Starfish

Anne-Marie Bokslag

Haarlem, Netherlands

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Found this starfish at Dingo Beach (close to Airlie Beach) in Queensland, Australia.
I really don’t know which specie out of 2.000 species of starfish this is.

Starfish or sea stars are echinoderms belonging to the class Asteroidea.2 The names “starfish” and “sea star” essentially refer to members of the class Asteroidea. However, common usage frequently finds “starfish” and “sea star” also applied to ophiuroids which are correctly referred to as “brittle stars” or “basket stars”.

There are 2,000 living species of starfish that occur in all the world’s oceans, including the Atlantic, Pacific, Indian as well as in the Arctic and the Southern Ocean (i.e., Antarctic) regions. Starfish occur across a broad depth range from the intertidal to abyssal depths (>6000 m).

Starfish are among the most familiar of marine animals and possess a number of widely known traits, such as regeneration and feeding on mussels. Starfish possess a wide diversity of body forms and feeding methods. The extent that Asteroidea can regenerate varies with individual species. Broadly speaking, starfish are opportunistic feeders, with several species having specialized feeding behavior, including suspension feeding and specialized predation on specific prey.

Most starfish typically have five rays or arms, which radiate from a central disk. However, several species frequently have six or more arms. Several asteroid groups, such as the Solasteridae, have 10-15 arms whereas some species, such as the Antarctic Labidiaster annulatus can have up to 50. It is not unusual for species that typically have five-rays to exceptionally possess six or more rays due to developmental abnormalities.

The bodies of starfish are composed of calcium carbonate components, known as ossicles. These form the endoskeleton, which takes on a variety of forms that are externally expressed as a variety of structures, such as spines and granules. The architecture and individual shape/form of these plates which often occur in specific patterns or series, as well as their location are the source of morphological data used to classify the different groups within the Asteroidea.

As echinoderms, starfish possess a hydraulic water vascular system that aids in locomotion.6 The water vascular system has many projections called tube feet on the ventral face of the sea star’s arms which function in locomotion and aid with feeding. Tube feet emerge through openings in the endoskeleton and are externally expressed through the open grooves present along the bottom of each arm.

The body cavity not only contains the water vascular system that operates the tube feet, but also the circulatory system, called the hemal system. Hemal channels form rings around the mouth (the oral hemal ring), closer to the top of the sea star and around the digestive system (the gastric hemal ring). A portion of the body cavity called the axial sinus connects the three rings. Each ray also has hemal channels running next to the gonads.

On the end of each arm or ray there is a microscopic eye (ocellus), which allows the sea star to see, although it only allows it to see light and dark, which is useful to see movement. Only part of the cells are pigmented (thus a red or black color) and there is no cornea or iris. This eye is known as a pigment spot ocellus.

The mouth of a starfish is located on the underside of the body, and opens through a short esophagus into firstly a cardiac stomach, and then, a second, pyloric stomach. Each arm also contains two pyloric caeca, long hollow tubes branching outwards from the pyloric stomach. Each pyloric caecum is lined by a series of digestive glands, which secrete digestive enzymes and absorb nutrients from the food. A short intestine runs from the upper surface of the pyloric stomach to open at an anus in the center of the upper body.10

Many sea stars, such as Astropecten and Luidia swallow their prey whole, and start to digest it in the stomachs before passing it into the pyloric caeca. However, in a great many species, the cardiac stomach can be everted out of the organism’s body to engulf and digest food. In these species, the cardiac stomach fetches the prey then passes it to the pyloric stomach, which always remains internal.

Sea stars and other echinoderms pump water directly into their bodies, via the water vascular system, as they find it. This makes them vulnerable to all forms of water pollution, as they have little ability to filter the water of toxins and contaminants. Oil spills and similar events often take a toll on echinoderm populations that carry consequences for the ecosystem.
(Source: Wikipedia)

Camera: Canon EOS D Mark II
Lens: Tamron Zoomlens AF 18-250mm
Soligor extender 1.4x
Exposure time 1/250s
Aperture value f/6.3
ISO 200
Focal length 250 mm

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