Life Cycles
A life cycle is defined as the developmental stages that an organism undergoes from its primary stage to the primary stage in its offspring. Life cycles range in complexity from extremely simple—as with unicellular bacteria that begin with fission forming the parent and ends when the parent asexually splits into two daughter cells—to increasingly complex—as with multicellular animals that begin with sexual reproduction and morph through developmental stages where their bodies undergo extreme changes.
The Marine Life Cycle
Although at the surface the ocean can appear calm and quiet, in fact there is an enormous amount of life activity taking place, particularly at certain times of the year. Reproductive strategies abound in the ocean where we have fission, budding, eggs hatching externally, eggs hatching internally, live births, some marine animals are born in freshwater, some are born on land, etc. The marine environment creates unique challenges to the cycle of life, some that have been met with amazing adaptability.
Because of the complexity of marine life, research on marine life cycles is important because it helps document how marine organisms cope with abiotic factors in the marine environment such as ocean currents, tides, light, temperature, and the many other abiotic factors that influence life cycles. For example, habitat plays an important role in the life cycle of many marine organisms. Some select different habitats for different stages of life such as breeding, nesting, juvenile development, and maturity. Tracking migratory marine life through its life cycle is used to understand how a given species survives in its changing environment.
Simple versus Complex Life Cycles
Life cycles are defined as either simple or complex. Many birds and mammals have simple life cycles as they do not undergo major morphological changes through their development.
A Simple Life Cycle
Bladder wrack algae can be used to illustrate a simple life cycle. This life cycle begins when reproductive receptacles form on the plants in autumn. Eggs form on the female while sperm forms on the male. The reproductive organs are fully mature the following summer, and the eggs and sperm are released into the water in great quantities. Females can produce more than one million eggs. The sperm cells find the eggs near the ocean floor, which are then fertilized and a new male or female plant starts to grow.
A Complex Life Cycle
Other types of algae have relatively complex life cycles where more than one generation can occur within a developmental stage and each generation changes radically in appearance. Surprisingly, the simplicity of the organism does not always correspond with the simplicity of the life cycle.
In complex life cycles, the larval or juvenile stages are longer, the developmental stages are more complicated, and the appearance of the organism throughout the developmental stages changes dramatically.
Jellyfish have a complex life cycle that progresses through what is called an alternation of generations where the organism takes two very different physical forms. Jellyfish are commonly recognized in their medusal stage, but they also undergo a polyp stage that occurs during the larval phase. Since jellyfish are either male or female, they reproduce sexually and have gonads found in the lining of their gut. The male transfers sperm by spitting it into the water where it is caught in the mouth of the female where her egg is fertilized. The embryo develops in “brood pouches” on the arms of the mouth structure inside the female jellyfish. The tiny larvae called planula swim out of the mouth or pouches into the water column where they attach at the bottom. After they are attached, the larvae become polyps and divide and bud into juvenile jellyfish, known as ephyra, which go on to develop into the adult medusa.
Although they are simple organisms, cnidarians (hydroids, jellyfish, anemones, and corals) alternate between sessile (or stationary) and mobile forms during their complex life cycle. Other simple animals alternate between a female form that reproduces asexually to male and female forms that reproduce sexually. The all female generation has the advantage of rapid reproduction during seasons where nutrients are plentiful.
Life Cycle Pressures
Many multicellular species must overcome ecological and practical obstacles to successfully complete a life cycle. For example, many species release eggs and sperm into the water where they must meet to form what are known as free-spawned fertilized gametes. The fertilized gametes, or zygotes, must then find nutrients for energy to grow into pelagic larvae. They must also find a benthic site to develop then survive as juveniles without being preyed upon by other marine life.
The way an organism copes with pressures throughout its lifecycle will influence how future life cycle strategies of the species are developed. Successful strategies will produce offspring that carry the life cycle strategy in their genetic structure. Unsuccessful strategies result in the death of offspring, which reduces the likelihood that the strategy will be passed on to the next generation. Marine turtles face enormous pressures throughout their life cycles. They have a particularly long life cycle because they grow very slowly; many take decades to reach adulthood. Juveniles either drift in the ocean currents or live in the same area for years before they return to their nesting beach to breed and lay eggs, often migrating up to 3,000 km. Males and females mate offshore with several partners about a month or two before the eggs are laid. The females store the sperm in their bodies and fertilize between 3-7 clutches (or sets) of eggs each season. At nesting time, the female uses her front flippers to navigate the sandy beach toward her nesting site where she digs a pit for the eggs, still using her front flippers, about 30-60 cm deep. The digging process takes between 30-45 minutes, after which the female spends 10-20 minutes laying hundreds of eggs. She then turns around to use her hind flippers to bury the eggs and returns to sea where she will begin fertilizing the next clutch of eggs. The turtles' sex is determined by the temperature of the nest during incubation. Eggs incubated in warm darkly colored sand produce mostly females, while eggs laid in cool, white sand take a little longer to hatch and produce mostly in males. In about 7-12 weeks the eggs begin to hatch and begin to emerge at night about 2 days later. The hatchlings then find their way to the sea where they feed on tiny marine organisms. It is thought that ocean currents and magnetic fields present help guide the hatchlings back to the nesting beach years later when its their time to mate.
Sexual or Asexual?
Marine organisms have great variation in their reproductive strategies. The evolution of a variety of life cycles among different species is a direct result of the need to reproduce efficiently and effectively so that genetic information is passed down. Asexual reproduction is an energy efficient method used in cases where resources are plentiful and conditions are stable; however, genetic information varies little from generation to generation. Sexual reproduction requires more energy (competition, mate selection, courtship, fertilization), however offspring are more genetically diverse, which facilitates long-term species survival in a changing environment.
Sponges reproduce both sexually and asexually. Sexual reproduction in sponges occurs when sperm is released into the water where it floats to another sponge containing that eggs, which are then fertilized. The fertilized eggs develop into the larval stage of the sponge. Sponges reproduce asexually through budding, a process where a new sponge is created from a piece of the parent sponge.
As mentioned earlier during a description of a complex life cycle, some types of algae switch between sexual and asexual reproduction. The same species of algae may reproduce asexually using spores in one generation, then switch to sexual reproduction using eggs and sperm.
Seasons and Rhythms
Many animals reproduce seasonally according to the time of year and phase of the moon, which can leave a short window of opportunity for sexual reproduction.
In the West Indies, the breeding ritual of a marine fireworm only takes place in spring or summer at sunset during a full moon before the moon rises. The fireworms emerge from crevices in the ocean floor to swim to the surface where eggs are floating. Because they have such specific breeding requirements, the fire worms only breed three or four days each year. Another organism confined to a short breeding period is the grunion, a fish commonly found on California's southern coast. The grunion comes out to mate at dark when the tide is high. They mate in the waves near the shoreline where the fertilized eggs are buried in the sand for two weeks when the next high spring tide washes the sand away.
Many animals breed seasonally so that the young are born during warmer months when more nutrients are available.
Reproductive Strategies
Simpler animals such as corals are often sessile (permanently attached or fixed; not free-moving), which presents a challenge to fertilization in those that reproduce sexually. One solution is to release clouds containing millions of gametes to increase chances of reproduction. At certain times of the year, the ocean is full of milky sperm clouds floating in search of eggs to fertilize. Gametes from the female are often larger, making it easier for the smaller more mobile sperm to find. Slow moving and sessile organisms often use hermaphroditism or parasitism as a reproductive strategy. Barnacles, for example, are hermaphroditic. They use extremely long sex organs to reach another for the transfer of sperm.
A wide variety of reproductive strategies exist in complex animals. Mobile animals often attract mates using signals such as pheromones, visual clues, sound, and competitive courtship behavior. Some species begin life as males and change into females as they mature. Many marine organisms produce large amounts of small eggs that hatch quickly producing large populations and therefore a greater chance for species survival. Other species maintain populations by reproducing multiple times during their life cycle, while other more complex animals, such as large mammals, reproduce less frequently because nursing and raising the young to survive requires time and energy.
Habitat use in Marine Organisms
Many marine species are highly mobile and often migratory; therefore they may rely on a number of different habitats throughout their developmental stages. For example, the South Atlantic peneaid shrimp utilizes marine habitats in deeper ocean waters as its spawning ground and estuarine waters in tidal wetlands for its nursing grounds. Salmon are a well-known known transient marine species. They spawn in freshwater rivers and migrate to the open ocean during the juvenile development stage where nutrients are abundant. Most reef fish, on the other hand, remain in sheltered coral reef ecosystems throughout their life cycles.
Types of Marine Life Cycles
Microscopic Life Cycles
Invertebrates such as jellyfish, sea anemones, ctenophores, sea worms, molluscs, sponges, and tunicates generally reproduce sexually, although some do reproduce asexually. The planktonic environment is extremely important for the life cycles of most of these organisms where they reside during their larval stages. Holoplankton are organisms that remain in the plankton for the duration of their life cycle. Meroplankton remain in the plankton for a portion of their life cycle while releasing larvae into the water column. Both holoplankton and meroplankton species have developed specific characteristics that make their reproduction in this habitat very efficient.
Planktonic and benthic organisms often use sexual reproduction and cling to mates using special appendages during fertilization. Some of the meroplanktonic and holoplanktonic invertebrates release sperm gametes into the water column that the female catches to fertilize her eggs. Other organisms release both sperm and eggs into the water column to form free-spawning gametes. Chemical, mechanical, and environmental cues trigger the release of gametes.
Marine Plant Life Cycles
The life cycle of a plant is multigenerational. Each plant starts with a spore that germinates and develops into an organism, known as a gametophyte that produces its own gametes. When the gametophyte reaches maturity, it can produce its own gametes and its offspring produce spores. These mature offspring are called sporophytes. The life cycle is complete when the sporophyte successfully produces spores. Fungi, protests, and plants have this multigenerational life cycle, which is also referred to as an alternation of generations.
Bacteria versus Multicellular Organisms
Bacteria undergo a haplontic life cycle where a single generation of haploid cells have one set of chromosomes. Many multicellular animals with a one generational life cycle have a diplontic life cycle in which cells have two sets of chromosomes. In diploid organisms each individual sex cell is haploid. When the haploid sperm and the haploid egg are joined, a diploid organism, with two sets of chromosomes, is formed. Plant life cycles are referred to as diplohaplontic because they develop from the gametophyte (haploid generation) to the sporophyte (diploid generation).
Fishes
Fish can have a range of life cycles where all stages of development take place in a small confined area like a pond or a stream to life cycles that take place over thousands of kilometers from streams to oceans and back to streams. The life cycles of Salmoniform fishes encompass almost all of the life cycle types and reproductive strategies. Some fish develop from the egg into the juvenile phase then into the adult phase. Some fish species, particularly those in the deep sea, have larval stages distinctly separate from the juvenile and adult stage. The larvae often look very different from the mature fish. Salmoniform fishes are usually either male or female, but some of the deep sea versions are hermaphroditic, a reproductive strategy common in the deep sea.
Because of the value of salmon and trout fisheries, the life cycles of these two fish are very well documented. The results of this research have been used to identify the origin of salmon captured in ocean water to resolve arguments between nations. This information helps to better manage this valuable commodity.
The reproductive strategies of fishes are varied. Sharks reproduce through internal fertilization and many shark species give birth to live young. Sharks that lay eggs produce large, tough shelled egg sacs often referred to as "mermaid's purses." Both sharks that give birth to live young and those that lay eggs produce relatively small numbers of young making it more important to preserve those species that are becoming overfished. Some bony fishes also bear live young, but most reproduce sexually through the fertilization of eggs joined by sperm in the water column. Females lay an enormous number of eggs to ensure fertilization as many eggs are eaten prior to encountering sperm in the water.
Like sharks, marine mammals also reproduce slowly and give birth to a limited number of young. Female whales, for example, give birth to a single calf and nurse for many months, in some species longer than a year. Female whales and their calves form a strong bond during the nursing period, which helps whale species ensure a high rate of survival as the mother protects her young until the calf has reached a level of development where it can survive on its own.
