Will a Chocolate Chip Starfish eat macro algae?

Common to both the pet fish and souvenir trade alike, the Chocolate Chip Starfish is well known among aquarists as well as to tourists and curio collectors. Despite this familiarity, this interesting and decorative species is much misunderstood. This article aims to summarize the biology and aquarium maintenance on the basis of observations of the species in nature and in captivity.

Chocolate Chip Starfish primarily inhabit sandy and muddy lagoons and seagrass beds, and are less common on the reefs themselves, preferring the back reef area. They are typically found in shallow water, at depths of 1-30 meters (3 to 100 feet); juveniles prefer even shallower water, and are most common at depths of less than 2 m (7 feet). In their favored sea grass biotopes, Chocolate Chip Starfish can occur at densities of up to 30 specimens per 100 square meters (1100 square feet).

This species occurs widely across the tropical Indo-Pacific region. It is found along the East Africa coastline including the island of Madagascar; Sri Lanka; Indonesia; as far north as southern Japan and as far south as the tropical northern coast of Australia; it is also found around some island groups of the Southern Pacific. Regionally it can be a very common starfish and is often collected, dried and sold as a souvenir.

The “chocolate chips” on the upper surface of Protoreaster nodosus can be rounded or pointed. The Chocolate Chip Starfish was among the first animals to achieve a formal scientific name, having been named by Linnaeus himself in 1758. It has several common names, being known primarily as the Chocolate Chip Starfish among aquarists but often as the Horned Starfish in the scientific literature and natural history books.

It is a member of the starfish family Oreasteridae, a group of starfish consisting of stout species with short, thick arms. Other members of the family are sometimes traded as aquarium specimens; these include the Red-Knobbed Starfish Protoreaster linkii and various species of Pentaceraster, sometimes called Knobby Starfish. Their aquarium care is similar to those of the Chocolate Chip Starfish, so much of what is said here applies equally to these starfish as well.

The eponymous “chocolate chips” or knobs are found only on the aboral surface, that is, the surface without the mouth. (Among echinoderm biologists, it is normal to refer to the upper and lower surfaces of the body as either the aboral or oral surface.) These knobs are believed to deter predators, perhaps by making the starfish too difficult to swallow whole; unfortunately for the Chocolate Chip Starfish, they do nothing to prevent humans from collecting them, the majority ending of these specimens ending up as dried souvenirs. Other starfish from the family Oreastidae require similar care as the Chocolate Chip Starfish.

According to the scientific literature the central disk can reach a diameter of up to 12 cm (5 inches) and the arms a length of about 14 cm (5.5 inches). Consequently the entire starfish can reach a diameter of up to 40 cm (16 inches). These maximum sizes are rarely reached in either nature or in captivity though, most specimens getting no larger than 20-25 cm (8-10 inches) in diameter.

In the Philippines, perhaps due to collecting pressure on the larger and more valuable specimens, Chocolate Chip Starfish larger than 14 cm (5.5 inches) are rare. The average diameter of adult specimens in the Philippines is about 10 cm (4 inches) and the average weight about 250 g (8.8 ounces). Chocolate Chip Starfish reach sexual maturity at a diameter of about 8 cm (3 inches); at this point they are usually 2-3 years old.

Most aquarium shops sell specimens about 10 cm (4 inches) in diameter, and these are about 5 or 6 years old. The bigger specimens sold measuring 14 cm (5.5 inches) or so are estimated to be around 17 years old. It is unclear if the large specimens (40 cm/16 inches) mentioned in the older literature are really Chocolate Chip Starfish or something else.

It could be that before they were seriously targeted by fisheries, Chocolate Chip Starfish in pristine habitats with lots of food could get very large indeed. Without selection pressure favoring specimens that became sexually mature when relatively small, the average adult size might have been larger than it is today. But it is also possible that collection by humans has had little impact on Chocolate Chip Starfish populations, and the big specimens mentioned in older books and scientific papers are wrongly identified starfish of other types.

Despite their name, the colors of Chocolate Chip Starfish can be quite variable. The ground color may be white, yellow, brown, red and even bluish, while the “chips” vary in color from grey to dark brown. The physiology of Protoreaster nodosus is similar to that of other starfish.

Food and waste enter and exit the same opening, the mouth. Chocolate Chip Starfish eat by everting their orange-colored stomach out of the mouth and onto their prey. As the prey is digested, the stomach is slowly pulled back into the mouth.

Because of the way they feed, it is quite easy to see what types of food they prefer, of which more will be said shortly. Seawater is pumped into the vascular system by the madreporite, a small, wart-like structure on the aboral surface of the starfish usually located to one side of the central disk. The madreporite supplies the various canals that make up the water vascular system responsible for the movement of the mouth and tube feed.

The water vascular system also works as the starfish’s circulatory system, doing many of the things that the bloodstream does in vertebrates. If air gets trapped in the water vascular system, the starfish can find itself unable to move and eventually dies. Shallow water starfish do sometimes find themselves above the waterline in the wild, and do have the ability to close the madreporite and thereby prevent the air bubble problem.

But this does assume they are not removed from the water too quickly, and aquarists should certainly avoid removing these animals from the water where possible, but if they must, they should do so slowly, exposing the arms to the air before the central disk, so that the starfish can react appropriately. Chocolate Chip Starfish are found in the intertidal zone and may sometimes leave the water voluntarily, in which case they will have closed the madreporite themselves. Short term exposure to air and sunlight does not appear to cause these starfish any harm.

The madreporite is visible as a dark grey spot on the central disk and is used to regulate flow of water into the vascular system. The water inside the vascular system of starfish is essentially identical to the seawater around them, and consequently sudden changes in salinity and temperature can be acutely stressful. It is essential that starfish are acclimated to the home aquarium slowly, so that they have time to adjust to differences in salinity and temperature compared to those experienced at the pet store.

The larger the difference in salinity and temperature, the slower the acclimation process should be; using the drip method of acclimation, half an hour to an hour should be sufficient in most cases. Curiously, these starfish possess some kind of memory, and are apparently are able to be trained to be fed at a specific point and at a specific time. In common with other starfish, they have a relatively simply nervous system without an obvious brain, so how they can exhibit such behavior remains unknown.

During the 1980s Chocolate Chip Starfish were at the center of research intended to benefit the biochemical industry. Several interesting chemicals were found in their bodies, including saponins, sterols and steroids. For whatever reason, this interest seems to have waned in recent years.

Chocolate Chip Starfish are dioecious, but the males and females cannot be distinguished without dissection. The two sexes occur in apparently similar numbers within each population. Spawning takes place between March and May, usually in the deeper water parts of their range.