Ophiothrix fragilis, commonly known as the fragile brittle star, is a species of echinoderms belonging to the class Ophiuroidea, within the family Ophiotrichidae. Its scientific name derives from the Greek ‘ophis‘, meaning ‘snake‘, and ‘thrix‘, meaning ‘hair‘, referring to its long slender arms covered with spines, while ‘fragilis‘ alludes to its ability to easily break or shed parts of its body as a defense mechanism. This species is common along the northeastern Atlantic coasts, from Norway and the British Isles to the Mediterranean, inhabiting subtidal zones and depths up to 200 meters. Ophiothrix fragilis is especially abundant on rocky bottoms and algal areas, often forming large aggregations in detritus-rich zones.
Ophiothrix fragilis has a small central disc no larger than 2 cm in diameter, from which emerge five long and extremely thin arms that can reach up to 10 cm in length. These arms are covered with movable spines, with a plumose or bristly appearance, giving it a fragile look and aiding in locomotion and feeding. Its coloration varies, commonly showing shades of gray, brown, or pink, often with darker bands on the arms. A distinctive feature that helps differentiate it from similar species such as Amphiura filiformis is the presence of longer spines and its tendency to form large aggregations. While Amphiura filiformis has smoother and shorter arms, Ophiothrix fragilis stands out due to its fine and flexible spines.
Regarding its feeding habits, Ophiothrix fragilis is a suspension feeder that filters organic particles and plankton from the water. It uses its spiny arms to capture suspended particles, which are then transported to the mouth located on the underside of the central disc. It is commonly found in areas with strong marine currents, which facilitate the capture of suspended particles. Additionally, by living in large aggregations, this species plays an important ecological role in nutrient cycling within the marine ecosystem.
Reproduction in Ophiothrix fragilis can be both sexual and asexual. In sexual reproduction, individuals release their gametes into the water where external fertilization occurs. The larvae are planktonic and pass through several developmental stages before settling on the seafloor and metamorphosing into juveniles. Asexual reproduction, through division of the central disc, is also possible, contributing to its ability to regenerate lost body parts, including arms. This high regenerative capacity also explains its fragility, as it can shed its arms as a defense mechanism when attacked by predators.
A curious fact about Ophiothrix fragilis is its ability to form dense aggregations of thousands of individuals, which can significantly influence the structure of the seafloor. These aggregations are known as ‘brittle star patches‘, and their density can completely cover the substrate, altering the dynamics of other benthic organisms. Furthermore, Ophiothrix fragilis is a bioindicator species, sensitive to changes in water quality, making it a key organism in environmental impact studies and marine ecosystem conservation.
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