Introduction to Ocean Sciences

Chapter 16: Marine Ecology

Guide to Reading and Learning

Perhaps among the motivations that brought you to study ocean sciences was a sense of wonder at the incredible profusion of strange and wonderful underwater life that you have seen on video. If so, then that sense of wonderment is likely to be even more intense after you have journeyed through this chapter. Chapter 16 needs little introduction, for it is simply an exploration of the fundamental ecological requirements that differentiate species, illustrated by descriptions of how selected species have adapted to these different requirements.

After you have explored this chapter, you may be lucky enough to go snorkeling or diving in some part of the oceans. If you do so, you will see the world around you through new eyes, for you will have an appreciation of how and why the many species that you encounter present such a tapestry of shapes and colors and why they vary so much. If you are not able to visit the underwater world yourself you can experience much of the same enlightenment and wonder just by visiting a rocky seashore or, with a little more difficulty, a mudflat or beach. You will also be able to enjoy those underwater videos far more than before. Finally, the section on fish adaptations is an excellent stimulant to a visit to an aquarium, aquarium store, or even the fish counter at your local supermarket. Look carefully at a number of fish species and you will be able to deduce a great deal about these species’ lifestyles.

Chapter 16 Essential to Know 

Critical Concepts used in this chapter

CC.14, CC.17

16.1 Ecological Requirements

  • The fundamental needs of all species are a habitat to live in, food to fuel their biochemical processes, a means for a sufficient number of the species to survive predators in order to successfully maintain the species, and a successful strategy for reproduction.

16.2 Habitats

  • There are two fundamentally different major habitats: pelagic and benthic. The benthic habitat is further split into benthic epifaunal and benthic infaunal habitat. Other restricted habitats with special characteristics include the surface microlayer and the intertidal zone.

Pelagic Habitat

  • The advantage of living in the pelagic zone is primarily the greater availability of food from primary producers. The disadvantages are that energy must be expended to provide buoyancy and to provide motility to avoid predators and find food.

Benthic Epifaunal Habitat

  • In the benthic habitat, species save energy by not having to control buoyancy and have a reduced need to move. The disadvantages are that food availability is less and depends primarily on organic matter transported from the overlying photic zone. Also, it is more difficult for benthic species, especially epibenthic species, to avoid detection by or escape from predators.

Benthic Infaunal Habitat

  • Infauna evade easy detection by living buried in the sediments but must use energy to dig into or move through the sediments and must adapt to variations in their environment due to chemical and biochemical processes that take place within the sediment. In particular, just below a sometimes shallow surface layer of sediments, oxygen is consumed and depleted and toxic hydrogen sulfide may be produced within the sediments.

Other Ocean Habitats

  • Limited special habitats include the surface microlayer where surface tension can inhibit sinking of eggs and larvae that are placed there and the intertidal zone and hydrothermal vent communities that take advantage of abundant food but must be able to tolerate variable environmental conditions.

16.3 Feeding

  • There are three major feeding strategies other than hunting other animals. These are suspension feeding, surface grazing, and deposit feeding. Suspension feeders must collect food particles that are dispersed in the water column and surface grazers and deposit feeders must rely on detritus falling from above except in shallow areas that are within the photic zone

Suspension Feeding

  • Suspension feeders consume suspended detritus particles and plankton. Many suspension feeders are filter feeders. Various adaptations including pumping, extension of feeding apparatus into currents, and weak swimming ability allow many filter feeders to filter greater volumes of water. Adaptations for filter feeding include various weblike structures and mucous sheets that filter out suspended particles as the water flows past them. Many suspension feeders have appendages that grasp and capture particles and many of these species feed selectively on particles of a specific type, usually a specific size range.

Pelagic Suspension Feeders

  • Copepods and euphausiids are pelagic suspension feeders that have delicate forelimbs adapted to capture small particles. Most of their close relative shrimp species have forelimbs that are more massive and adapted for capturing or crushing larger food items. Other pelagic filter feeders such as pteropods and salps capture food on mucous sheets and some salps form colonies that cooperate to pump water through the colony. Most jellyfish are suspension feeders.

Benthic Suspension Feeders

  • In soft sediments, infaunal suspension feeders generally either extend tubes to the sediment surface and pump water in filter it and pump it out again(for example, many bivalve mollusks such as cockles) or they extend feeding tentacles up into the water column to capture particles as they drift by (for example, many tube worms, anemones, and sea pens).
  • Benthic epifaunal suspension feeders include a wide variety of types of species including barnacles, which capture particles in their weblike legs that they extend when their shell plates are opened; many species of corals, anemones and related types, which are all polyps where a ring of tentacles equipped with stinging cells surrounds the mouth. Polypoid species opportunistically capture detritus or plankton that drift by them. Many epibenthic suspension feeders including some sea cucumbers, basket stars, and coral seafans enhance their chances of capturing food by extending themselves or their colonies out from the reef or sea floor and orienting themselves so that the currents flow through the extended structure.
  • There are many epibenthic filter feeders including oysters and mussels
  • Epibenthic sponges and tunicates actively pump water through their bodies to enhance the number of particles that are captured by their filtering mechanisms or mucous coatings and often form colonies that can further enhance the water flow.

Surface Grazing

  • Surface grazers consume microalgae and macroalgae that grow on substrates in the photic zone and organic detritus, bacteria, and other animal species that are found on the substrate both in and below the photic zone. Surface grazers, many of which have mouths on their underside, include many snail-like mollusks, both gastropods that have a shell and nudibranchs which do not, and sea urchins.

Deposit Feeding

  • Deposit feeders sift through the sediments and consume organic particles, bacteria, and organic coatings on the particles. Many suspension feeders do this by passing sediment through their gut. Infaunal suspension feeders normally must obtain oxygen from the water column above. Two methods are common: extending tubes upward to the sediment surface through which oxygenated water can be pumped, and living in burrows through which water can be pumped from an intake hole to the animal and then out of an output hole.

16.4 Hunting and Defense

  • Each species has adapted one or more of several strategies for hunting and/or defense. These include speed, lures, camouflage and mimicry, concealment, spines and armor, poisons, and group cooperation.


  • Speed can be used in different ways including sustained swimming, short bursts, and speed of turning. Sustained speed is used for hunting or to avoid prey by relatively few species because only large predators or prey such as sharks, marine mammals, and large fishes can invest the considerable energy used in sustained swimming. Many predators, including a variety of fishes including frogfishes, lizardfishes, and hawkfishes hunt by remaining still and use a short burst of speed only to capture prey that venture too close. Many fish species use short burst of speed and/or their rapid turning ability to evade predators, and many invertebrates such as tube worms and anemones can use speed when a potential predator approaches to withdraw almost instantly from their exposed feeding position into a protective tube or the sand.


  • Lures can be used to attract prey that believes the lure is a food item. For example, frogfishes wiggle a lure on the end of a modified dorsal fin ray. Lures can also be used to confuse potential predators. For example, many butterflyfish and other fishes have a large false eyespot on the rearward half of their body making them appear larger. Many fishes also have color bands or other confusing markings around their eye. The false eyespot and concealed eye may lure the predator to anticipate the wrong direction in which the fish will attempt to escape.

Camouflage and Mimicry

  • Camouflage is an especially favored strategy in the benthic marine habitat and takes many forms. Most often, camouflage is used by prey to avoid detection by predators. It is also used by predators to lie in wait for unsuspecting prey to approach. Camouflage is achieved in different species by both color blending and modification of body shape.
  • Countershading, in which the upper body surface is dull and nonreflective and the lower body surface is white or silvery and highly reflective, is used by many pelagic species to camouflage themselves from potential predators looking from above or below them..
  • Mimicry is used by some species to avoid predators by appearing to be an undesirable or poisonous species, and by predators to approach potential prey by appearing to be a nonthreatening species, sometimes a cleaner species.


  • Concealment is an especially favored defensive strategy by many benthic species that live or have refuges in burrows in the substrate or holes in a reef. Predators may also conceal themselves in or partially buried by substrate until prey approach. Many species are nocturnal, using concealment by day to avoid predators. Many nocturnal and deep species are red in color because red light is absorbed strongly by water and many marine species have eyes that are not sensitive to red wavelengths.

Spines and Armor

  • Many species have evolved thick shells (for example, many mollusks), sharp spines (for example, sea urchins), or hard exoskeltons (for example, shrimp and lobsters) as protection from predators. Certain species also build armored burrows in the sediment or drilled into the reef.


  • Poisons are used by many species either to make themselves unpalatable to predators or to stun or kill their prey. Many species, such as many nudibranchs, sponges, and tunicates, that are poisonous when eaten by potential predators are brightly colored. Many species, including anemones and corals, either stun or kill their prey by injecting the poison with dartlike projectiles fired from nematocysts. Some species, including cone shells and other predatory mollusks, inject venom through a long, slender tube that they can extend to reach their victim. Other species inject venom in their bite and yet others simply excrete the toxic substance to the water. Certain dinoflagellate species produce toxins that can adversely affect humans through ingestion of shellfish and probably by ingestion of aerosols in areas close to coastal and estuarine waters sustaining dinoflagellate blooms.

Group Cooperation

  • Schooling and other group cooperation can be used to confuse predators or to overcome the defenses of stronger but lone competitors. However, the principal advantage of schooling may be that it limits predation because predators can eat less if they periodically find and gorge on a school than they could if their food supply was constantly available.
  • Synchronized mass spawning is a group cooperation strategy used by many marine fishes and invertebrates to ensure a high rate of fertilization but also to enhance survival by limiting the period of time that predators have to gorge on the eggs and larvae.

16.5 Selected Adaptations in Fishes

  • There is an endless variety of different adaptations among marine species of animals. Fishes provide some examples.

Swimming Adaptations

  • Fishes are adapted to reduce surface, form, and turbulent drag in different ways for different species according to their swimming habits. The most efficient body shape for fast, continuous swimming is a rounded body narrowed at the tail and fatter near the front. Tuna, mackerel, and other pelagic predators all have this general body form.
  • Lateral compression of the body aids in making quick stops and turns and is a feature of many fishes that live in coral reefs and other areas where there are many places to shelter once a quick turn has evaded a predator.

Adaptations of Fins

  • Fish fins are adapted in many forms for different lifestyles. The caudal fin has five general types: rounded, truncate, forked, lunate (which, in this sequence, are adapted for faster and more continuous swimming as opposed to slow maneuverability), and heterocercal (which is a form found in sharks that provides lift to avoid sinking).
  • Most fishes swim by undulating their body and caudal fin but many species are adapted to swim primarily by oarlike motions of their pectoral fins (for example, wrasses), or back and forth motions of specially adapted anal and dorsal fins (for example, triggerfishes), which enable them to maneuver better at slow speeds and even to swim in reverse. There are also many other swimming variations used by particular species such as the upright swimming posture of sea horses and ghost pipefish.
  • In some fish species, fins are adapted for other special purposes such as for resting on the seafloor (as in sea perches), for swimming through narrow tortuous holes in a reef (as in the elongated moray eels), for flying out of the water (as in flying fishes), and for attaching to the seafloor or another organism (as in clingfishes and remoras). In other fish species (such as lionfishes and scorpionfishes) the dorsal fins is adapted to inject poisons.


  • Ocean fish species counteract loss of water from their bodies by osmosis by constantly drinking seawater, excreting salt through a special gland in the gills, and producing low volumes of urine. Freshwater fishes gain water constantly by osmosis, and counteract this by not drinking water, absorbing salt through the gills, and producing large volumes of urine.

Swim Bladders and Buoyancy

  • Most pelagic fishes have swim bladders filled with gas to provide buoyancy. Some species, like moray eels, have a special duct so they can rapidly “burp” gas from the swim bladder as they change depth. Others without such a duct cannot change depth quickly and will die if brought to the surface quickly, as the gas in their swim bladder expands at the lower pressure. Other species, especially deep water species, have sacs filled with oil rather than gas. Tuna, mackerel, and other constantly swimming pelagic species have no swim bladder and must constantly expend swimming energy to keep from sinking.

16.6 Reproduction

  • Reproduction of ocean species can take place by separate-sex reproduction, hermaphroditism, asexual reproduction, or by egg laying. Many species can change sex during their lifetimes.

Separate-Sex Reproduction

  • The majority of marine species reproduce be sexual interactions between male and female, which increases genetic diversity.
  • Sexual reproduction occurs by direct transfer of sperm from male to female in some species, which minimizes the number of eggs and sperm that must be produced but requires that a mate be located.
  • Mate location in the oceans is often difficult and some species, such as the anglerfish and some barnacles, avoid this problem by living with a small male permanently attached to a much larger female.
  • Many species of fishes and invertebrates release eggs and sperm to the water column where the eggs are fertilized if they encounter sperm. Synchronous spawning can enhance the probability of fertilization but this strategy requires that large numbers of eggs and sperm be produced to ensure adequate fertilization of eggs.


  • A number of species, including nudibranchs, posses both male and female reproductive organs and can perform either role or both when reproducing.
  • Most barnacles are also hermaphrodites, each having ovaries and a penis with which they can fertilize their neighbor’s eggs.
  • Anemonefish and other species are sequential hermaphrodites. A family living in a particular anemone has a large aggressive male, a smaller female, and a number of immature subadults or juveniles. If the male dies, the female grows and becomes the male and one of the immature individuals matures to become the female.
  • Female to male transition is preferred in species that lay fewer eggs and defend them until they mature. Male to female transition occurs when large numbers of pelagic eggs are produced, such as in the common eastern oyster lifecycle.

Asexual Reproduction

  • Asexual reproduction can take place as in diatoms through binary fission when one individual splits to become two. Asexual reproduction can also take place by fragmentation, where a multicelled organism, such as some macroalgae and some worms and sea stars, can develop a new individual from a fragment broken off the organism. In vegetative reproduction a single individual may divide into many individuals, as is common with many colonial species including some corals, sponges, and anemones.
  • Asexual reproduction produces genetically individual clones, which reduces genetic diversity but may be beneficial in colonial species or species that live in dense congregations, as clones are less aggressive to each other than are sexually reproduced individuals.

Egg Laying

  • Egg laying species are oviparous if they lay large numbers of eggs that hatch on the seafloor or in the water column. They are ovoviviparous if they retain the fertilized eggs until they hatch. The first strategy has the advantage that the species can survive even if there is a very large mortality of eggs and larvae due to predators or lack of suitable habitat, because large numbers of eggs are laid and fertilized. The ovoviviparous strategy has the advantage that eggs and larval mortality may be kept small and fewer eggs must be laid. However, energy must be used to maintain the fertilized eggs until they hatch.
  • Most marine fishes are oviparous. Many release very large numbers of eggs and spawn to the water column. However, in some species such as anemonefishes and damselfishes, the need to produce very large numbers of eggs is reduced by laying eggs on the seafloor and protecting them until they hatch. In other species such as sharks, nudibranchs, octopi, and squid the eggs are encased in tough envelopes or in a gel.
  • Viviparous species, such as marine mammals, nurture their offspring until they are fully developed, but this is energy costly and generally only one or two young can be born in each reproductive cycle.


  • Timing of reproduction during the seasonal cycle can be of great importance. Many species spawn at precise times that ensure that seasonally available food sources such as diatoms in the spring bloom are available when the larvae are at the correct size to utilize this food. If their food source species is not present at the right time this can result in the loss of an entire year class of the spawning species.
  • Timing of reproduction within the life cycle is also important. In species with high adult mortality and relatively low egg and larval mortality, natural selection favors early and one-time reproduction. In species with high egg and larval mortality and relatively low adult mortality, natural selection favors late maturation and reproduction more than once in the life cycle.


  • Many species migrate at different times of the year or at different times within the reproductive cycle. The migration generally can place the adults where major food sources exist at certain times of year and return them to more favorable locations for reproduction. Anadromous and catadromous species use the extreme migration strategy of migrating between freshwater and the oceans, spawning in one and living their adult lives in the other.

16.7 Associations

  • Many species live in some form of symbiotic association with another species. The association can be mutualistic (of mutual benefit), commensal (one species benefits, the other does not but is not disadvantaged), or parasitic (one species is advantaged, the other is disadvantaged). The association of zooxanthellae with corals, which is probably mutualistic, is among the most important in the oceans.
  • There are a large number of species of marine parasites, many of which have multiple life stages and adult forms that are degenerated to little more than a digestive system and reproductive tract.

16.8 Communication and Navigation

  • Many marine species communicate, at least on a rudimentary level, using sound and probably electric fields and other as-yet unknown means. Many marine mammals and certain other species use sound to locate objects and to stun prey.
  • Many pelagic species navigate using as-yet poorly understood abilities to sense chemical compounds or electrical and magnetic fields and they perhaps use other sensing mechanisms as yet unknown.


Critical Concept Reminders:

CC.14 Photosynthesis, Light, and Nutrients (p. 446)

  • Photosynthesis and chemosynthesis are two processes by which simple chemical compounds are made into the organic compounds of living organisms and upon which all species are ultimately dependent. Photosynthesis depends on the availability of light and can only take place in a shallow upper layer of water or on the shallow seafloor. Chemosynthesis does not use light energy but instead uses chemical energy from reduced compounds. Therefore, chemosynthesis can occur in all ocean environments in which oxygen is depleted, but these environments are very limited in extent in the present day oceans. To read CC14 go to page 46CC.

CC.17 Species Diversity and Biodiversity (pp. 447, 477)

  • Biodiversity is an expression of the range of genetic diversity; species diversity; diversity in ecological niches and types of communities of organisms (ecosystem diversity); and diversity of feeding, reproduction and predator avoidance strategies (physiological diversity), within the ecosystem of the specified region. Species diversity is a more precisely-defined term and is a measure of the species richness (number of species) and species evenness (extent to which the community has balanced populations with no dominant species). High diversity and biodiversity are generally associated with ecosystems that are resistant to change. To read CC17 go to page 53CC.


Reduce Text Size Increase Text Size Email Print Page

Chapter Features

Purchase the Introduction to Ocean Sciences, Second Edition ebook

Need technical support? Please visit our helpdesk.

Questions of comments about the site content? Contact the author.

Visit the author’s photo archive

Norton Gradebook

Instructors now have an easy way to collect students’ online quizzes with the Norton Gradebook without flooding their inboxes with e-mails.

Students can track their online quiz scores by setting up their own Student Gradebook.