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 copula-tors 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.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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