pelagic zone as opposed to the benthic zone. Holoplankton include both phytoplankton and zooplankton and vary in size. The most common plankton are protists.

Forage fish, also called prey fish or bait fish, are small pelagic fish that feed on planktons (i.e. planktivores) and other small aquatic organisms (e.g. krill). They are in turn preyed upon by various predators including larger fish, seabirds and marine mammals, this making them keystone species in their aquatic ecosystems.

The typical ocean forage fish feed at the lower trophic level of the food chain, often by filter feeding. They include particularly fishes of the order Clupeiformes (herrings, sardines, shad, hilsa, menhaden, anchovies, and sprats), but also other small fish, including halfbeaks, silversides, smelt such as capelin and goldband fusiliers.

A sympagic environment is one where water exists mostly as a solid, ice, such as a polar ice cap or glacier. Solid sea ice is permeated with channels filled with salty brine. These briny channels and the sea ice itself have its ecology, referred to as "sympagic ecology".

Residents of temperate or tropical climates often assume, mistakenly, that ice and snow are devoid of life. In fact, a number of varieties of algae such as diatoms engage in photosynthesis in arctic and alpine regions of Earth. Other energy sources include Aeolian dust and pollen swept in from other regions. These ecosystems also include bacteria and fungi, as well as animals like flatworms and crustaceans. A number of sympagic worm species are commonly called ice worms.