''Macrohectopus branickii'' is a species of

amphipod Amphipoda () is an order of malacostracan crustaceans with no carapace and generally with laterally compressed bodies. Amphipods () range in size from and are mostly detritivores or scavengers. There are more than 10,700 amphipod species cur ...

(a group containing the

scud A Scud missile is one of a series of tactical ballistic missiles developed by the Soviet Union during the Cold War. It was exported widely to both Second and Third World countries. The term comes from the NATO reporting name attached to the m ...

s) living in the

pelagic zone The pelagic zone consists of the water column of the open ocean and can be further divided into regions by depth. The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or water column between the sur ...

Lake Baikal Lake Baikal is a rift lake and the deepest lake in the world. It is situated in southern Siberia, Russia between the Federal subjects of Russia, federal subjects of Irkutsk Oblast, Irkutsk Oblasts of Russia, Oblast to the northwest and the Repu ...

, believed to be the only species of amphipod in this niche within

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include non-salty mi ...

(many more amphipods are pelagic/

plankton Plankton are the diverse collection of organisms that drift in Hydrosphere, water (or atmosphere, air) but are unable to actively propel themselves against ocean current, currents (or wind). The individual organisms constituting plankton are ca ...

ic in the sea, such as the Hyperiids). It is the only known species 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 ecosystem Freshwater ecosystems are a subset of Earth's aquatic ecosystems that include the biological communities inhabiting freshwater waterbodies such as lakes, ponds, rivers, streams, springs, bogs, and wetlands. They can be contrasted with marine ec ...

s. These pelagic amphipods are prey to many animals within its ecosystem, including the top aquatic predator of the lake, the

Baikal seal The Baikal seal (''Pusa sibirica''), also known as Lake Baikal seal or Baikal nerpa (), is a species of earless seal endemic to Lake Baikal in Siberia, Russia. The Baikal seal is one of the smallest earless/true seals, and the only exclusively ...

Description

''Macrohectopus branickii'' was described by

Benedykt Dybowski Benedykt Tadeusz Dybowski (12 May 183331 January 1930) was a Polish naturalist and physician. Life Benedykt Dybowski was born in Adamaryni, within the Minsk Governorate of the Russian Empire to Polish nobility. He was the brother of naturalis ...

, who diagnosed it as possessing two pairs of antennae covered in

seta In biology, setae (; seta ; ) are any of a number of different bristle- or hair-like structures on living organisms. Animal setae Protostomes Depending partly on their form and function, protostome setae may be called macrotrichia, chaetae, ...

e, with the upper pair more robust and elongate, kidney-shaped

eyes An eye is a sensory organ that allows an organism to perceive visual information. It detects light and converts it into electro-chemical impulses in neurons (neurones). It is part of an organism's visual system. In higher organisms, the ey ...

that each occupy half of the head, smooth body segments, along with keeled and spined tail segments. The laminae of the

gill A gill () is a respiration organ, respiratory organ that many aquatic ecosystem, aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow r ...

s and brood plates are not covered, and are visible from the side. The

chitin Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is the second most abundant polysaccharide in nature (behind only cell ...

ous segments are near-transparent, making the animal "clear as water"; which makes the

muscle fiber A muscle cell, also known as a myocyte, is a mature contractile cell in the muscle of an animal. In humans and other vertebrates there are three types: skeletal, smooth, and cardiac (cardiomyocytes). A skeletal muscle cell is long and threadl ...

s visible without

dissection Dissection (from Latin ' "to cut to pieces"; also called anatomization) is the dismembering of the body of a deceased animal or plant to study its anatomical structure. Autopsy is used in pathology and forensic medicine to determine the cause of ...

. This is likely to be a means of

camouflage Camouflage is the use of any combination of materials, coloration, or illumination for concealment, either by making animals or objects hard to see, or by disguising them as something else. Examples include the leopard's spotted coat, the b ...

; '' 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 Sexual dimorphism is the condition where sexes of the same species exhibit different morphological characteristics, including characteristics not directly involved in reproduction. The condition occurs in most dioecious species, which consis ...

, with some considering the males to be " dwarfed". The

mitochondrial A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is used ...

genome A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...

of ''M. branickii'' was analyzed in 2021; containing 42,256

base pair A base pair (bp) is a fundamental unit of double-stranded nucleic acids consisting of two nucleobases bound to each other by hydrogen bonds. They form the building blocks of the DNA double helix and contribute to the folded structure of both DNA ...

s, it is the longest sequence known for amphipods and one of the largest genomes within the animal kingdom.

Ecology

''Macrohectopus branickii'' is

endemic Endemism is the state of a species being found only in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also foun ...

to Lake Baikal, with the "center" of its range being the deepest, central part of the lake. It prefers waters colder than . Lake Baikal contains a great diversity of amphipod species; 40% of all known species of gammaroid amphipods live within this lake, and they have undergone significant

adaptive radiation In evolutionary biology, adaptive radiation is a process in which organisms diversify rapidly from an ancestral species into a multitude of new forms, particularly when a change in the environment makes new resources available, alters biotic int ...

into a plethora of morphological forms. Out of all these species, ''M. branickii'' is the only pelagic member of the group;

molecular A molecule is a group of two or more atoms that are held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemistry, ...

and morphological analysis, however, indicates that the genus originates within Micruropodidae, a family of

burrow file:Chipmunk-burrow (exits).jpg, An eastern chipmunk at the entrance of its burrow A burrow is a hole or tunnel excavated into the ground by an animal to construct a space suitable for habitation or temporary refuge, or as a byproduct of Animal lo ...

ers, 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 Phototaxis is a kind of taxis, or locomotory movement, that occurs when a whole organism moves towards or away from a stimulus of light. This is advantageous for phototrophic organisms as they can orient themselves most efficiently to receive ...

may make them vulnerable to

light pollution Light pollution is the presence of any unwanted, inappropriate, or excessive artificial Visible spectrum, lighting. In a descriptive sense, the term ''light pollution'' refers to the effects of any poorly implemented lighting sources, during the ...

littoral zone The littoral zone, also called litoral or nearshore, is the part of a sea, lake, or river that is close to the shore. In coastal ecology, the littoral zone includes the intertidal zone extending from the high water mark (which is rarely flood ...

s close to settlements. Macrohectopus branickii

The pelagic amphipod migrates throughout the lake all year round, continuing to do so in their peak

breeding season Seasonal breeders are animal species that successfully mate only during certain times of the year. These times of year allow for the optimization of survival of young due to factors such as ambient temperature, food and water availability, and ch ...

. This species also undergoes

diel vertical migration Diel vertical migration (DVM), also known as diurnal vertical migration, is a pattern of movement used by some organisms, such as copepods, living in the ocean and in lakes. The adjective "diel" ( IPA: , ) comes from , and refers to a 24-hour ...

, 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 The deep scattering layer, sometimes referred to as the sound scattering layer, is a layer in the ocean consisting of a variety of marine animals. It was discovered through the use of sonar, as ships found a layer that scattered the sound and w ...

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.

Trophic ecology

''Macrohectopus branickii'' is the main species of

macroplankton Plankton are the diverse collection of organisms that drift in water (or air) but are unable to actively propel themselves against currents (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they pr ...

within the lake, being an important part of the

food chain A food chain is a linear network of links in a food web, often starting with an autotroph (such as grass or algae), also called a producer, and typically ending at an apex predator (such as grizzly bears or killer whales), detritivore (such as ...

; the microcrustaceans '' Epischura baikalensis'' and '' Cyclops kolensis'' feed on

phytoplankton Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater Aquatic ecosystem, ecosystems. The name comes from the Greek language, Greek words (), meaning 'plant', and (), mea ...

(predominantly '' Aulacoseira baicalensis''). They then fall prey to the pelagic amphipod, which in turn feed various fish such as omul ('' Coregonus migratorius''), sculpins ('' Comephorus spp.'', ''

Cottocomephorus ''Cottocomephorus'' is a genus of freshwater ray-finned fishes belonging to the Family (biology), family Cottidae, the typical sculpins. This genus is endemism, endemic to Lake Baikal and its surrounding tributaries in Russia. They have relativel ...

spp.''), and the

seal Seal may refer to any of the following: Common uses * Pinniped, a diverse group of semi-aquatic marine mammals, many of which are commonly called seals, particularly: ** Earless seal, also called "true seal" ** Fur seal ** Eared seal * Seal ( ...

, which feeds on the fish and the amphipods. During the amphipod's diel migration, some of them may rise up too fast and become temporarily stuck on the water's surface after breaching; they must break through the

surface tension Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension (physics), tension is what allows objects with a higher density than water such as razor blades and insects (e.g. Ge ...

to resubmerge. Before resubmerging, they are vulnerable to the water bat ''

Myotis petax The eastern water bat or Sakhalin bat (''Myotis petax'') is a species of mouse-eared bat. It was for a long time considered to be a subspecies of ''Myotis daubentonii''. Distribution The Eastern water bat is a forest bat found in Russia (south a ...

'', which " trawl up" the amphipods for an easy meal. On occasion they may also rise up to the surface during daytime, and

seagulls Gulls, or colloquially seagulls, are seabirds of the subfamily Larinae. They are most closely related to terns and skimmers, distantly related to auks, and even more distantly related to waders. Until the 21st century, most gulls were placed ...

are quick to take advantage of this opportunity. Its role within the lake's ecosystem has been compared to that of

Antarctic krill Antarctic krill (''Euphausia superba'') is a species of krill found in the Antarctica, Antarctic waters of the Southern Ocean. It is a small, swimming crustacean that lives in large schools, called swarms, sometimes reaching densities of 10,000� ...

and

mysis shrimp Mysida is an order of small, shrimp-like crustaceans in the malacostracan superorder Peracarida. Their common name opossum shrimps stems from the presence of a brood pouch or "marsupium" in females. The fact that the larvae are reared in this p ...

, being an important lower-level consumer 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 Biomass is a term used in several contexts: in the context of ecology it means living organisms, and in the context of bioenergy it means matter from recently living (but now dead) organisms. In the latter context, there are variations in how ...

. Annual biomass production of ''M. branickii'' is calculated to be around .

Notes

References

{{Taxonbar, from1=Q18606058, from2=Q13805074 Gammaroidea Endemic fauna of Russia Crustaceans of Russia Crustaceans described in 1874 Fauna of Lake Baikal Amphipod genera Monotypic crustacean genera