''Hydra oligactis'', also known as the brown hydra, is a species of

hydra Hydra generally refers to: * Lernaean Hydra, a many-headed serpent in Greek mythology * ''Hydra'' (genus), a genus of simple freshwater animals belonging to the phylum Cnidaria Hydra or The Hydra may also refer to: Astronomy * Hydra (constel ...

found widely dispersed in the northern

temperate zone In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges throughout t ...

. It is a common organism found in still waters from early Spring to late Autumn. It is commonly found attached to the stems of water plants, the undersides of leaves, submerged twigs and on the surface of stones. When disturbed it retracts to a small brown blob which is easily overlooked. Gently sweeping through a clean weedy

pond A pond is an area filled with water, either natural or artificial, that is smaller than a lake. Defining them to be less than in area, less than deep, and with less than 30% emergent vegetation helps in distinguishing their ecology from th ...

and allowing the collected water and leaves to stand in a jar will often reveal Hydra emerging after only a few minutes. When in feeding mode, the fully extended

tentacle In zoology, a tentacle is a flexible, mobile, and elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work main ...

s are very long and may exceed in length. In this condition the tentacles are very difficult to see with the naked eye and are often only revealed when a

prey Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill the ...

animal such as Daphnia is caught. The relative length of the tentacles compared to the body is characteristic of the species and serves to differentiate it from any other brown Hydra of cool temperate waters. When a cold sensitive strain of ''H. oligactis'' is subjected to temperature stress, it undergoes a shift from asexual budding to

sexual reproduction Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote tha ...

. This shift involves an upreglation of genes involved in gametogenesis,

cellular senescence Cellular senescence is a phenomenon characterized by the cessation of cell division. In their experiments during the early 1960s, Leonard Hayflick and Paul Moorhead found that normal human fetal fibroblasts in culture reach a maximum of approxi ...

apoptosis Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes (morphology) and death. These changes incl ...

and

DNA repair DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA dam ...

, and a downregulation of genes involved in

stem cell In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type o ...

maintenance. ''Hydra oligactis'' is preyed upon by the flatworm ''Microstomum lineare'', which incorporates ''Hydra'' stenoteles into its own epidermis.

References

Hydridae Animals described in 1766 Taxa named by Peter Simon Pallas {{Anthoathecata-stub