Asteronotus cespitosus is a species of nudibranch belonging to the family Discodorididae within the order Nudibranchia. The genus name ‘Asteronotus‘ comes from the Greek ‘aster‘, meaning star, and ‘notos‘, meaning dorsum, referring to the shapes of the structures covering its body. The epithet ‘cespitosus‘ alludes to its hairy appearance or the presence of projections on its surface, similar to tufts of grass. Asteronotus cespitosus is widely distributed in the Indo-Pacific, from the coasts of East Africa, the Red Sea, to Japan, Australia and some central Pacific islands. It is found mainly on sandy seabeds, seagrass beds, coral reefs and intertidal zones, at depths ranging from a few meters to about 30 meters.
Asteronotus cespitosus is a medium to large nudibranch, which can reach up to 15-25 centimeters in length. Its body is oval and slightly flattened, with a rough and rough texture due to the presence of conical tubercles scattered all over its dorsal surface. These tubercles give it a hairy or ‘caespitose‘ appearance, which contributes to its camouflage among algae and corals. Its coloration can vary from dark brown, yellowish to grayish tones, sometimes with cream or white spots that provide additional camouflage in its environment. It has a pair of retractable rhinophores on the anterior part of its body, which are sensory organs used to detect chemicals in the water, and a branched duster-like gill on the posterior part of the body, which is used for respiration.
As for its feeding habits, Asteronotus cespitosus is a specialized predator that feeds mainly on marine sponges. It uses its radula (a tongue-like structure with small chitinous teeth) to scrape and consume sponge cells. This sponge-rich diet also contributes to the toxicity of the species, as it can store toxins derived from its prey in its own tissues, providing some protection against potential predators. This nudibranch is selective with respect to the species of sponges it consumes, preferring certain specific species found in its habitat, such as Dysidea herbacea.
Reproduction of Asteronotus cespitosus is sexual and, like all nudibranchs, it is a simultaneous hermaphrodite, meaning that each individual possesses both male and female reproductive organs. During mating, two individuals line up head to tail and both exchange sperm. Subsequently, both will lay eggs in characteristic gelatinous ribbons on the substrate. The eggs are generally deposited in coils or in white to pink masses and, after a period of development, hatch as planktonic larvae that disperse in the water column before settling and metamorphosing into juveniles.
A curious fact about Asteronotus cespitosus is its ability to camouflage itself effectively in its environment. Its coloration and body texture mimic the sponges and algae among which it lives, allowing it to avoid predators. In addition, this nudibranch can regenerate parts of its body if damaged, an ability it shares with other members of its group. Although it is not a highly toxic species for humans, it is recommended to avoid direct contact, as some toxins accumulated from the sponges it feeds on can cause skin irritation.
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