Sea apple is the

common name In biology, a common name of a taxon or organism (also known as a vernacular name, English name, colloquial name, country name, popular name, or farmer's name) is a name that is based on the normal language of everyday life; and is often contrast ...

for the colorful and somewhat round

sea cucumber Sea cucumbers are echinoderms from the class Holothuroidea (). They are marine animals with a leathery skin and an elongated body containing a single, branched gonad. Sea cucumbers are found on the sea floor worldwide. The number of holothuria ...

s of the

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

''Pseudocolochirus'', found in

Indo-Pacific The Indo-Pacific is a vast biogeographic region of Earth. In a narrow sense, sometimes known as the Indo-West Pacific or Indo-Pacific Asia, it comprises the tropical waters of the Indian Ocean, the western and central Pacific Ocean, and the ...

waters. Sea apples are

filter feeder Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feedin ...

s with tentacles, ovate bodies, and tube-like feet. As with many other holothurians, they can release their internal organs or a toxin into the water when stressed.

Physiology

Sea Apple at Cannibal Rock Indonesia 2002

Sea apples are holothuroids, and as such share many of the same physical characteristics. A few notable characteristics are discussed below.

Anatomy and feeding

The ovate body of an adult sea apple can grow up to long. A central mouth-like cavity is surrounded by feathery tentacles, which add additional length. Sea apples, like many

echinoderms An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the sea li ...

, have rows of

tube feet Tube feet (technically podia) are small active tubular projections on the oral face of an echinoderm, whether the arms of a starfish, or the undersides of sea urchins, sand dollars and sea cucumbers; they are more discreet though present on britt ...

which help them move over and adhere to structures. The bodies and tentacles of sea apples come in many different colorings. The Australian species has a primarily purple body, red feet, and purple and white tentacles. The sea apple feeds primarily on

plankton Plankton are the diverse collection of organisms found in Hydrosphere, water (or atmosphere, air) that are unable to propel themselves against a Ocean current, current (or wind). The individual organisms constituting plankton are called plankt ...

, which it filters from the water with its tentacles. It alternately brings each tentacle to its mouth, scraping off the captured plankton. Sea apples usually feed at night, a time when their delicate tentacles are less at risk from predators.

Defense

When disturbed, sea apples, like other holothuroids, can violently extrude their entrails from both ends. In addition, sea apples can release a toxic

saponin Saponins (Latin "sapon", soap + "-in", one of), also selectively referred to as triterpene glycosides, are bitter-tasting usually toxic plant-derived organic chemicals that have a foamy quality when agitated in water. They are widely distributed ...

called

holothurin The holothurins are a group of toxins originally isolated from the sea cucumber '' Actinopyga agassizii''. They are contained within clusters of sticky threads called Cuvierian tubules which are expelled from the sea cucumber as a mode of sel ...

into the water as a defense mechanism. In addition, if threatened or in an unsuitable environment, sea apples can consume large amounts of surrounding seawater to swell to nearly double their original size, this allows them to be moved to a new area by water currents, and much more quickly than they could walk.

Problems in captivity

Because of their interesting appearance and behaviour, sea apples are often widely desired as specimen for display

marine aquaria A marine aquarium is an aquarium that keeps marine plants and animals in a contained environment. Marine aquaria are further subdivided by hobbyists into fish only (FO), fish only with live rock (FOWLR), and reef aquaria. Fish only tanks often sh ...

. They are considered

reef safe Reef safe is a distinction used in the saltwater aquarium hobby to indicate that a fish or invertebrate is safe to add to a reef aquarium. There is no fish that is completely reef safe. Every fish that is commonly listed as reef safe are species th ...

as far as their compatibility with other species. However, they can be considered unsafe for reef aquaria for multiple reasons:

Starvation

Sea apples often starve to death in display aquaria. Levels of plankton in aquaria are often lower than optimal, and sea apples are often seen attempting to feed not only at night, as in their natural habitat, but also in the daytime. With only low levels of food available, these sea apples often starve, becoming progressively smaller as this happens. To try to circumvent these problems, hobbyists attempt to give the sea apple specimens supplemental feedings of plankton and liquid food.

Harassment and predation

Sea apples are often harassed by many aquarium inhabitants.

Crustaceans Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group ...

, such as

hermit crabs Hermit crabs are anomuran decapod crustaceans of the superfamily Paguroidea that have adapted to occupy empty scavenged mollusc shells to protect their fragile exoskeletons. There are over 800 species of hermit crab, most of which possess an a ...

, and fish often peck or pick at sea apple's feathery tentacles. This may be for predatory purposes, or simply to steal trapped particles and plankton from the tentacles. Occasionally, sea apples use their defense mechanisms in response to harassment. The release of their toxin can poison other aquarium inhabitants, and is one of the reasons they are not commonly seen in aquariums."Saltwater Invertebrates for Marine Reef Aquariums: Sea Apple." Aquarium Fish: Tropical Freshwater Fish and Saltwater Fish for Home Aquariums. 2008. .

References

External links

* {{Taxonbar, from=Q2265800 Cucumariidae Invertebrate common names