Sea apple is the
common name for the colorful and somewhat round
sea cucumbers of the
genus
Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...
''Pseudocolochirus'', found in
Indo-Pacific waters.
Sea apples are
filter feeders 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 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, 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 br ...
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 water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a crucia ...
, 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 called
holothurin 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. They are considered
reef safe 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 gro ...
, such as
hermit crabs, 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
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{{Taxonbar, from=Q2265800
Cucumariidae
Invertebrate common names