This week’s Wet Beast Wednesday is going to be stellar, as we’re reaching for the stars and talking about starfish. Also known as sea stars and asteroids, these echinoderms are a classic in tidepools and touch tanks, so much so that many people don’t think much about them. If that’s you, you’re missing a lot, since there’s plenty of interesting things to learn about our radially symmetrical brethren.
(Image: a common starfish (
Asterias rubens) resting on sand, seen from above. It is a star-shaped animal with five distinct rays connecting at the base. Its body is orangeish, with small white slumps all over its surface. End ID)
Starfish are echinoderms of the class Asteroidea and when I called them our brethren, I wasn’t entirely kidding. Echinoderms are duterostomes, the clade of bilaterally symmetrical animals whose anus forms before the mouth while in embryo. Chordates, which include humans, are also duterostomes. This means you are more closely related to starfish than you are to arthropods, molluscs, or various worms. As with the other echinoderms, starfish are radially symmetrical as adults, but bilaterally symmetrical as larvae, indicating that they evolved from bilaterally symmetrical ancestors. Starfish should not be confused with brittle stars and basket stars (which you can read about here), though they do have the same common ancestor.
I know what you’re thinking and shame on you. (Image: the granulated starfish (
Choriaster granulatus). It has short rays with rounded heads that have vertical slits on the tips. Its body is a reddish-orange with hard markings, but the tips of the rays are a more pale color. End ID)
Starfish consist of a central disc with appendages called arms or rays extending from it. The term ray is often preferred because they are not actually limbs. They are flexible, though, and are used in locomotion and object manipulation. Most species have five rays, but some can have more. The number of rays is usually a multiple of five (and can reach up to 50 in
Labidiaster annulatus), but species with other numbers of rays do exist. The underside of the rays are lined with rows of tube feet. These hollow tubes can be filled with water to extend out of the body and are maneuvered with a complex system of muscles. Each tube foot can be individually controlled and a starfish can have hundreds of them. Tube feet are used for locomotion and object manipulation. While the ends are often suction-cup shaped, they attach to objects using chemical adhesives rather than suction. Tube feet are used to drag the starfish forward and they typically will have one ray that points in the direction they are moving, possibly a remnant of their bilaterally symmetrical ancestors having a front end. Starfish typically move fairly slowly, with about 15 cm (6 in) a minute being a typical top speed. There are exceptions, though, with
Luidia foliolata being able to reach almost 3 m (9 ft) per minute. Tube feet are also filled with sensory cells to help the starfish examine its environment. The flexible arms also help starfish flip themselves over if they end up upside down.
(Image: a starfish of the order Brisingida. It is orange and has numerous very long and flexible arms that are held up in the water column. The arms are covered with elongated spiny protrusions used to filter feed. End ID)
The central disc contains the mouth at the center that is opened and closed with a sphincter. The moth leads into a short esophagus which leads to a stomach divided into two segments: the larger cardiac stomach and smaller pyloric stomach. In primitive starfish, food is swallowed hole and passed to the cardiac stomach, where digestion begins, then passed to the pyloric stomach where digestion finishes. In most species, however, the cardiac stomach has been adapted to be ejected out through the mouth to engulf prey and begin digesting it outside of the body, passing broken-down food into the internal pyloric stomach. This allows starfish to consume prey considerably larger than they are. Starfish famously can consume bivalves by using their arms and tube feet to pry the shellfish open and eject their stomachs into the shell. Not all starfish are carnivores. Many will feed partially or totally on algae and detritus and some have adapted to be filter feeders that use their rays to catch plankton and carry it to their mouths. Some species use modified pedicellariae to capture small fish and crustaceans. Pedicellariae are pincer-like structures found on the skin of some species of starfish that have a number of uses, including aiding in feeding and removing objects and small animals from the starfish.
(Image: a close-up of the underside of a starfish. On the bottom of each ray is a channel filled with tube feet, which appear as small, reddish tubes with sucker-shaped endings. The channels meet in the middle where the mouth is visible as a small hole. End ID)
Internally, the starfish is supported by an endoskeleton made of honeycomb-like calcite structures called ossicles. Most ossicles fit together to form a protective yet flexible shell on the top of the starfish. Ossicles are often the only parts of a starfish that fossilize, leaving starfish with a sparse fossil record. Beneath the ossicles are the digestive system (which extends into the arms) as well as the nervous system and water vascular system. The nervous system consists of a nerve ring that surrounds the mouth and branches off into radial nerves that run down the rays. A par of nerve nets run under the skin and in the water vascular system. Starfish are known to sense by touch, smell, and chemoreception, and though they do not have eyes, they do have light-sensitive eyespots at the tips of the rays. The water vascular system is used both for circulation and movement. Water is drawn into the body through a modified ossicle called the madreporite and into a series of canals that run through the body. Muscular action can open or close valves leading to the tube arms. This causes the tube arms to either fill with water and extend, or lose water and contract. The water vascular system is also used to dispose of some waste and to circulate oxygen through the body. The circulatory system (consisting of a heart and 3 ring canals) does not circulate oxygen, only nutrients. Starfish hearts beat at an average of 6 times per minute.
(Image: a sunflower star (
Pycnopodia helianthoides) on the seafloor. It is a large, reddish-orange starfish with a large central disc and 20 long, slender arms. End ID)
The majority of starfish are dioecious, meaning they have separate males and females, but some species are hermaphroditic, either simultaneous (both male and female gonads at the same time) or sequential (will transition from one sex to another). Paired gonads are located at the base of each arm and release gametes through gonopores on the discs. Some species will engage in behavior where a male will climb on top of the female and overlap her arms, then they will release gametes together to maximize the chance of fertilization. Most fertilized eggs are released into the water, but some will be placed under rocks and some species will brood the eggs using species structures. Brooding species have larger eggs with lots of yolk that skip the larval stage and hatch as small adults. Most species hatch into a planktonic larval stage called the scaphularia, which is equivalent to the blastula stage of vertebrate embryo development. The scaphularia then develops into a bipinnaria, which has bands of cillia on its body used for movement and feeding, as well as stubby rays. The next larval stage is the brachiolaria, which has more developed rays and attaches itself to the substrate through a stalk. Up until now, the larva has been bilaterally symmetrical, but this is where that changes via a radical metamorphosis. The body rearranges itself so that the left side of the brachiolara becomes the bottom of the starfish and the right side becomes the top. The body cavities are rearranged into the circulatory and water vascular system while the gut, mouth, and anus rearrange themselves. The starfish is now a tiny (usually 1 millimeter) radially symmetrical adult that drops off of the stalk.
(Image: a starfish’s larval development from an egg through an amorphous, tentacled brachiolara larva, and to a half- developed juvenile starfish that has not yet formed distinct rays. End ID)
Starfish are famous for their regenerative ability. A starfish that loses rays to predators can grow them back in a process that can take over a year. In many species, a starfish split in half can regenerate into two complete starfish. There are different types of regeneration and different species are capable of different types. The most common is unidirectional regeneration, where a starfish needs the majority of its disc to regenerate. If it is cut in half, only the the piece with over half of the disc will regenerate. Rarer is disc-dependent bidirectional regeneration. This allows a severed ray with at least part of the disc attached to regenerate into a full starfish. Part of the central disc is needed to provide access to the digestive system and mouth. The rarest and most extensive form is disc-independent bidirectional regeneration. This allows a severed ray with none of the original disc to regenerate a full starfish. The severed arm must rely on stored nutrients until the digestive tract regenerates, so only very healthy limbs will last long enough to do so.
(Image: a red starfish regenerating. It has three large, normal rays ans three very small rays branching off of the disc. End ID)
Regeneration occurs in three stages. The first is the repair stage, where the initial wound is healed and the body prepares to the generation of new tissues. This stage is where the starfish is at its most vulnerable to infection or succumbing to the injury. Next is the early regenerative phase, where undifferentiated cells and body structures move toward the regenerating surface. Finally is the advanced regenerative phase, where massive cell replication and differentiation occurs. During this phase, the new ray will grow as a miniature version of the originals and will gradually enlarge until reaching the adult size. This is a vastly oversimplified explanation of regeneration because most of it is cell biology that goes way over my head. Severed rays regrowing a body are sometimes called comets due to having one ray significantly larger than the others. Some species of starfish will deliberately drop a ray if threatened by a predator. This is called autotomy and relies on the predator favoring the easy meal of the dropped body part over continuing to attack the main body.
(image: a comet starfish attached to a glass tank wall, seen from below. It looks like a normal starfish, but with one disproportionately large ray. End ID
Starfish can use their regenerative powers for asexual reproduction. Certain species will engage in fission, splitting themselves apart so both parts will regrow into a full starfish. Some will split off a large section of disc while others can drop a single ray to regenerate. Fission seems to have evolved independently in multiple lineages and presents differently in different species. Some species will only do it as young, while others will do it their entire life. Some species will rarely do it, while others will drop limbs throughout their lives. In at least once species, only males will split themselves. Females of the species
Nepanthia belcheri can split into two males. Asexual reproduction usually occurs in adults, but some species can reproduce as larvae in good conditions, either through fission or budding off clones.
(Image: a chocolate chip starfish (
Protoreaster nodosus) on sand. It is a five-rayed, white starfish with multiple prominent black spines surrounded by red skin on its top end. End ID)
Starfish are found in every ocean and from the intertidal zone to the abyssal depths. Because they do not have the ability to regulate their internal salt content, starfish are not found in fresh water and only relatively few of their nearly 2000 species live in brackish water. Being relatively large generalist predators or omnivores generally occupying the middle of the food chain, starfish are often keystone species for their environments. Fun fact: the term keystone species was originally used to describe a starfish. Starfish play a large role in regulating benthic micro- and macro-organism densities while also being a food source for larger animals. Places where starfish have been removed have seen population explosions of bivalves and other prey species that lead to an overall decrease of biodiversity. On the other hand, the crown-of-thorns starfish (
Acanthaster planci) has seen multiple population explosions due to loss of their predators and are posing a major threat to coral reefs throughout the Indo-Pacific due to their diet of coral. There are also a few invasive species of starfish.
Asterias amurensis is on the list of the world’s 100 most invasive species. Echinoderms are sensitive to pollution and some species of starfish are used as a bioindicators of the health of their ecosystems. Starfish are threatened by pollution and habitat loss, but appear to be more resistant to ocean acidification than other species with calcareous skeletons.
(Image: a crown-of-thorns starfish on bleached coral. It is a large, purple starfish with 15 rays covered with spines all over. End ID)
