Sirenia

(redirected from Sirenians)
Also found in: Dictionary, Thesaurus.
Related to Sirenians: Pinnipeds

Sirenia

An order of herbivorous aquatic placental mammals, commonly known as sea cows, that includes the living manatees and dugongs and the recently exterminated Steller's sea cow. The order has an extensive fossil record dating from the early Eocene Epoch, some 50 million years ago.

The earliest known sirenians were quadrupedal and capable of locomotion on land. Fossils clearly document the evolutionary transition from these amphibious forms to the modern, fully aquatic species, which have lost the hindlimbs and transformed the forelimbs into paddlelike flippers. The living species have streamlined, fusiform bodies with short necks and horizontal tail fins like those of cetaceans, but no dorsal fins. The skin is thick and nearly hairless. The nostrils are separate, and the ears lack external pinnae.

Sirenians typically feed on aquatic angiosperms, especially seagrasses, but in ecologically marginal situations they also eat algae and even some animal material. They are normally found in tropical or subtropical marine waters, but some have become adapted to fresh water or colder latitudes. Body sizes have ranged from less than 3 m up to 9–10 m (30–33 ft). Sirenians mate and give birth in the water, bearing a single calf (occasionally twins) after about 13–14 months of gestation and then nursing it from one pair of axillary mammae. The closest relatives of sirenians among living mammals are the Proboscidea (elephants).

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Sirenia

 

(sirens), an order of aquatic mammals comprising three families: Manatidae (three species of manatees), Dugon-gidae (with one species—the dugong), and Hydrodamalidae (with one extinct species—Steller’s sea cow—which was exterminated in the 18th century). Sirens are adapted to an aquatic mode of life and never emerge onto land. Their torpedolike body ends in an evenly rounded or two-lobed tail fin, which serves as the main organ of locomotion. The head is bluntly truncated, and the neck is short but movable. The forelimbs are in the form of large flippers that articulate at the ulnar and carpal joints. The rough skin is dark brown and sparsely covered with hair; there is a thick subcutaneous layer of fat. Paired nostrils are situated at the end of the muzzle. The females have two mammae, located in the chest region. The teeth and digestive organs are adapted to a diet of aquatic plants. Modern sirens have two to eight simultaneously functioning cheek teeth in each half of each jaw. Male dugongs have a pair of incisors in the upper jaw that resemble small tusks. In the course of a lifetime sirens have 30 cheek teeth. The palate and lower jaw in Steller’s sea cow were covered with horny plates. The stomach is capacious and consists of two sections. The intestine is long with a developed cecum.

Sirens, which are rare everywhere, live in small groups. A single young is born at a time, after a gestation period of five or six months in manatees and 11 months in dugongs. Since their population is diminishing, sirens require protection.

REFERENCE

Mlekopilaiushchie Sovetskogo Soiuza, vol. 2, part 1. Edited by V. G. Geptnerand N. P. Naumov. Moscow, 1967.

S. V. MARAKOV

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

Sirenia

[sī′rē·nē·ə]
(vertebrate zoology)
An order of aquatic placental mammals which include the living manatees and dugongs; these are nearly hairless, thick-skinned mammals without hindlimbs and with paddlelike forelimbs.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
Several bone fragments are well preserved in-situ (in the original place), showing broad and re-curved ribs typical of Sirenians,' it said.
This is clearly shown by the sirenians, as well as the crocodiles and turtles.
A puzzling feature of sirenian physiology is a surprisingly small brain.
The Sirenians were formed 30 years ago this year, first as a ladies choir and later including male voices.
The act of cetaceans (whales and dolphins), sirenians (manatee) and pinnipeds (seals, sea lions and their relatives) coming on to land, dead or alive, accidental of intentional, is known as a stranding (Jefferson et al.
Barnacles of the superfamily Coronuloidea live as obligate commensals on sea turtles, cetaceans, sirenians, sea snakes, and crustaceans (Newman and Ross, 1976).
Sirenians' baritone and deputy conductor Oliver Sammons married alto Judith Eastham at All Saint's Church, Gresford earlier this month - with the choir on hand to sing as the couple tied the knot.
Report of the Workshop: Age Determination of Toothed Whales and Sirenians. Report of the International Whaling Commission, Special Issue 3:2.
Jean Stanley Jones, recently awarded the MBE, said: "The Sirenians and I have enjoyed many successful visits to European cities such as Budapest, Prague, Riva del Garda etc, but this concert tour has proved an amazing experience."