14.9: Chapter Summary

Ecological Requirements.

The fundamental needs of all species are a place to live, food, safety from predators, and successful reproduction.

Habitat.

The water column, seafloor, and sediments are three fundamentally different habitats. Pelagic organisms must avoid sinking, and most pelagic animals must actively seek food. Benthic epifauna save energy because they do not have to control buoyancy, but they have difficulty avoiding predators and, in most areas, must rely on detrital food that rains down from above or on predation. Benthic infauna have some protection from predators and do not require buoyancy control, but because food in sediments is limited, they must expend energy to move or dig through the sediment, and they must obtain oxygen that is depleted in most sediments below the upper few centimeters. The surface microlayer, intertidal zone, and hydrothermal vents are special habitats that present unique problems and opportunities.

Feeding.

Suspension feeders eat particles suspended in the water column, including living phytoplankton and zooplankton, and detritus. Many suspension feeders are filter feeders that strain water through a mesh-like structure to capture food. Other suspension feeders use mucus to capture food particles, or they are able to grab or grasp particles with arm-like appendages. Many suspension feeders increase their feeding efficiency by pumping water through their collection apparatus, moving the apparatus through the water, or placing it in currents. Many suspension feeders are present among both pelagic fauna and benthic epifauna, and a few benthic infaunal species suspension-feed using a feeding apparatus that they extend above the sediment surface or by pumping water through their tubes or burrows.

Surface grazers consume algae, small sedentary animals, and detritus from the seafloor. Food is abundant only where the seafloor is within the photic zone, where the substrate may be covered with benthic microalgae, macroalgae, and sponges, tunicates, and other animals. In the deep sea, surface grazers eat primarily detritus or bacteria. Surface grazers may feed by using specially adapted mouths on the underside of the body that can either rasp off food organisms or grasp food as they sift through the surface sediment. Others suck off and sift surface sediment to obtain food particles. Some surface grazers also suspension-feed or hunt.

Most deposit feeders move through the sediment, taking sediment into the gut, where they digest organic matter from detritus or coatings. In sediments with anoxic pore waters, deposit feeders must obtain oxygen from the water above or remain in the oxygenated upper sediment layer.

Hunting and Defense.

Speed is used by both hunter and hunted; the hunter chases, and the potential prey seeks to escape. Prolonged movement at high speed in the oceans is energy-intensive because of water resistance. Consequently, all but a few of the largest ocean animals use speed in short bursts rather than in prolonged chases. Other strategies are used to get close to the prey, and then a short burst of speed over the remaining small distance is used to capture the prey. In defense, speed generally is used for immediate escape, followed by other strategies to ensure ultimate safety.

Many marine hunters use lures to attract prey. The lure is typically an appendage on or near the mouth that resembles a bite-sized meal to the prey. Some species use false markings such as eyespots to lure predators into attacking in the wrong direction or at the wrong place as the potential prey speeds off in the opposite direction.

Hunters use camouflage to lie in wait unseen by their prey, or they mimic species that clean other species so that they can approach unsuspecting prey. Many species use camouflage to conceal themselves from predators, or they mimic fiercer or poisonous species to deter predators. Concealment in sediment, within other species, or in holes in reefs and rocks is a much-used defensive strategy. Many species conceal themselves by day and emerge to hunt only at night. Hunters also conceal themselves to ambush prey.

Spines and armored bodies are used by many marine species as defensive mechanisms, and in some cases as weapons. Hunters use crushing claws and jaws to overcome the armor of prey species. Poisons are used by hunters that inject venom into their victims to stun or kill them, and by potential prey species that inject venom into attacking species or make themselves toxic to eat. Many species that are poisonous to predators are brightly colored to advertise their toxicity.

Schooling or cooperation can be used to overcome defenses of a confused prey species, and to confuse predators or frighten them off by creating the appearance of a target much larger than any one individual.

Selected Adaptations in Fishes.

Fish body shapes differ by species to reflect swimming habits. Species that swim continuously have teardrop shapes—a compromise that minimizes total drag, including surface, form, and turbulent drag. Fishes that make fast turns are flattened so that the flat sides can act like a rudder. Fishes that specialize in a quick burst of speed are thickened in the middle by the needed musculature.

Fin shapes also vary with swimming habits. The caudal fin is rounded in fast-maneuvering and -accelerating species, truncated and forked in faster-swimming species, and lunated in the fast, continuous swimmers. Heterocercal caudal fins and pectoral fins in sharks provide lift to maintain buoyancy. Fins are also adapted for special purposes, such as “walking,” defense and venom injection, attachment to substrates or hosts, and even gliding through the air.

Unlike invertebrates, bony fishes have internal body fluids that have lower salinity than seawater, and therefore they must prevent the continual loss of water by osmosis. Marine fishes osmoregulate by drinking seawater and excreting salts through a special gland. Freshwater fishes gain water by osmosis and osmoregulate by producing large volumes of urine. Some species that live where salinity is variable must be able to osmoregulate in both ways.

Most fish species maintain buoyancy by producing and storing low-density oils or by filling a swim bladder with gas. Gas must be released from or added to the swim bladder as the fish changes depth. Oils are favored in abyssal species because gas compressed at depths below about 7000 m is more dense than oil. Some species have no swim bladders, must swim continuously to avoid sinking, and have fins and bodies adapted to provide lift.

Reproduction.

Many species reproduce sexually, either by the direct transfer of sperm from male to female or, more often, by the release of sperm, or of eggs and sperm, to the water column for fertilization. Mating congregations and synchronous spawning by all males and females of a species maximize the probability of egg fertilization, improve egg and larval survival, and maximize genetic diversity.

Because mates may be difficult to locate, many species are hermaphroditic, and some can even function as both sexes at the same time. Asexual reproduction is common in algae and in colonial invertebrates.

To ensure egg fertilization and survival, either very large numbers of eggs must be produced and released or smaller numbers of eggs must be defended against predators. Many fishes and invertebrates release large numbers of eggs to the water column, where they hatch to become meroplankton. Other species lay eggs on the substrate and protect them until they hatch, and still others retain fertilized eggs in or on their bodies until they hatch. Spawning often occurs at times and locations that match the availability of suitable food for larvae. Species with high adult mortality and relatively low larval mortality tend to reproduce early and only once. Species with high egg and larval mortality tend to mature late and spawn more than once.

Many species migrate during their life cycle. Most often they move from adult or seasonal feeding areas to mating or spawning areas. Anadromous and catadromous fishes migrate between freshwater and the ocean.

Associations.

Symbiosis between species is very common in the marine environment. In some associations, one species benefits and the other is disadvantaged (parasitism). In other associations, both species benefit (mutualism), or one species gains and the other is neither advantaged nor disadvantaged (commensalism). Benefits gained from associations include food, living space, camouflage, protection from predators, removal of parasites, and transportation.

Communication and Navigation.

Many marine species have and use vision, but other senses are often more important. These senses are used to locate prey, mates, and predators and for navigation. Sensing of dissolved chemicals, akin to smell or taste, is especially well developed in many species. Certain species generate sounds that are used for echolocation, to stun prey, or to communicate. Many species can sense small changes in pressure, electrical fields, and probably the Earth’s magnetic field.