Hydra 4b4k6j

Phylum: Cnidaria Order: Hydroida Class: Hydrozoa Genus: Hydra Description: One of the few Cnidarians that is found in freshwater around the globe, on the underside of leaves or aquatic plants in clean streams and ponds where they feed on a variety of small crustaceans, insect larvae, and annelid worms. It exists only in the polyp form (no medusa stage). Moreover, it is a solitary polyp which can extend its body up to a length of 25-30 mm. It is a cylindrical tube with the lower (aboral) end drawn out into a slender stalk, ending in a basal (or pedal) disc for attachment. This basal disc has gland cells have adhesive properties that enable a Hydra to a substratum and also to secrete a gas bubble for floating. The mouth, located on a conical elevation called the hypostome, is encircled by six to ten hollow tentacles. In some individuals buds may project from the sides. Testes or ovaries, when present, appear as rounded projections on the surface of the body. Generally, the body of a Hydra may consists of about 100 000 cells of roughly a dozen different types; Epitheliomuscular cells, Interstitial cells, Gland cells, Cnidocytes, Sensory cells, Nerve cells, Nutritive-muscular cells and Gastrodermal cells. However, the two main cellular layers of the Hydra is the epidermis (outer cell layer-ectodermal) and Gastrodermis (inner cell layerendodermal) which is separated by the mesoglea (gelatinous, or jellylike and has no fibers or cells). Gastrodermis- made up of large, ciliated, columnar epithelial cells with irregular flat bases. Cells of the gastrodermis include nutritive-muscular, interstitial, and gland cells. Epidermis- contains epitheliomuscular, interstitial, gland, cnidocyte, and sensory and nerve cells. Epitheliomuscular cells make up most of the epidermis and serve both as a cover and muscular contraction. Interstitial cells- are undifferentiated stem cells found among the bases of the epitheliomuscular cells. Differentiation of interstitial cells gives rise to cnidoblasts, sex cells, buds, nerve cells, and others, but generally not to epitheliomuscular cells. Hydra reproduces both asexually by budding from the side of the polyp and sexually. Hydra are somewhat unusual in the way that sexual reproduction occurs in the polyp stage. Both the testis and ovary arises from the interstitial cells. Hydra, ectodermal cells are continuously produced just below the mouth region and migrate to the tentacle tips or to the foot, where they are sloughed off. At a certain distance from the tentacular region, cells can move out and divide to form a bud. This ability to move and differentiate gives the polyp the ability to regenerate.

Diagram showing a Hydra with the different cell layers:

Cross-section of the body wall of a Hydra showing the different cell types:

Asexual reproduction in Hydra: It is a process called budding which lasts two to four days, ending with the daughter Hydra detaching from the parent to become a separate organism. It usually occurs in winter when conditions are favourable. Certain cells divide rapidly and develop on the body surface to form an external bud (1 or 2 in number). The bud cells proliferate to form a cylindrical structure, which develops into a young Hydra that that pinches off from the parent and live independently. 

Bud is formed by dermis, so buds are endodermal so Hydra is formed generally by dermal epithelium.

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In starting bud is solid.

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Soon coelenteron is formed & bud becomes hollow, mouth & tentacles are formed.

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At base constriction is formed. Young Hydra is separated from parental Hydra.

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Parent Hydra remains the same. Process is completed in 3–4 days. Budding is exogenous (budding in porifera is endogenous in the form of gemmule).

Diagram illustrating asexual reproduction in Hydra.

When conditions deteriorate, Hydra can reproduce sexually, forming resistant zygotes that remain dormant until conditions improve.

Sexual Reproduction in Hydra: Sexual reproduction usually occurs in the autumn when environmental conditions are favourable. Gonads usually appear in autumn stimulated by lower temperatures and possibly also by reduced aeration of stagnant waters or high levels of carbon dioxide concentrations in the water. Testes (6-8 in number) form from interstitial cells that produce a swelling in the upper third of the body. Spermatogenesis results in flagellated spermatozoa. Ovary (also formed from interstitial cells) is 1 or 2 in number and oval shaped. By oogenesis, 1 ovum is formed in 1 ovary, remaining ova are degenerated & form deutoblast. Ovum is covered by gelatinous layer, which is protective in nature. During egg formation, yolk is incorporated into the egg cell from gastrodermal cells. As ovarian cells disintegrate, a thin stalk of tissue attaches the egg to the body wall. Egg is large in size without any membrane. Life span of ovum is of 24 hrs. If it gets sperm, then it converts into zygote, otherwise it degenerates. After the epidermis ruptures over the mature ovum, it is fertilized by spermatozoa released into the surrounding water. The fertilized ovum remains on the parent’s body, where it undergoes cleavage and gastrulation to become an embryo. A protective shell (enables survival of embryo in winter) forms around the embryo by the epithelial cells, after which it detaches from the parent. The egg may remain dormant for some time before the embryo emerges as a small immature polyp with a mouth, body, and tentacles. Young Hydra hatch out in spring when the weather is favorable.

Diagram illustrating sexual reproduction in Hydra:

Diagram illustrating life cycle of Hydra (Both sexual and asexual reproduction)

(A) Life cycle of Hydra. Sperm produced by the male polyp (a) fertilizes the eggs of the female polyp (b). During cleavage, the eggs secrete a chitinous theca about themselves. After hatching, the embryos (c) grow into polyps that reproduce asexually by budding (d), until environmental conditions again trigger sexual reproduction.

References: Brusca. Invertebrates (2 ed). Sinauer associates; 2003 Milley-Harley. Zoology (5 ed). McGraw Hill companies; 2001. Moore. An introduction to the Invertebrates. Cambridge University Press; 2006. Hickman. Animal diversity (3 ed). McGraw Hill companies; 2002. http://resources.wardsci.com/livecare/working-with-hydra/ http://www.saburchill.com/ans02/chapters/chap051.html http://www.saburchill.com/ans02/chapters/chap051.html

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