Reproductive biology

Monogeneans are hermaphrodites, with the male reproductive system usually first to mature. This characteristic is known as protandry. Mutual or unilateral insemination may occur, although self-insemination also takes place. The tanned eggs are assembled in an egg mold or ootype; the vitelline cells provide the raw material for the shell and resources for the developing embryo. With the exception of the viviparous gyrodactylids, the eggs are released into the environment and produce infective larvae (oncomiracidia) which are able to swim freely with the help of cilia. The larvae of many mono-geneans hatch spontaneously at a particular time of day, which often coincides with times when the host is particularly vulnerable to invasion. Hatching may also be triggered by such host-derived cues as chemicals, mechanical disturbance, or shadows. The oncomiracidia do not feed until they reach the host, which means that their survival as free-living organisms and their potential for host infection are limited, usually to a period of several hours. The oncomiracidia throw aside their ciliated cells when they reach the host. Entobdella soleae can infect new hosts by direct transfer of adults or juveniles, as well as by eggs and oncomiracidia. The juveniles of a related parasite can swim and may reach new hosts in this way.

The gyrodactylids are unique among monogeneans. They have abandoned freely deposited eggs and free-swimming on-comiracidia; they are viviparous, producing offspring that are usually full size at birth. They increase their reproductive rate by telescoping generations; their offspring already contain a partly developed embryo at birth and sometimes a second smaller embryo within the first. The appearance of the male reproductive system is delayed until after the female reproductive system is operational. This characteristic, which is known as protogyny, helps to concentrate the organism's resources on embryo development. The first two offspring are probably produced asexually. Gyrodactylids usually spread to new hosts by direct transfer when infected hosts make contact with one another. In addition, however, the hosts may be infected by gyrodactylids drifting freely in the water column or by making contact with parasites attached to the substrate. There is also one record of a gyrodactylid that can swim.

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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