by Megan Piechowski, RJD Intern
Have you ever wondered how small, sparsely populated fish find each other to mate in the deep, dark ocean? Due to the unusual light organ that is found dangling over the mouth of a certain deep-sea creature it is likely one of the most well-known and recognizable fish that dwells in the deep sea (Picture 1). These fish were discovered ninety years ago and have been studied extensively due to their use of an extremely unique and successful reproduction strategy. Over sixty years ago this discovery remained unfathomable to many. The first scientist to descend into the deep abyss, William Beebe, likely faced much disbelief about these unknown organisms:
“To be driven by impelling odor headlong upon a mate so gigantic, in such immerse and forbidding darkness, and willfully to eat a hole in her soft side, to feel the gradually increasing transfusion of her blood through one’s veins, to lose everything that marked one as other than a worm, to become a brainless, senseless thing that was a fish – this is sheer fiction, beyond all belief unless we have seen the proof of it.”
This deep-sea resident is classification in the suborder Ceratioidei, a division of the order commonly referred to as anglerfish. The anglerfish in Ceratioidei provide a major contribution to the biodiversity of the deep-sea as a primary carnivore in their communities. Due to the difficulties they face finding a mate in their environment these fish have gone to extremes to guarantee successful reproduction upon the discovery of the opposite sex. This group of anglerfish has adapted a strategy allowing the dwarfed males to attach to the females, either temporarily or permanently (Mead). The combination of this drastic size difference and the presence of the female-only light organ demonstrate exceptional sexual dimorphism (Pietsch).
Male anglerfish are one of the smallest known vertebrates and possess large eyes and nostrils, which are instrumental to locate a female emitting species-specific pheromones. The most extreme example of the enormous size difference is found in the species Ceratias holboelli, where the females are half a million times heavier than the males (Pietsch, Orr). This drastic size difference is crucial to the relationship between male and female to minimize the impact to the female once they are attached. However, the process of attachment entails a series of drastic morphological changes to the male. He is able to better latch onto the female’s body by replacing the normal jaw teeth with the development of a pair of pincer-like denticles at the ends of his jaw (Pietsch). In many species the male permanently attaches to the female and becomes a parasite. The outgrowths used to hold onto the female eventually fuse with her skin (Picture 2).
Ultimately, this allows the male to accesses the female’s circulatory system where he is able to survive by removing nutrients from her blood. This relationship creates a situation where the “female appears as a kind of self-fertilizing hermaphrodite”, and the male is been reduced to an organ with a single function of producing sperm (Mead).
This parasitic relationship creates a beneficial situation for both sexes. While the female must feed more frequently to account for the male removing nutrients, she is guaranteed a lifetime of successful spawning seasons. Each season she will produce a large quantity of small eggs, where it is hypothesized that the same male is responsible for fertilizing each season of spawning until the female dies. The permanent attachment allows for an economic use of the small amount of semen present in the dwarfed male. Due to the miniscule size of male anglerfish this relationship benefits him as a source of protection from predators and a free food source (Mead).
Once the pair has successfully reproduced and released fertilized eggs the young spend the majority of their larval development in the epipelagic waters, rarely found below two hundred meters. This is beneficial to the larvae as this zone in the ocean is much more productive and contains more nutrients than the depths where they will spend the rest of their lives. As the young mature their sex typically determines where they are found within the water column. All of the young anglerfish tend to sink to depths between one and three thousand meters, where females are typically distributed in the deeper end of that range (Mead).
Surprisingly, scientists like William Beebe already knew much of the strategies used by these fish. However, recent genetic analysis has shined a light on the dark and bizarre history and distribution of anglerfish. Their parasitic relationship proves to be a successful method of reproduction for these deep-sea creatures as it is understandably difficult to locate a member of the opposite sex every spawning season at two thousand meters. Much of the deep sea has yet to be explored and there are likely many more unfathomable species with extreme survival adaptations that will inspire future generations of marine biologists to explore the depths of our oceans for answers.
REFERENCES
Mead, Giles W., E. Bertelsen, and Daniel M. Cohen. “Deep Sea Research and Oceanographic Abstracts.” SciVerse 11.4 (1964): 569-96. http://iiiprxy.library.miami.edu:2172/science/article/pii/0011747164900038#. Web. 17 Feb. 2013.
Pietsch, Theodore W. “Dimorphism, Parasitism, and Sex Revisited: Modes of Reproduction among Deep-sea Ceratioid Anglerfishes (Teleostei: Lophiiformes).” Ichthyological Research (2005): 207-36. ProQuest. Web. 17 Feb. 2013. <http://search.proquest.com/docview/821813922>.
Pietsch, Theodore W., and James W. Orr. “Phylogenetic Relationships of Deep-Sea Anglerfishes of the Suborder Ceratioidei (Teleostei: Lophiiformes) Based on Morphology.” Copeia 2007 (2007): n. pag. JSTOR. American Society of Ichthyologists and Herpetologists (ASIH), 28 Feb. 2007. Web. 17 Feb. 2013.
By the end of this section, you will be able to:
Some animals produce offspring through asexual reproduction while other animals produce offspring through sexual reproduction. Both methods have advantages and disadvantages. Asexual reproduction produces offspring that are genetically identical to the parent because the offspring are all clones of the original parent. A single individual can produce offspring asexually and large numbers of offspring can be produced quickly; these are two advantages that asexually reproducing organisms have over sexually reproducing organisms. In a stable or predictable environment, asexual reproduction is an effective means of reproduction because all the offspring will be adapted to that environment. In an unstable or unpredictable environment, species that reproduce asexually may be at a disadvantage because all the offspring are genetically identical and may not be adapted to different conditions.
During sexual reproduction, the genetic material of two individuals is combined to produce genetically diverse offspring that differ from their parents. The genetic diversity of sexually produced offspring is thought to give sexually reproducing individuals greater fitness because more of their offspring may survive and reproduce in an unpredictable or changing environment. Species that reproduce sexually (and have separate sexes) must maintain two different types of individuals, males and females. Only half the population (females) can produce the offspring, so fewer offspring will be produced when compared to asexual reproduction. This is a disadvantage of sexual reproduction compared to asexual reproduction.
Asexual reproduction occurs in prokaryotic microorganisms (bacteria and archaea) and in many eukaryotic, single-celled and multi-celled organisms. There are several ways that animals reproduce asexually, the details of which vary among individual species.
Fission, also called binary fission, occurs in some invertebrate, multi-celled organisms. It is in some ways analogous to the process of binary fission of single-celled prokaryotic organisms. The term fission is applied to instances in which an organism appears to split itself into two parts and, if necessary, regenerate the missing parts of each new organism. For example, species of turbellarian flatworms commonly called the planarians, such as Dugesia dorotocephala, are able to separate their bodies into head and tail regions and then regenerate the missing half in each of the two new organisms. Sea anemones (Cnidaria), such as species of the genus Anthopleura (Figure 13.2), will divide along the oral-aboral axis, and sea cucumbers (Echinodermata) of the genus Holothuria, will divide into two halves across the oral-aboral axis and regenerate the other half in each of the resulting individuals.
Budding is a form of asexual reproduction that results from the outgrowth of a part of the body leading to a separation of the “bud” from the original organism and the formation of two individuals, one smaller than the other. Budding occurs commonly in some invertebrate animals such as hydras and corals. In hydras, a bud forms that develops into an adult and breaks away from the main body (Figure 13.3).
View this video to see a hydra budding.
Fragmentation is the breaking of an individual into parts followed by regeneration. If the animal is capable of fragmentation, and the parts are big enough, a separate individual will regrow from each part. Fragmentation may occur through accidental damage, damage from predators, or as a natural form of reproduction. Reproduction through fragmentation is observed in sponges, some cnidarians, turbellarians, echinoderms, and annelids. In some sea stars, a new individual can be regenerated from a broken arm and a piece of the central disc. This sea star (Figure 13.4) is in the process of growing a complete sea star from an arm that has been cut off. Fisheries workers have been known to try to kill the sea stars eating their clam or oyster beds by cutting them in half and throwing them back into the ocean. Unfortunately for the workers, the two parts can each regenerate a new half, resulting in twice as many sea stars to prey upon the oysters and clams.
Parthenogenesis is a form of asexual reproduction in which an egg develops into an individual without being fertilized. The resulting offspring can be either haploid or diploid, depending on the process in the species. Parthenogenesis occurs in invertebrates such as water fleas, rotifers, aphids, stick insects, and ants, wasps, and bees. Ants, bees, and wasps use parthenogenesis to produce haploid males (drones). The diploid females (workers and queens) are the result of a fertilized egg.
Some vertebrate animals—such as certain reptiles, amphibians, and fish—also reproduce through parthenogenesis. Parthenogenesis has been observed in species in which the sexes were separated in terrestrial or marine zoos. Two female Komodo dragons, a hammerhead shark, and a blacktop shark have produced parthenogenic young when the females have been isolated from males. It is possible that the asexual reproduction observed occurred in response to unusual circumstances and would normally not occur.
Sexual reproduction is the combination of reproductive cells from two individuals to form genetically unique offspring. The nature of the individuals that produce the two kinds of gametes can vary, having for example separate sexes or both sexes in each individual. Sex determination, the mechanism that determines which sex an individual develops into, also can vary.
Hermaphroditism occurs in animals in which one individual has both male and female reproductive systems. Invertebrates such as earthworms, slugs, tapeworms, and snails (Figure 13.5) are often hermaphroditic. Hermaphrodites may self-fertilize, but typically they will mate with another of their species, fertilizing each other and both producing offspring. Self-fertilization is more common in animals that have limited mobility or are not motile, such as barnacles and clams. Many species have specific mechanisms in place to prevent self-fertilization, because it is an extreme form of inbreeding and usually produces less fit offspring.
Mammalian sex is determined genetically by the combination of X and Y chromosomes. Individuals homozygous for X (XX) are female and heterozygous individuals (XY) are male. In mammals, the presence of a Y chromosome causes the development of male characteristics and its absence results in female characteristics. The XY system is also found in some insects and plants.
Bird sex determination is dependent on the combination of Z and W chromosomes. Homozygous for Z (ZZ) results in a male and heterozygous (ZW) results in a female. Notice that this system is the opposite of the mammalian system because in birds the female is the sex with the different sex chromosomes. The W appears to be essential in determining the sex of the individual, similar to the Y chromosome in mammals. Some fish, crustaceans, insects (such as butterflies and moths), and reptiles use the ZW system.
More complicated chromosomal sex determining systems also exist. For example, some swordtail fish have three sex chromosomes in a population.
The sex of some other species is not determined by chromosomes, but by some aspect of the environment. Sex determination in alligators, some turtles, and tuataras, for example, is dependent on the temperature during the middle third of egg development. This is referred to as environmental sex determination, or more specifically, as temperature-dependent sex determination. In many turtles, cooler temperatures during egg incubation produce males and warm temperatures produce females, while in many other species of turtles, the reverse is true. In some crocodiles and some turtles, moderate temperatures produce males and both warm and cool temperatures produce females.
Individuals of some species change their sex during their lives, switching from one to the other. If the individual is female first, it is termed protogyny or “first female,” if it is male first, it is termed protandry or “first male.” Oysters are born male, grow in size, and become female and lay eggs. The wrasses, a family of reef fishes, are all sequential hermaphrodites. Some of these species live in closely coordinated schools with a dominant male and a large number of smaller females. If the male dies, a female increases in size, changes sex, and becomes the new dominant male.
The fusion of a sperm and an egg is a process called fertilization. This can occur either inside (internal fertilization) or outside (external fertilization) the body of the female. Humans provide an example of the former, whereas frog reproduction is an example of the latter.
External fertilization usually occurs in aquatic environments where both eggs and sperm are released into the water. After the sperm reaches the egg, fertilization takes place. Most external fertilization happens during the process of spawning where one or several females release their eggs and the male(s) release sperm in the same area, at the same time. The spawning may be triggered by environmental signals, such as water temperature or the length of daylight. Nearly all fish spawn, as do crustaceans (such as crabs and shrimp), mollusks (such as oysters), squid, and echinoderms (such as sea urchins and sea cucumbers). Frogs, corals, mayflies, and mosquitoes also spawn (Figure 13.6).
Internal fertilization occurs most often in terrestrial animals, although some aquatic animals also use this method. Internal fertilization may occur by the male directly depositing sperm in the female during mating. It may also occur by the male depositing sperm in the environment, usually in a protective structure, which a female picks up to deposit the sperm in her reproductive tract. There are three ways that offspring are produced following internal fertilization. In oviparity, fertilized eggs are laid outside the female’s body and develop there, receiving nourishment from the yolk that is a part of the egg (Figure 13.7 a). This occurs in some bony fish, some reptiles, a few cartilaginous fish, some amphibians, a few mammals, and all birds. Most non-avian reptiles and insects produce leathery eggs, while birds and some turtles produce eggs with high concentrations of calcium carbonate in the shell, making them hard. Chicken eggs are an example of a hard shell. The eggs of the egg-laying mammals such as the platypus and echidna are leathery.
In ovoviparity, fertilized eggs are retained in the female, and the embryo obtains its nourishment from the egg’s yolk. The eggs are retained in the female’s body until they hatch inside of her, or she lays the eggs right before they hatch. This process helps protect the eggs until hatching. This occurs in some bony fish (like the platyfish Xiphophorus maculatus, Figure 13.7 b), some sharks, lizards, some snakes (garter snake Thamnophis sirtalis), some vipers, and some invertebrate animals (Madagascar hissing cockroach Gromphadorhina portentosa).
In viviparity the young are born alive. They obtain their nourishment from the female and are born in varying states of maturity. This occurs in most mammals (Figure 13.7 c), some cartilaginous fish, and a few reptiles.
Reproduction may be asexual when one individual produces genetically identical offspring, or sexual when the genetic material from two individuals is combined to produce genetically diverse offspring. Asexual reproduction in animals occurs through fission, budding, fragmentation, and parthenogenesis. Sexual reproduction may involve fertilization inside the body or in the external environment. A species may have separate sexes or combined sexes; when the sexes are combined they may be expressed at different times in the life cycle. The sex of an individual may be determined by various chromosomal systems or environmental factors such as temperature.
Sexual reproduction starts with the combination of a sperm and an egg in a process called fertilization. This can occur either outside the bodies or inside the female. The method of fertilization varies among animals. Some species release the egg and sperm into the environment, some species retain the egg and receive the sperm into the female body and then expel the developing embryo covered with shell, while still other species retain the developing offspring throughout the gestation period.
Answers
asexual reproduction: a mechanism that produces offspring that are genetically identical to the parent
budding: a form of asexual reproduction that results from the outgrowth of a part of an organism leading to a separation from the original animal into two individuals
external fertilization: the fertilization of eggs by sperm outside an animal’s body, often during spawning
fission: (also, binary fission) a form of asexual reproduction in which an organism splits into two separate organisms or two parts that regenerate the missing portions of the body
fragmentation: the breaking of an organism into parts and the growth of a separate individual from each part
hermaphroditism: the state of having both male and female reproductive structures within the same individual
internal fertilization: the fertilization of eggs by sperm inside the body of the female
oviparity: a process by which fertilized eggs are laid outside the female’s body and develop there, receiving nourishment from the yolk that is a part of the egg
ovoviparity: a process by which fertilized eggs are retained within the female; the embryo obtains its nourishment from the egg’s yolk, and the young are fully developed when they are hatched
parthenogenesis: a form of asexual reproduction in which an egg develops into a complete individual without being fertilized
sex determination: the mechanism by which the sex of individuals in sexually reproducing organisms is initially established
sexual reproduction: a form of reproduction in which cells containing genetic material from two individuals combines to produce genetically unique offspring
viviparity: a process in which the young develop within the female and are born in a nonembryonic state