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Macrohectopus branickii is a species of Amphipoda (a group containing the scuds) living in the pelagic zone of Lake Baikal, believed to be the only species of amphipod in this Ecological niche within Fresh water (many more amphipods are pelagic/ in the sea, such as the Hyperiidea). It is the Monotypic taxon within the genus Macrohectopus and the only member of the family Macrohectopidae.
The sheer scale of Lake Baikal allows many species, including M. branickii, to occupy such niches which do not exist in other freshwater ecosystems. These pelagic amphipods are prey to many animals within its ecosystem, including the top predator of the lake, the Baikal seal.
The segments are near-transparent, making the animal "clear as water"; which makes the visible without dissection. This is likely to be a means of camouflage; Cystisoma, a marine amphipod, is similarly transparent to avoid predation. Indeed, Takhteev noted that M. branickii exhibits some ecological parallels with other marine amphipods, those being the Vitjazianidae and Hyperiopsidae families.
Male M. branickii are up to long, immature females are , and mature females reach up to ; they exhibit significant sexual size dimorphism, with some considering the males to be "Dwarfism".
The mitochondrial genome of M. branickii was analyzed in 2021; containing 42,256 , it is the longest sequence known for amphipods and one of the largest genomes within the animal kingdom.
Lake Baikal contains a great diversity of amphipod species; 40% of all known species of Gammaroidea live within this lake, and they have undergone significant adaptive radiation into a plethora of Body plan. Out of all these species, M. branickii is the only pelagic member of the group; molecular and morphological analysis, however, indicates that the genus originates within Micruropodidae, a family of , though Micruropus wahli is a capable swimmer and may be caught at the surface through light-fishing; this attraction to light is also observed for the pelagic amphipod. Phototaxis may make them vulnerable to light pollution in close to settlements. The pelagic amphipod Animal migration throughout the lake all year round, continuing to do so in their peak breeding season. This species also undergoes diel vertical migration, moving to the surface at night from their daytime depth of . The larger mature females inhabit the deeper range of this depth compared to smaller individuals. During nighttime, they may be especially abundant in the water column above underwater slopes. The density of amphipods is such that it creates a noticeable scattering layer when using hydroacoustic sensors, akin to the deep scattering layer in the ocean. These sensors are one method used to survey the population of these crustaceans. The pelagic amphipods tend to feed more on zooplankton the larger they grow, though the exact ratios vary between individuals; these prey items are consistently located above depth throughout the day.
Its role within the lake's ecosystem has been compared to that of Antarctic krill and mysis shrimp, being an important Trophic level due to its abundance; the density of pelagic amphipods has been estimated at tens of grams per , with total numbers reaching several million animals and constituting 90-99% of the zooplanktonic biomass. Annual biomass production of M. branickii is calculated to be around .
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