Reproductive biology

Nothing is known regarding the courtship and mating. The eggs are brought in masses on female medusae or in female gonophores. According to the species, the eggs can be small and in great numbers, or they can be large and few, even a single one per gonophore.

In the Automedusae, with no polyp stages, there is little or no asexual reproduction in larval medusae (known as act-inulae in the Narcomedusae) so that each fertilization event leads to one or a few adult medusae. In the Hydroidomedusae, the widespread asexual reproduction of the polyp stage can be considered as a polyembryony or as a larval amplification, as it happens in some parasites such as trematodes. Each fertilization event, therefore, leads to a single planula that produces a polyp colony that, in its turn, will produce many adult medusae. Since the lifespan of polyp colonies can last many years, each planula can lead to the production of hundreds or thousands medusae.

The so-called planula larva is nothing more than a gas-trula, thus being more an embryo than a larva. Since gametes are shed before or soon after fertilization, embryonic development takes place outside the maternal body. Planulae can be solid (stereogastrula) or hollow (coeloblastula); usually the species with medusae in the lifecycle have hollow planulae that live part of their life in the water column, swimming with cilia or flagella to reach the settling sites. The species with suppressed medusae usually produce solid planulae that fall to the bottom and settle near the parent colony. Adults, by definition, are the sexually reproductive morphs in a lifecy-cle; if a medusa is present, it is the adult in the lifecycle. The polyp stage, in this framework, is a specialized (and perennial) larva that produces a great number of adults throughout its long life. In many species, however, the medusa stage can be reduced or even suppressed, so that the larvae, by paedo-morphosis, become the sexually mature morphs of the lifecy-cle. Almost half of the species of Hydroidomedusae have suppressed or reduced medusae; the group is, therefore, the most paedomorphic of the whole animal kingdom.

Some medusae (e.g., Eleutheria) have special brood pouches where they safeguard developing medusae. Some hydroids have gonothecae with apical brood chambers that retain the planulae for a certain period.

Many species are sharply seasonal, being active only in narrow periods of time. The medusae can be present for a few weeks or months, completely disappearing from the water column and being represented by the correspondent polyps in the benthos. Polyp colonies can regress to resting hydrorhizae for long periods, reactivating at the onset of favorable conditions. Planulae can become encysted and remain dormant just as the resting hydrorhizae, being covered by a chitinous sheath.

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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