Reproduction, development, and life history

What is a life history?

Each animal species can be viewed as a collection of individuals, all sharing a common pool of genes. The genes determine all of the animal's characteristics, and are made of the complex molecule known as DNA. The genes are exchanged among the members of the species through a variety of mechanisms, all of which are related in some way to the process of reproduction. For this reason, the ability to reproduce is often considered to be one of the most important defining characteristics of life. But reproduction, like most basic biological functions, occurs primarily at the level of a single individual that may interact with another individual to exchange genetic information through the transfer of DNA. This individual parent animal reproduces to form another new individual offspring. The offspring, however, begins its individual life as a single cell rather than as a multicellular animal like its parent. If it is to reproduce, it must usually become multicellular as well, and must ultimately mature to be anatomically similar to its parent. This entire cycle of development is generally known as the life history of an individual animal. In other words, the life history of any individual animal comprises the processes of reproduction and development; in turn, each of these processes is broken down into other more specific processes, each designed to accomplish a particular function.

Asexual reproduction

Fundamentally, reproduction is the copying of an individual animal's DNA combined with transferring the copy into a newly formed individual. In some cases, the copy of the DNA is nearly exact, and the offspring develop from a single parent. This is typically the case in what is known as asexual reproduction. In its truest form, asexual reproduction involves simple cell division, or mitosis, without any reorganization of DNA fragments during the process. Asexual reproduction also involves only one parent, and thus involves no new combinations of genes resulting from mixing complementary genes from two parents. Asexual reproduction has the advantage of allowing a very fast rate of reproduction, with a resulting rapid increase in the population of a given species. The primary disadvantage of asexual reproduction is that it does not permit much genetic variability; as a result, the population as a whole is relatively unable to adapt to diverse or changing environmental conditions.

Sexual reproduction

In most cases, the DNA copy is not exact, so that the genetic makeup of the newly formed offspring differs from that of its parent. This programmed variability is accomplished primarily by sexual reproduction. The genetic process that defines sexual reproduction occurs only during a very brief specialized phase of cell division, and only within cells belonging to the germ cell line. A germ cell is defined as a cell belonging to a cellular lineage that, at some point, will deviate from normal cell division (i.e., mitosis, which results in exact duplicate copies of DNA) to engage in meiosis. Meiosis is often known as reductional division, because it results in reducing the number of chromosomes by half in preparation for an exchange with the complementary chromosome of a mating partner that restores the full set. The most important aspect of meiosis, however, occurs long before this reduction of chromosome number, which occurs late in meiosis. Early in meiosis, gene segments are actually recombined by the exchange of DNA sequences, so that the individual chromosomes are transformed into genetically unique combinations of genes. This process is known as crossing over; it is the defining event that distinguishes sexual from asexual reproduction. In this way, germ cells become genetically unique while they are still in the parent animal, long before they become fully formed sperm or oocytes, and even longer before they fuse with gametes from another parent. At the later point of gamete fusion, the recombined genes in the chromosomes from two parents will fertilize each other to produce new chromosome combinations. Thus, the genetic recombination that takes place during meiosis, combined with the reaggregation of the chromosomes that occurs during fertilization, results in the distinctive offspring that characterize most animals. It is important, however, to recognize that sexual reproduction does not require either fertilization or two parents. If meiosis takes place, the reproduction is sexual. In some lower metazoans, a single parent animal may produce gametes by meiosis, which can then develop into fully formed offspring by various processes known collectively as sexual parthenogenesis.

Reproductive duality

Many animals, particularly the lower metazoans, actually use both asexual and sexual reproduction at various times. This reproductive duality gives them the advantages of both modes. It is rare for both asexual and sexual reproduction to occur simultaneously, however. In many lower metazoans, the asexual and sexual processes are cyclical, occurring in different seasons. Examples include many marine sponges (phylum Porifera), in which a single individual may alternate between reproductive modes. Another example may be seen among the freshwater rotifers (phylum Rotifera), in which the two reproductive modes will be restricted to sequential generations that are otherwise anatomically similar. In other lower metazoan groups, asexual and sexual processes are segregated into two distinct stages in the life history, with radically different anatomical and behavioral traits, or even different habitats. The best examples of these stages are found among the hydroids and jellyfishes (phylum Cnidaria), and among the parasitic flatworms (phylum Platyhelminthes). Since the life history stages responsible for asexual and sexual phases are so distinctive in these groups, and occur in a regularly alternating pattern, this general reproductive strategy is often called alternation of generations.

Gametogenesis

During sexual reproduction, each parent animal must form specialized cells known as gametes, which are genetically recombined and in which the chromosome number is reduced by half from a diploid double set to a haploid single set. Both processes occur during meiosis, which is the first stage of gamete formation, or gametogenesis. In virtually all animals that reproduce sexually, the gametes occur in two morphologically distinct forms corresponding to male and female. These distinctions in form and structure are related to the specific functions of each gamete. The differences become apparent during the latter stages of spermatogenesis (for male gametes) and oogenesis (for female gametes).

After spermatogenic meiosis, the morphological transformation of the male gamete generally includes development of a small motile sperm. The sperm's function is to move toward and ultimately meet the female gamete, beginning a sequence of events that ends in the fusion of the two gametes. After the gametes fuse, the sperm's role is essentially complete except for the final genetic contribution from its half of the new offspring's genome. Thus, the primary structures developed by the sperm are concerned with movement and with engaging the female gamete and its coatings. The specific locomotory structures of sperm vary among the lower metazoans, ranging from pseudopodia (temporary extensions of cell material) resembling amoebas in the roundworms (phylum Nemata) to flagella (long whiplike projections) in most other groups.

After oogenetic meiosis, the morphological transformation of the female gamete generally includes development of a large oocyte that does not move around. The oocyte's functions are far more numerous than those of the sperm. For most lower metazoan groups, they include equal or greater participation in the process of gamete fusion. Following fusion, however, the oocyte must provide the coordination of and materials for all the early stages of embryo and larval development.

To carry out these functions, the oocyte must build up large stores of energy-rich nutrients (e.g., carbohydrates in the form of glycogen, and lipid or proteinaceous yolk); phospholipid stores for membrane synthesis; extra nucleotides or redundant DNA; transcripts of RNA for protein synthesis; extra regulatory and structural proteins; and occasionally materials for eventual eggshell formation. In most cases, the oocyte must fulfill all these functions by itself. Some animals, however, use other germ cells to assist the oocyte. Perhaps the best example of this assistance is found among the parasitic flatworms (phylum Platyhelminthes). These organisms have special vitelline (resembling yolk) germ cells that never become gametes but instead supply the gametes with needed materials. The oogenic stage that is most often involved in fertilization is the oocyte; however, there are some exceptions to this generalization. The ambiguous term "egg" is often applied to oocytes and other fertilizable stages of female gametes. "Egg" may, however, also refer to fully formed embryos or juveniles within various embryonic coverings, so the word should be avoided in most instances.

Spermatogenesis and oogenesis most often occur in different individual animals known as males and females respectively. This differentiation of sexes is known as gonochorism. Alternatively, it is quite common for the same individual to produce both sperm and oocytes. This condition is known as hermaphroditism; it may involve either simultaneous or sequential production of sperm and oocytes. Oogenesis and spermatogenesis may occur in different gonads, namely ovaries and testes, or may occur in a single hermaphroditic gonad. Such lower metazoans as sponges (phylum Porifera) may lack distinct gonads, with gametes developing in normally somatic regions of the body. Whether through gonochorism or hermaphroditism, gametogenesis may occur throughout the adult life of the animal, as in parasitic flatworms (phylum Platyhelminthes). The more common pattern among the lower metazoans, however, is one of seasonal reproduction.

Gamete exchange

Gametes come together in a variety of ways among the animals in the lower metazoan groups. Sperm are generally motile and engage in oocyte-seeking behavior of some sort. The small size and short-term motility of the sperm, however, mean that their efforts are effective for only a very short time; thus sperm motility is only effective for meeting the oocyte within very small spaces. Bringing the sperm and oocyte into these small spaces depends on the behavior of the parent animal, which must engage in some form of gamete exchange.

Adult animals fall into two broad categories in relation to gamete exchange: spawners and copulators. Lower metazoans demonstrate a broad range of variations within both of these categories. The vast majority of spawners release their gametes directly into the surrounding water, an activity known as broadcast spawning. Broadcast spawning is the most common method of gamete exchange in free-living marine invertebrates, but is rare among most freshwater groups. In the very lowest phyla (e.g., Porifera, Placozoa, Cnidaria) spawning is the only method of gamete exchange. In some groups, only the males spawn while the females take up the sperm while retaining their oocytes for internal fertilization. In others, both sperm and oocytes are spawned, resulting in external fertilization. For broadcast spawning with external fertilization to be successful, the parent animal must use some strategy to increase the chances of the gametes coming together. The most common strategy involves simultaneous spawning, in which all gametes are released at the same time by all members of a population. Other strategies include producing gametes with similar densities, adhesive properties, or other features that cause them to settle out into the same general parts of the water column or substrate.

Copulation occurs in many groups, and varies considerably among the lower metazoans. In all cases, there is some mechanism for transferring sperm directly from the male to the female. This transfer may occur by direct injection or by transfer of sperm packets known as spermatophores. Most copulators have specialized genital structures for transferring the sperm. Most often these structures inject the sperm directly into the female's reproductive system, but they may also inject them through the body wall, as in the hypodermic traumatic insemination seen in some turbellarian flatworms (phylum Platyhelminthes).

In many animals, the males and females are morphologically distinct, thus exhibiting sexual dimorphism. Sexual dimorphism is, however, more common among higher animals. The males and females of many lower metazoans are sexually monomorphic, and thus are distinguishable only microscopically by the nature of their gametes. Generally speaking, most broadcast spawners are sexually monomorphic, while copulators tend to be sexually dimorphic. The dimorphism may relate only to differences in copulatory structures or genitalia, but in some cases it may relate to differences in other habits or roles of the two sexes. Marked somatic dimorphism is much more common among higher animals, but does occur in some lower metazoans; examples include the schistosomatid flukes (phylum Platyhelminthes) and certain roundworms (phylum Nemata). In both of these cases, the dimorphism reflects differences in size as well as form.

Fertilization

After the gametes come together, they must fuse with one another to form a diploid zygote, which is the genetically complete new animal in a unicellular (single-celled) form. Fertilization is not a single event; rather, it is a complex series of events in which both sperm and oocyte actively participate. It begins with simple recognition among the sperm and oocytes of a given species, and concludes with the fusion of the haploid pronucleus of the sperm and the haploid pronucleus of the oocyte. Between these two events, there is considerable variation among the lower metazoans. In some cases, the oocytes have a covering that must be penetrated by the sperm. In most cases, the sperm must initiate the actual process of fusion of the cell membranes of the two gametes; however, it is the oocyte that generally is most active in directing the fusion and actually incorporating the sperm into its cytoplasm. The actual fusion of the two cells, known as syngamy, is accomplished largely by the oocyte. After syngamy occurs, pronuclear fusion may follow rapidly or be delayed up to several days, depending on the species. At this point, the new genetically unique and genetically complete animal, in the form of a single-celled zygote, is ready to complete the development of its body.

Embryogenesis

The development of a fully functional animal body begins with the transformation of the single-celled zygote into a multicellular embryo. Part of the definition of an animal (i.e., a metazoan) is that it possesses true multicellularity, which it acquires during the process of embryogenesis. True multicellularity is defined not only as the possession of multiple cells in the body, but also a specific division of labor among those cells. In particular, the body of an animal must have somatic (body) cells separated from the reproductive (germ) cells. But, beyond that the somatic cells must be differentiated into different functional groups. During the process of embryogenesis, the new individual first develops numerous cells, then differentiates them according to space, structure, and function. Among the lower metazoans, we find a range of structural complexity, from simple double-layered groups of cells in the phylum Placozoa to complex organ systems in such phyla as the Platyhelminthes, Nemertea, and Nemata. Many other lower metazoan phyla fall at various places along this spectrum.

The first stage of embryogenesis is cleavage, which creates the simplest form of multicellularity. Cleavage is simply a series of mitotic cell divisions that take the new individual from a unicellular zygote to a multicellular mass of cells generally known as a morula. Depending on the phylum and species, this cell mass may consist of a few dozen to hundreds of cells. The actual pattern of cleavage varies in terms of the cells' spatial relationships to one another, the plane of the cell axis at which cleavage occurs, and the degree to which the cytoplasm is divided. The subject of cleavage is complex, but the type of cleavage is important in defining a group's evolutionary relationships to other groups. There is also great variation in the degree of cellular differentiation at this stage. In most lower metazoans, however, the cells do differentiate during cleavage into large macromeres and small micromeres; some groups also have intermediate-sized mesomeres. In most cases, these classes of cells are destined to become specific layers within the later embryo. In turn, each layer will give rise to certain tissues of the adult body. Prior to forming layers, most embryos go through a stage of minor reorganization known as blastulation, which provides the spatial framework in which the actual layering takes place.

The vast majority of animals develop either two or three layers of cells, known as germ layers. These germ layers develop out of the dramatic reorganization of the cells that were formed during cleavage and slightly reorganized during blastulation. The stage of radical reorganization is called gastrulation. Possession of two or fewer germ layers is found only in certain lower metazoan phyla. A very few simple phyla (e.g., the Placozoa, Monoblastozoa, Orthonectida, and Rhombozoa) do not have specific germ layers. Sponges (phylum Porifera) are often interpreted as having no germ layers, but some biologists regard them as having two. Members of the phyla Cnidaria (jellyfishes, corals, etc.) and Ctenophora posses two well-developed germ layers, and thus are described as diploblastic. All other lower metazoans, and indeed all other animal phyla, possess three well-developed germ layers, and are correspondingly described as triploblastic. The actual mechanisms of layering vary widely among different phyla; they range from the migration of cells into the interior of the cell mass to the infolding of an entire hemisphere of the spherical blastula. In all cases, however, the embryo is left with an endoderm layer on the inside and an ectoderm layer on the outside. In triploblastic phyla, a layer of mesoderm forms between the other two. The endoderm typically develops into the digestive system of the adult animal, while the ectoderm develops into the epidermis and nervous system. The mesoderm, if one is present, gives rise to such structures as excretory systems and the tissues that line body cavities. Other organ systems develop from various layers, depending on the phylum, and most often involve contributions from two germ layers working cooperatively.

At the end of gastrulation the embryo is fully formed; embryogenesis is complete. The new organism now looks a bit like an animal with a skin and a gut; all the basic layers are present, ready to differentiate further into a more definitive animal having the recognizable characteristics of its taxonomic group.

Most higher animals, above the platyhelminthes, can be divided into two groups based primarily on embryonic features. These two major branches are known as the protostomes and the deuterostomes. Protostomes undergo determinate cleavage or mosaic development, in contrast to the indeterminate cleavage or regulative development of deuterostomes. The determinate cleavage of protostomes results from a plane of cell division, usually visible after the second division, that cuts diagonally across the original zygote axis, thus compartmentalizing different regulative and nutritive chemicals in each of the resulting cells. This is referred to as spiral cleavage, since the cells dividing diagonally appear under the microscope to spiral around the original axis. In contrast, the indeterminate cleavage of deuterostomes results from a planes of cell division that cut alternatively longitudinally along the zygote axis, then transversely across the axis, thus leaving each resulting tier of cells with similar regulative and nutritive chemicals. This is referred to as radial cleavage, since the cells dividing at alternating parallel and right angles to the original axis appear under the microscope to radiate in parallel planes from that axis. The most important thing is not whether the resulting cell masses appear to spiral or to radiate, but that only the spiraling cells of the protostomes show determination of specific germ layers as early as the first cell division, and almost universally by the third. Thus, at the very earliest stages of cleavages, specific cells of protostomes have already been determined to a fate of forming a specific one of the three germ layers. During gastrulation, the embryo is left with an opening to the outside called the blastopore, which will develop into an opening into the gut in the adult animal. The precise nature of the opening is the second major feature differentiating deuterostomes from protostomes. In protostomes, the blastopore becomes the adult mouth, whereas in deuterostomes, the blastopore becomes the anus. Shortly or immediately after gastrulation is complete, higher animals form their body cavity, the coelom. By definition, the true coelom is always a body cavity within mesodermal tissue. The mechanism by which the coelom is formed is the third primary distinction between protostomes and deuterostomes. In most deuterostomes, the coelom forms by outpocketing from the original archenteron, a process known as enterocoely since the coelomic cavities are thus derived directly from embryonic enteric cavities. In protostomes, the coelom forms from a split in the previously solid mass of mesodermal cells, a process thus known as schizocoely. There are some exceptions to this rule, but it applies well to most.

Postembryonic development

Most animal embryos look rather similar to one another up through the stage of gastrulation. It is during the important stages of postembryonic development, however, that the characteristic features of specific phyla, classes, and orders finally emerge. For this reason, much of postembryonic development is said to involve morphogenesis, or the establishment of the animal's definitive body form. Along with the completion of form comes the establishment of function, so that the end result is a fully functional animal. For some species, this fully functional individual will be a juvenile, which resembles an adult in form but lacks a mature reproductive system. Development that proceeds from embryo to juvenile with no intervening stage is known as direct development. Direct development occurs in some lower metazoans, including the nematodes, gnathostomulids, rotifers, and gastrotrichs. In contrast, most lower metazoans undergo indirect development, in which a larval stage is inserted between the embryo and the juvenile or adult.

A larva is a fully functional animal, generally feeding and moving about independently. The larval form of a given species is generally as characteristic of the species as the adult, and may complete some critical parts of the life history strategy for its species. The most common task of larvae is long-distance migration in order to colonize new environments for the species. This phenomenon, known as planktonic dispersal, is especially critical in marine species whose adult forms have limited or no mobility, such as corals, sponges, ribbon-worms, and polyclad flatworms. Larvae may also make use of food resources that differ from those needed by the parent, thus avoiding competition within the species. Because of the critical and distinctive attributes of larvae, their formation is frequently referred to as larvigenesis, and represents a discrete (separate) stage of postembryonic morphogenesis.

By definition, larvae and adults are dissimilar in structure and function. For this reason, the transition from larva to adult requires radical changes in the morphological, behavioral, and physiological characteristics of the animal. This transformation between successive postembryonic forms is known as metamorphosis. As with cleavage and gastrulation, the exact mechanism of metamorphosis varies widely among different lower metazoan phyla. In all, it involves the loss of some specifically larval structures and the development of new adult structures from groups of undifferentiated cells.

Sexual maturation

After the embryo or larva is transformed into a juvenile form, all that remains for the individual is maturation of its sexual reproductive systems, accompanied or followed by meiotic maturation of the gametes to form fully functional and fertilizable sperm and oocytes. The mechanisms for maturation vary widely among animal phyla, but are especially diverse among the lower metazoans. Such phyla as placozoans and sponges have no discernible gonads; the gametes simply form out of previously undifferentiated somatic cells. In such others as the diploblastic cnidarians, the gametes develop from specialized cells within one of the germ layers, but no other gonadal tissues are present. At the other end of the scale of complexity, such animals as roundworms and flatworms develop elaborate gonads and ducts; these structures include discrete testes with sperm ducts, discrete ovaries with oviducts, and even numerous types of specialized reproductive glands. In some respects, the parasitic flatworms have the most structurally sophisticated reproductive systems in the animal kingdom, even though as a group they are considered primitive organisms.

Another variation that occurs among the lower metazoa relates to reproductive seasonality. While some groups, such as tapeworms and flukes, reproduce continually and thus retain all of their sex organs, such others as hydroids and turbellarian flatworms reproduce only temporarily, but repeatedly. These organisms often lose their reproductive organs during their nonreproductive periods and redevelop them during the next mating season.

Structural maturation of the sex organs and gametes is often accompanied by certain changes in behavior as well as the development of special structures that are not gonadal but are nonetheless related to reproduction. The most obvious behavioral changes involve mate-seeking and copulatory behaviors. Structural changes include the development of special genitalia for coupling. The latter includes the copulatory cirrus (flexible penis) of parasitic flatworms and the copulatory spicules of many roundworms. Other related behavior may include various forms of brooding or other parental care strategies. While parental care is not generally as common or as well developed among lower metazoans as in higher animals, some examples do exist. Among the lower metazoans, most of these are more structural than behavioral. Examples include the maintenance of amphiblastula larvae within specially adapted radial canals of calcareous sponges; the development of special egg-enclosure organs in several tapeworms; the retention of fully-formed juveniles within the uterus of some roundworms; and the retention of successive generations as colonies in many hydroids and other cnidarians.

Phylum summaries

Brief summaries of the primary reproductive and developmental strategies of each lower metazoan and deuterostome phyla follow. The variations are within each phylum are great, however, and the short summaries below are intended only to situate each phylum within the overall context of reproductive strategies and processes discussed earlier. Interested readers should consult the references listed for further details and analyses.

Phylum Placozoa

The limited information on Trichoplax adhaerens, the only known species of Placozoa, indicates that the organism has no capacity for sexual reproduction. Asexual reproduction, however, is very effective and diverse, occurring in three distinct modes. Simple fission, or division of the simple body by cell separation, is the most common mode. More rarely, two types of budding occur. In one form, hollow swarmers bud off the parent organism and may swim to remote locations to develop further. In other individuals, the attached buds may stretch and attach themselves to adjacent substrates before detaching from the parent. In both cases, cellular rearrangements similar to gastrulation occur.

Phylum Monoblastozoa

This phylum, represented only by the single genus Salinella, has been observed and described by only one author, and some researchers question its existence. The original description is vague, but describes asexual reproduction by a sort of transverse fission similar to that of placozoans.

Phylum Porifera

Sponges may be either gonochoristic or hermaphroditic, but most undergo some form of sexual reproduction. The exact origin of germ cells varies somewhat among species, but most sperm and oocytes develop from undifferentiated cells known as archeocytes in the central mesohyl (connective tissue) layer. In some species, sperm may develop through transformation of the flagellated collar cells that line the sponge's chambers and create the water currents responsible for the exchange of all materials within the sponge. Males are broadcast spawners, but most sponges undergo internal fertilization. Fertilization is followed by internal brooding of larvae in many sponges, including most marine calcareous sponges and the spongillid family of freshwater sponges. Whether sponges have true germ layers is often debated, since some cells can transform into any cell type even in the adult; however, cellular rearrangements comparable to gastrulation take place at the end of embryogenesis. Sponges are perhaps the most efficient phylum in the animal kingdom for asexual reproduction. They employ a number of different strategies ranging from simple fragmentation of the adult body to formation of specialized gemmules (reproductive buds), the latter being more common in the overwintering stages of freshwater species.

Phylum Cnidaria

Sexual reproduction is well developed throughout the phylum. Gonochoristic and hermaphroditic species are known to occur; however, the true jellyfishes (class Scyphozoa) and colonial hydroids (class Hydrozoa) are primarily gonochoristic. Germ cells develop in either the ectoderm or endoderm, depending on the class, but always originate from undifferentiated cells known as interstitial cells. Most species of cnidarians are broadcast spawners, but in some, such as the freshwater Hydra, the oocyte may be retained for internal fertilization. In most species, embryonic development leads to a planula larva that settles to the substrate for metamorphosis into the adult cnidarian. Asexual reproduction is very common and takes many different forms. Many species undergo alternation of generations, with asexually produced medusae (free-swimming jellyfish) alternating with sexually produced polyps attached to the substrate. Sexual and asexual mechanisms of reproduction may occur in either stage, however, depending on the species.

Phylum Ctenophora

Comb jellies are primarily hermaphroditic, with only a few gonochoristic species. They have simple gonads resembling those of the closely related cnidarians. Most are broadcast spawners, but some are fertilized internally and may even brood their larvae. Embryogenesis results in a cydippid larva that swims freely during its metamorphosis into an adult. Asexual reproduction is not known to occur in this phylum.

Phylum Rhombozoa

The dicyemid mesozoans are all parasites, and alternate between sexual stages in the adult host and asexual stages in the juvenile host. The sexual forms are hermaphroditic. These animals are structurally simple, barely qualifying as truly multicellular. They lack layers comparable to the germ layers of other animal phyla.

Phylum Orthonectida

Orthonectids are parasites that alternate between sexual and asexual stages within their host animal. Asexually produced plasmodia may develop into sexual forms, most of which are gonochoristic, with a few hermaphroditic species. Copulation is followed by internal fertilization, and the larva ultimately leaves the parent to seek a new host.

Phylum Platyhelminthes

The vast majority of flatworms are hermaphroditic, but some gonochoristic forms occur, including the medically important schistosome flukes. The reproductive systems of predominantly free-living turbellarians are simple and transient (temporary), whereas those of the parasitic tapeworms and flukes are complex and permanent, with many specialized organs. Copulation is the rule for reproduction in this phylum, followed by internal fertilization and either internal or external development. Fertilization generally involves incorporation of the full sperm into the oocyte. Internal development often takes place within specialized structures for maternal care of the larvae. Cleavage and embryogenesis occur in patterns unique to this phylum, especially among the tapeworms and flukes; there are many different forms of larvae and patterns of metamorphosis in this group. In addition, the tapeworms and flukes engage in regular alternation of sexual and asexual generations, perhaps producing the greatest number of progeny in the animal kingdom.

Phylum Nemertea

Nemerteans are primarily gonochoristic (except for the few freshwater and terrestrial species), with large but simple gonads. Most species are marine, and reproduce by broadcast spawning followed by external fertilization and embryonic development. They undergo spiral cleavage, and thus are generally considered to be related to the protostomes. Postembryonic development leads to formation of a pilidium larva in most nemerteans. Some species may reproduce asexually by fragmentation, but this pattern is uncommon.

Phylum Nemata (Nematoda)

Sexual reproduction is the rule among the roundworms; most species are gonochoristic with some sexual dimorphism. Some hermaphroditic species do exist. Reproductive systems are tubular; in copulation, the male introduces amoeboid sperm into the vagina of the female. The embryogenetic process begins with an unusual form of bilateral cleavage, which ends with the direct development of a juvenile form (often incorrectly called a larva) that is structurally like a miniature adult. There are five juvenile molts before the adult form is reached. Asexual reproduction is extremely rare, and only involves the mitotic division of female germ cells.

Phylum Nematomorpha

Horsehair worms are exclusively sexual and gonochoristic. The gametes develop in long strands attached to support cells. Adult nematomorphs copulate, often in large masses; fertilization is either external or internal, depending on the species. Little is known about embryogenesis in this phylum, but it culminates in a distinctive free-swimming larva that must invade an arthropod host before it can transform itself into a juvenile. The juvenile in turn must leave the host before maturing into an adult. Asexual reproduction is unknown in this phylum.

Phylum Priapulida

Priapulids are all gonochoristic, with no known mode of asexual reproduction. Their reproductive systems are poorly described, but are similar to those of nematomorphans in being formed as strands of oocytes attached to a common stalk. Most priapulids are broadcast spawners, with external fertilization and embryogenesis. Postembryonic development involves a distinctive larva that undergoes metamorphosis to become an adult.

Phylum Acanthocephala

Thorny-headed worms are gonochoristic, with complex reproductive systems, copulation, and internal fertilization. They develop through an intricate series of stages, including an acanthor larva and a cystacanth juvenile. The various stages of development occur inside different hosts of these parasitic animals. Asexual reproduction does not occur in this phylum.

Phylum Rotifera

As a group, rotifers exhibit a variety of reproductive strategies, with the three classes distinguished by either hermaphroditic or gonochoristic sexuality. Some species, especially in freshwater habitats, alternate between generations produced by parthenogenesis, in which no fertilization occurs, and typical generations produced by copulation and fertilization. The gonads of these tiny animals consist of only a few gametes enclosed by a thin sac. Embryogenesis culminates in direct development of a juvenile that quickly matures into an adult.

Phylum Gastrotricha

Gastrotrichs are generally hermaphroditic, with sperm and oocytes generally forming within the same gonad. Adults reciprocally inseminate each other during copulation. Fertilization is internal, but embryonic development is external. Postembryonic development is direct, and there is no known example of asexual reproduction in this phylum.

Phylum Loricifera

The loriciferans are exclusively sexual and gonochoristic in their reproduction. Little is known about their embryonic development, but it ends with the formation of a distinctive Higgins larva, or perhaps juvenile, that is similar to the adult.

Phylum Kinorhyncha

All known kinorhynchs are sexual and gonochoristic. Copulation, fertilization, and embryonic development are poorly known. Postembryonic development appears to be direct.

Phylum Gnathostomulida

Gnathostomulids are primarily hermaphroditic, and none are known to reproduce asexually. Simple reproductive systems, copulation, and internal fertilization characterize this group. The spiral cleavage is similar to that of protostome animals, and development progresses directly into a juvenile and then an adult form.

Phylum Chaetognatha

Adult arrow worms are hermaphroditic, with well-developed male and female gonads in separate body cavities. Fertilization is internal, but development of the embryos is external, though some species brood their young. Cleavage is radial, and the coeloblastula undergoes gastrulation similar to that of echinoderms and other deuterostomes. Asexual reproduction is not known to occur.

Phylum Hemichordata

Acorn worms are gonochoristic, and gametes are spawned into the open seawater. Fertilization and embryonic development, beginning with radial cleavage, occur in the plankton, and are similar to the patterns of echinoderms. Embryos develop directly or through a distinctive tornaria larva. Some species may reproduce asexually by fragmentation.

Phylum Echinodermata

Starfishes, sea urchins, sea lillies, and their relatives are extremely diverse, and exhibit a variety of reproductive and development modes. In all, however, typical deuterostome development is the rule, beginning with radial cleavage. Larval forms are varied, and tend toward different forms in different classes. Both sexual and asexual reproduction occur within the phylum, but broadcast spawning and planktonic development are the most common patterns.

Phylum Chordata

The invertebrate chordates, including tunicates, lancelets, and their relatives, generally reproduce by spawning and development planktonically in the seawater. Sexual and asexual reproduction may occur in the urochordates, but cephalochordates only undergo sexual processes. Cleavage is generally radial, but may be more mosaic than that of other deuterostomes.

Phylum Entoprocta

Various entoprocts may be either gonochoristic or hermaphroditic, depending on the species. A few may reproduce asexually by budding. Males generally spawn into open water, but the sperm are usually taken up by females for internal fertilization. Cleavage is spiral, possibly indicating some relationship to protostomes.

Phylum Ectoprocta (Bryozoa)

Most bryozoans are hermaphroditic. As colonial animals, all reproduce by asexual means as well. Males spawn sperm, which females take up for internal fertilization. Cleavage is radial, and most species have planktonic larvae.

Phylum Brachiopoda

Lampshells are primarily gonochoristic. Fertilization is variable, but cleavage is always radial. Depending on the class, they may undergo planktonic development through a larva, or may develop directly. Asexual reproduction has not been described for the group.

Phylum Phoronida

The vast majority of phoronids are hermaphroditic, with female and male gonads functioning simultaneously. Fertilization is usually internal. Cleavage is radial, followed by planktonic development in most species, generally through a distinctive actinotroch larva. Asexual reproduction by budding or fission occurs in a few species.

Phylum Cycliophora

Cycliophorans alternate between sexual and asexual stages. They are gonochoristic, and the male attaches to the female for insemination followed by internal fertilization and development. The modified trochophore larva is somewhat similar to that of some protostome groups.

References

Books

Conn, David Bruce. Atlas of Invertebrate Reproduction and Development. 2nd ed. New York: Wiley-Liss, 2000.

Gilbert, Scott F., and Anne M. Raunio, eds. Embryology: Constructing the Organism. Sunderland, MA: Sinauer Associates, 1997.

Strathmann, Megumi F., ed. Reproduction and Development of Marine Invertebrates of the Northern Pacific Coast. Seattle: University of Washington Press, 1987.

Wilson, W. H., Stephen A. Sticker, and George L. Shinn, eds. Reproduction and Development of Marine Invertebrates. Baltimore: Johns Hopkins University Press, 1994.

Young, Craig M., M. A. Sewell, and Mary E. Rice, eds. Atlas of Marine Invertebrate Larvae. San Diego: Academic Press, 2002.

Periodicals

Fautin, Daphne Gail. "Reproduction of Cnidaria." Canadian Journal of Zoology 80 (2002): 1735–1754.

Leys, Sally P., and Bernard M. Degnan. "Embryogenesis and Metamorphosis in a Haplosclerid Demosponge: Gastrulation and Transdifferentiation of Larval Ciliated Cells to Choanocytes." Invertebrate Biology 121 (2002): 171–189.

Slyusarev, George S., and Marco Ferraguti. "Sperm Structure of Rhopalura littoralis (Orthonectida)." Invertebrate Biology 121 (2002): 91–94.

Swiderski, Zdislaw, and David Bruce Conn. "Ultrastructural Aspects of Fertilization in Proteocephalus longicollis, Inermicapsifer madagascariensis, and Mesocestoides lineatus (Platyhelminthes: Cestoda)." Acta Parasitologica 44 (1999): 19–30.

Weiss, Mitchell J. "Widespread Hermaphroditism in Freshwater Gastrotrichs." Invertebrate Biology 120 (2001): 308–341.

David Bruce Conn, PhD