Cushion star

Cushion stars are any of the species of starfish belonging to the taxonomic genus Culcita. These sea-dwelling invertebrates of the Indo-Pacific have several characteristics that set them apart from other species of starfish. One key distinguishing factor is the Cushion star's appearance. With an inflated, rather rotund body and nearly invisible arms, the Cushion star is quite different from a typical sea star. However, its unique appearance actually aids in its protection because it is difficult for predators to grasp its body. This species is furthermore fascinating to look at; this quality is clearly displayed in the numerous patterns, colors, and designs of the Cushion star. Its varied coloring also matches its habitat; the Cushion star will often be found scavenging near and around coral reefs for its meal. The Cushion star shares common characteristics with other starfish, including its method of reproduction and the incredible water-vascular system the sea star is famous for. However, these similarities do not take away from the uniqueness of the Cushion star.

Body Design


The members of the genus Culcita, normally referred to as Cushion stars, are a rather unique genus of starfish--the most obvious reason being their clearly distinct appearance. Unlike most species of starfish, Cushion stars are rather round and globular, with short arms that can be very difficult to distinguish from their body. They have even been called "spineless sea urchins" because of this reason. However, if one turned the Cushion star upside down, they would see the recognizable pattern of a starfish's five rows of tube feet and central mouth. These hydraulic tube feet are a significant part of its incredible water-vascular system, which the Cushion star and other starfish species use for locomotion and food capture. It can also use its tube feet to right itself if it happens to be turned on the wrong side.

The endoskeleton of a Cushion star is made up of ossicles (plates) containing calcium carbonate. These ossicles are supported by structures that also brace the ambulacra--the grooves in the rays of the starfish from which the tube feet protrude and retract. Although the underside of the Cushion star is flat, the dorsal side is more spherical in nature. Small bumps dot the surface, and circular clumps of papulae cover the star. The transparent, finger-like papulae, also known as skin gills, aid in the respiratory and excretory processes of the Cushion star. They are most easily seen when the Cushion star is submerged underwater.

A Cushion star exhibits radial symmetry, with a diameter that can grow up to a maximum of 30 centimeters. The thick calcareous walls of its body, along with its inflated design, make it challenging for fish and other predators of the Cushion star to grip and bite it. The rounded pentagonal shape of a Cushion star not only serves as a method of defense, but can also be admired simply for its aesthetic appearance. A Cushion star can be covered with an extensive variety of patterns and colors, including nearly every color of the rainbow: red, orange, yellow, green, fawn, brown, and the less common purple and blue.

An interesting fact about the Cushion star's design is that as a juvenile, it looks vastly different from the adult. So different, in fact, that it had been previously classified as a separate species. An adolescent Cushion star is much flatter in appearance, with more defined star-shaped arms (although they are still rather short). They also possess a border-like plate running along the edge of their bodies. Because of their different characteristics, young Cushion stars are often mistaken for other large species of starfish. Between the species of the Culcita genera, there are generally three types of basic appearances: leathery, spiny, and pillow-like. These descriptions correspond with their names: coriacea, schmideliana, and novaeguineae, respectively.

Life Cycle


As in most sea stars, the fertilization of the family Oreasteridae, and thus the Cushion star, takes place externally between separate sexes. The male and female cushion stars synchronize the release of their gametes (eggs and sperm) into the water. Often the sea stars will gather in groups to reproduce by free-spawning so the eggs will have a greater chance of survival. After the eggs hatch, the very young Cushion stars emerge as free-swimming larvae. The ciliated and bilaterally symmetrical larvae will drift around the ocean for days or even weeks until coming to settle on the seafloor where they will transform into juvenile Cushion stars. When Cushion stars first become juveniles, they are about 0.5 mm in diameter with an underwater weight of 18µg, or 18 micrograms (in other words, extremely tiny). This stage in a Cushion star's life cycle lasts for about 2 years.

During the juvenile period the Cushion star's appearance lacks the usual characteristics of the adult. This quality is known as a goniasterid phase--when the young of a species appears to be a member of a different family. Juvenile Cushion stars are also known to display cryptic behavior-that is, they can be very difficult to detect. The more vulnerable youth are often found under rocks, hiding from predators such as fish.

After about six and a half months after it becomes a juvenile, the young Cushion star grows to a weight of 1 g; it takes another 9.6 months to grow to a size of 6g, and another 8.7 months to reach the weight of an adult. When the juvenile has grown to a diameter of about 50 mm, it undergoes metamorphosis, expanding and inflating from the flat-shaped youth into the cushion like adult. The newly transformed adult weighs around 23g underwater. Once the adult Cushion star reaches sexual maturity, its life cycle begins again.

Ecology


Cushion stars belonging to the genus Culcita are primarily found in their native region of the Indo-Pacific. Favoring warm tropical waters, the Cushion star can commonly be found among coral reefs where they feed on the live coral. Cushion stars have also been known to eat certain species of hard coral. Cushion stars also consume mollusks, other sessile invertebrates, sponges, and worms. Another way the Cushion star obtains food is by scavenging for detritus along the sea floor. They have been recorded to eat sediments to sift for any edible organic particles as well. Part of the Cushion star's nutrition also comes from coral polyps and seaweed. A Cushion star eats by everting its stomach out through its mouth and onto its prey. The stomach then secretes digestive juices, softening the food. The everted stomach absorbs this softened material and carries it back inside the sea star for digestion to be completed.

Cushion stars can inhabit waters anywhere from low tide to the depths of 92 meters. They are typically found in groups around coral or rocky reefs, although they are also known to lead solitary lives. While Cushion stars are still young, they may be subjected to predation from the Triton's trumpet snail, but after they mature into adults this risk is greatly reduced. Because of the Cushion star's unique body design, they are usually safe staying out in the open. To compensate for their lack of protection, juvenile Cushion stars often hide under rocks to escape predators. Cushion stars also act as hosts for certain organisms, such as the sea-star shrimp and the pearl fish. The tiny sea star shrimp (Periclimenes soror), is a common organism to find on starfish. These tiny creatures do not harm their host in any way; instead, they hide underneath the starfish and change their color to match that of the Cushion star. The pearl fish (Carapus mourlani) sometimes shares a commensal relationship with the Cushion star by residing within its body cavity. Every now and then the pearl fish will emerge from the Cushion star to feed.

Relationship with Coral Reefs
The Cushion star shares a unique relationship with coral reefs. These vast beds of coral located in warm tropical waters are home to the majority of Cushion stars. They can be found anywhere from offshore reefs and islands in northwestern Australia to habitats in the eastern Indian Ocean, as well as the western and central Pacific Ocean. Within these regions Cushion stars inhabit reef flats, patch reefs, and protected areas. Because of their inflated and calciferous skeleton, Cushion stars have a lower chance of being attacked; thus, they are commonly found on fully exposed reef terraces. While the juvenile form of the sea star hides underneath rocks and feeds on algae, adult Cushion stars constantly roam around the corals and rocks, scavenging for food and consuming coral polyps.

Since the Cushion star possesses rather short arms, its speed is considered rather slow, even for a starfish. Its nearly non-existing rays also restrict its ability to climb. As a result, the Cushion star usually preys on developing coral colonies of lower elevation. A unique behavior of the Cushion star is that it feeds predominantly at night. The Cushion star has also been observed to display selective predation; some of the Cushion star's favorite coral species are Pocillopora, Acropora, Porites, and Faviids. This behavior could alter the size and structure of the reef colonies by limiting the population of those select species, resulting in other coral species dominating the reef community. This in turn could affect other organisms within the coral reef ecosystem as the change makes its way up the food chain. However, although the Cushion star has this tendency, it is not considered a major threat to coral reefs in comparison to other species of starfish. In fact, the decline in the abundance of coral reefs may influence the population of the Cushion star.

Video
Scuba Diver handling a Cushion star, allowing one to see the relative size compared to a human. PIDIMrxobHk