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24 June 2017
Morning Sedition

Lots of Green, Leafy… Sea Dragons

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Leafy Sea Dragon (Phycodurus equus)

The Leafy Sea Dragon (Phycodurus equus) is a relative of sea horse and pipe fish. It is found only in the southern waters of South Australia, where it lives in sea grass. These are fairly small, as ocean creatures go, typically growing to between 9 and 23 inches (20cm to 50cm) in about two to four years.

Evolution has equipped them with a body resembling seaweed, rendering them virtually invisible as they move among the sea grass on the ocean's floor. Notice the tiny fins on the back and head; these provide the propulsion, while the tail acts as a rudder, steering it. Their movement is normally dainty, but when threatened their fins are flapped as the body undulates like a dolphin. You can see their normal movement in a video (12 MB) at Dive Gallery, which has wonderful pictures and videos. (Far better than the Australian aquariums.)

Although they lack teeth or a stomach, the leafy sea dragon is a voracious predator. If you're a tiny food source, that is. Their main food source are the tiny mysid shrimps, colloquially called so-called "sea lice" or "brine shrimp". When born, they subsist upon the yolk in their egg sack until large enough to hunt rotifers and copepods, eventually graduating to the small shrimp. Their voracious appetite makes them an expensive species for an aquarium.

Sea Dragons are arguably the most spectacular and mysterious of all ocean fish. Though close relatives of sea horses, sea dragons have larger bodies and leaf-like appendages which enable them to hide among floating seaweed or kelp beds. Sea dragons feed on larval fishes and amphipods, such as and small shrimp-like crustaceans called mysids ("sea lice"), sucking up their prey in their small mouths. Many of these amphipods feed on the red algae that thrives in the shade of the kelp forests where the sea dragons live.

Dive Gallery

The leafy sea dragon's reproductive life is quite interesting. Like seahorses, the female lays eggs under the male's tail; from this point forward the male cares for the egs, for about two months, until they hatch. But that's the short version, and the full version conveys this creature's peculiar evolution:

Unlike seahorses, sea dragons do not have a pouch for rearing the young. Instead, the male carries the eggs fixed to the underside of his tail from where they eventually hatch. When male sea dragons are ready to receive eggs from the female, the lower half of the tail on the male appears wrinkled.

During mating, the female lays 100-250 eggs onto a special 'brood patch' on the underside of the male's tail, where they are attached and fertilized. This brood patch, consisting of cups of blood-rich tissue each holding one egg, and is specifically developed by the male for use during the breeding season of August-March. The bright pink eggs become embedded in the cups of the brood patch, receiving oxygen via the cups' blood vessels.

During each breeding season, male Leafy sea dragons will hatch two batches of eggs. After a period of about 4-6 weeks from conception, the male 'gives birth' to miniature juvenile versions of sea dragons. As soon as a baby sea dragon leaves the safety of its father's tail, it is independent and receives no further help from its parents. For 2-3 days after birth, the baby sea dragons are sustained by their yolk sac. After this, they hunt small zooplankton, such as copepods and rotifers, until large enough to hunt juvenile mysids.

Sea dragons grow to a length of 20 cm after one year, reaching their mature length at two years. In the wild, young sea dragons are preyed upon by other fish, crustaceans and evn sea anemones. Young sea dragons look more delicate, and are often differently colored than adults, and may hide in different types of seaweeds.

MarineBio.org

The species, however, has been threated with extinction through a combination of factors: The biggest are pollution (fertilizer runoff), collecting for home aquariums or idiotic "alternative medicine" and storms that move them between water pressures, rupturing their swim bladders.

Unique to the southern waters of WA and South Australia, the leafy sea-dragon's home is inshore areas of seagrass. Unfortunately these are under increasing threat from pollution and excessive fertiliser run-off.

This is not the only danger faced by the sea-dragon. Although having no known predators amongst the marine world, it has become the target of unscrupulous 'collectors' who have denuded the more accessible seagrass areas of this amazing creature.

In 1991, the Department of Fisheries, concerned by the rapidly decreasing numbers of the leafy sea-dragon, declared it a totally protected species.

Aquarium of Western Australia

Sources and Further Reading

  1. Dive Gallery's Pictures and Video (gorgeous!)
  2. Melbourne Aquarium
  3. Aquarium of Western Australia
  4. MarineBio.org

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