The Eastern Crescentjack

Order: Remipinniformes

Family: Lunatopteridae

Genus: Lunatopterus

If Alladoras had an definition to the word 'adaptable', then the crescentjack would be the answer to that word. 

The crescentjacks (Lunatopteridae) are among the most common predators in Alladoras, with their distinctive crescent-shaped dorsal/ventral fins being one of their most recognisable traits, which aid them in manoeuvrability while swimming at high speeds. They are defined as generalist, medium-sized predators, averaging at roughly 50-80 cm, with the largest reaching 2 metres. Often seen swimming in small groups of up to 20 individuals, they can be seen off seamounts, plateworm reefs and carpgrass meadows, with all 30 species in 4 genera living worldwide except for the poles. The most common genus, Lunatopterus (which the species shown here, the eastern crescentjack, is in) have a virtually circumtropical distribution, with the eastern crescentjack, shown here chasing a collared chovy (Engrauloides collaris), found in Ichthyonesia and Pulau Kecil.

They are easily distinguished from other pelagys by their sharp, sickle shaped dorsal and ventral fins, which paired together, resemble a crescent. This remarkable feature is an evolutionary holdover from their ancestors, which had enlarged dorsal/ventral fins, which were actually used in swimming, in a method similar to how an ocean sunfish swims, called median paired fin swimming, still used by some species, but repurposed in the crescentjacks to aid in manoeuvrability, with the flexible joints at the base of the fins capable of stiffening it up to stop quickly, redirect themselves to aid in turning, allowing it to essentially turn on a dime, stopping rapidly while swimming and accelerate rapidly, employing this to their advantage, darting into schools of small pelagys, chasing targetted pelagys and outmanoeuvring them, eating their prey.

This unparalleled agility is unrivaled, with crescentjacks often evading grapsharks and large aeunas despite their superior speed, often twisting and turning away at high speeds, often out of sight from predators. They also express little specialisation for a niche, instead being largely generalist predators who will eat any kind of schooling pelagys, cuttlers or skrim, and are not picky with what they eat, even dabbling in scavenging on occasion if the option arises.

Crescentjacks particularly frequent shallow habitats, seen close to seamounts, reefs and coastlines in tropical areas, grouping together in loose groups, and when not hunting, often swim leisurely together, without expressing much aggression towards others, either looking for cleaner pelagys; pelagys which specialise for eating dead skin and parasites off skin, sometimes even resorting to rubbing themselves on rough surfaces, such as rocks, sand and even gallantee skin. The eastern crescentjack is somewhat fond of lagoons and estuaries, being the most prolific predator of young juvenile pelagys, taking advantage of the young, naïve prey, with their smaller size compared to other predators allowing them to enter weed choked estuaries and hunt prey. Aside from hunting prey, however, the eastern crescentjack also spends a lot of its juvenile life in estuaries and lagoons, as they breed here in order to provide safe nursery to their young, which will spend their time here eating on worms and small skiddlecrabs. When the young grow older, they head into carpgrass prairies to hunt small pelagys, and eventually head out into the open ocean to join adult crescentjacks in the open seas of Alladoras.