Animal Diversity Web

Ge­o­graphic Range

The brown hydra is com­monly found in the North­ern Hemi­sphere and parts of Aus­tralia.

• Biogeographic Regions
• nearctic
  • native
• australian
  • native

Habi­tat

Hy­dras are are well known as the fau­nas of ponds, spring brooks, un­pol­luted streams, rivers, and the lit­toral zone of lakes.

• Aquatic Biomes
• lakes and ponds
• rivers and streams

Phys­i­cal De­scrip­tion

• Other Physical Features
• ectothermic
• radial symmetry

Re­pro­duc­tion

The com­mon asex­ual method of re­pro­duc­tion by hy­dras is bud­ding. Buds orig­i­nate at the junc­tion of the stalk and gas­tric re­gions. The bud be­gins as a hemi­spher­i­cal out­pouch­ing that even­tu­ally elon­gates, be­comes cylin­dri­cal, and de­vel­ops ten­ta­cles. The bud then pinches off and a new in­di­vid­ual be­comes in­de­pen­dent. Buds are pro­duced every two to three days under fa­vor­able con­di­tions. Fol­low­ing un­fa­vor­able con­di­tions, such as in­juries or pe­ri­ods of scarce re­sources, hy­dras oc­ca­sion­ally re­pro­duce through trans­verse and lon­gi­tu­di­nal fis­sion.

• Parental Investment
• no parental involvement

Be­hav­ior

Hydra oli­gac­tis is mostly ses­sile. Hy­dras at­tach to stones, twigs, veg­e­ta­tion, or de­bris. The brown hydra rarely is found at depths ex­ceed­ing 1.5 m. Spon­ta­neous move­ments are few. When the hydra re­mains undis­turbed, its body is ex­tended and the ten­ta­cles spread. For no ap­par­ent rea­son, con­trac­tions and ex­pan­sions of the body occur at in­ter­vals and the ten­ta­cles are con­stantly in mo­tion. Free hy­dras can move from place to place by basal glid­ing. The hy­dras usu­ally move by loop­ing and som­er­sault­ing, by at­tach­ing the ten­ta­cle ends and pulling them­selves along. When there is an in­suf­fi­cient sup­ply of oxy­gen, hy­dras move to re­gions of higher oxy­gen con­tent. In gen­eral, the be­hav­ior is char­ac­ter­ized by its me­chan­i­cal na­ture, great in­de­pen­dence of parts, lack of in­te­gra­tion, and lack of exact re­sponses.

Food Habits

Hydra oli­gac­tis, as in all Cnidaria, are strictly car­niv­o­rous and eat many dif­fer­ent kinds of small meta­zoans, in­clud­ing an­nelids, cope­pods, clado­cer­ans, and in­sects. Hydra cap­ture their food by par­a­lyz­ing and killing the food or­gan­ism by means of ne­ma­to­cysts, which are dis­charged into the prey. The prey is brought to the mouth (proc­tostome) by the ten­ta­cles, a re­sponse that is in­duced glu­tathione. This is con­sid­ered the key mech­a­nism in di­ges­tion. The or­gan­ism is then taken in through the mouth, which is star-shaped or cir­cu­lar. Hy­dras have been known to feed on the or­ganic ma­te­r­ial of the sub­strate when the food sup­ply is in­suf­fi­cient. This be­hav­ior, how­ever, is not con­sid­ered nor­mal. Di­ges­tion is both extra and in­tra­cel­lu­lar.

Eco­nomic Im­por­tance for Hu­mans: Neg­a­tive

Hydra oli­gac­tis is a hatch­ery nui­sance be­cause it may kill fish fry.

Other Com­ments

Hy­dras have well de­vel­oped pow­ers of re­gen­er­a­tion and graft­ing. Very lit­tle is known about the length of life under nor­mal con­di­tions. In the lab, hy­dras have been known to live for three to twelve months or more. Males out­nu­mer fe­males, an un­usual situ­ta­tion in most fresh water in­ver­te­brates.

Con­trib­u­tors

Jana D. Wein­berger (au­thor), Uni­ver­sity of Michi­gan-Ann Arbor.

Ref­er­ences

Hyman, Lib­bie Hen­ri­etta. 1940. The In­ver­te­brates: Pro­to­zoa through Ctenophora. Mc­Graw Hill Book Co., New York.

Pearse, Vicki and John and Mil­dred and Ralph Buchs­baum. 1987. Liv­ing In­ver­te­brates. Black­well Sci­en­tific Pub­li­ca­tions, Palo Alto.

Br­usca, Richard C. and Gary J. Br­usca. 1990. In­ver­te­brates. Sin­auer As­so­ci­ates, Inc., Sun­der­land.

Pen­nak, Robert W. 1989. In­ver­te­brates of the United States: Pro­to­zoa to Mol­lusca. John Wiley and Sons, Inc., New York.