Any of 20–30 species of freshwater cnidarians (genus Hydra). The polyp-type body is a thin, usually translucent tube that measures up to slightly more than 1 in. (25 mm) long. Food is ingested and wastes are ejected from the open, tentacled end. Individuals are usually hermaphroditic (see hermaphroditism). Reproduction by budding is also common. Species differ in colour, tentacle length and number, and gonad position and size. All hydras feed on other small invertebrates (e.g., crustaceans).
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Hydra (Ύδρα, ) is one of the Saronic Islands of Greece, located in the Aegean Sea between the Saronic Gulf and the Argolic Gulf. It is separated from the Peloponnese by the narrow Hydra Gulf. Hydra is also a municipality and one of the few provinces in Greece to have two municipalities---which includes the island of Dokos (pop. 43)---yet fewer than five communes. In ancient times, the island was known as Hydrea (Υδρέα), which was a reference to the springs on the island.
An island dependent upon tourism, Athenians comprise a sizeable segment of Hydra's visitors. Hydra is served by high speed hydrofoils and catamarans from Piraeus plus daily island tour boats. Besides garbage trucks, motor vehicles are not permitted on the island, which leaves the bulk of public transportation up to horses, donkeys and water taxis. The inhabited area, however, is so compact that most people walk everywhere.
There is one main town, known simply as "Hydra port" (pop. 2,526 in 2001). It consists of a crescent-shaped harbor, around which is centered a strand of commercial establishments (restaurants, shops, markets, and galleries), all of which cater to tourists and locals (Hydriots). Steep stone streets lead up and outwards from the harbor area. Most of the local residences, as well as the hostelries on the island are located on these streets. Other small villages or hamlets on the island include Mandraki (pop. 33), Kamini, Vlychos (28), Palomitha (18), Episkopi (23), and Molos (2).
Although the island's name is derived from ancient springs, it is now almost dry. While Hydra previously had wells, these were drained by seismic activity around the mid-20th century. Today, the island's water is imported by boat from the Greek mainland. The dominant geographic feature of Hydra is its rocky hillsides, which are bare, save for the occasional farmhouse and six Orthodox monasteries.
Hydra's harbor, market, and buildings were perhaps an architectural influence for resort developer Charles Fraser to create Harbour Town, the well known village at Sea Pines Resort on Hilton Head Island, South Carolina, USA.
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There are no municipal boundaries on the island and its smaller islands.