{{Short description|Pelagic freshwater amphipod}} {{Speciesbox | image = Macrohectopus branickii.jpg | image2 = Constantia branickii.png | display_parents = 3 | genus = Macrohectopus | parent_authority = Stebbing, 1906 | species = branickii | authority = (Dybowsky B.N., 1874) | synonyms = ''Constantia branickii'' Dybowsky, 1874 (Unavailable) | synonyms_ref = <ref name="WoRMS">{{Cite WoRMS|last=Lowry|first=Jim|url=https://www.marinespecies.org/amphipoda/aphia.php?p=taxdetails&id=531153|title= Amphipoda taxon details Macrohectopus branickii (Dybowsky, 1874)|access-date=18 May 2025}}</ref> | range_map = Benedictiinae map.png | range_map_caption = Lake Baikal within Eastern Siberia }}
'''''Macrohectopus branickii''''' is a species of amphipod (a group containing the scuds) living in the pelagic zone of Lake Baikal, believed to be the only species of amphipod in this niche within freshwater (many more amphipods are pelagic/planktonic 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'''.{{Efn|The fossil genus ''Hellenis'', previously considered the only other genus in the family, was removed from Macrohectopidae after a 2024 study evaluating it.<ref>{{Cite journal |last1=Copilaş-Ciocianu |first1=Denis |last2=Ionesi |first2=Viorel |date=2024-03-29 |title=New Miocene fossil taxa illuminate the evolution and paleobiogeography of the Ponto-Caspian gammaroid amphipod radiation |url=https://brill.com/view/journals/ctoz/aop/article-10.1163-18759866-bja10061/article-10.1163-18759866-bja10061.xml |journal=Contributions to Zoology |pages=1–21 |doi=10.1163/18759866-bja10061 |issn=1383-4517|doi-access=free }}</ref><ref name="Hellenis">{{cite WoRMS|last=Horton|first=Tammy|date=12 April 2024|title=''Hellenis'' Petunnikov, 1914 †|id=531144|access-date=23 May 2025}}</ref>}}
The sheer scale of Lake Baikal allows many species, including ''M. branickii'', to occupy niches which do not exist in other freshwater ecosystems. These pelagic amphipods are prey to many animals within its ecosystem, including the top aquatic predator of the lake, the Baikal seal.
==Description== {{See also|Amphipoda#Anatomy}} {{Expand section|descriptions of the genus (p. 50) and the limbs (original paper is in german technical terms)|date=May 2025}} ''Macrohectopus branickii'' was described by Benedykt Dybowski, who diagnosed it as possessing two pairs of antennae covered in setae, with the upper pair more robust and elongate, kidney-shaped eyes that each occupy half of the head, smooth body segments, along with keeled and spined tail segments. The laminae of the gills and brood plates are not covered, and are visible from the side.<ref name="OG"/>
The chitinous segments are near-transparent, making the animal "clear as water";{{Efn|{{Lang|de|Der Körper dieser schönen, zarten Thiere ist wasserhell, woher man sie beim Schwimmen nur an ihren schwarzen Augen warnehmen kann.<ref name="OG"/>|italic=yes}}}} which makes the muscle fibers visible without dissection.<ref name="OG">{{cite journal |last1=Dybowsky |first1=B.N. |title=Beiträge zur näheren Kenntniss der in dem Baikal-See vorkommenden niederen Krebse aus der Gruppe der Gammariden |journal=Horae Societatis Entomologicae Rossicae, Beiheft |date=1874 |volume=10 |language=German |pages=186–188 |doi=10.5962/bhl.title.9945 |url=https://www.marinespecies.org/aphia.php?p=sourceget&id=21064 |access-date=18 May 2025}}</ref><ref>{{cite journal |author=Polina Drozdova |author2=Alexandra Saranchina|author3=Mariya Morgunova|author4=Alena Kizenko|author5=Yulia Lubyaga|author6=Boris Baduev|author7=Maxim Timofeyev|title=The level of putative carotenoid-binding proteins determines the body color in two species of endemic Lake Baikal amphipods |journal=PeerJ |date=19 June 2020 |volume=8 |article-number=e9387 |doi=10.7717/peerj.9387 |doi-access=free |pmid=32596057 |pmc=7307558 }}</ref> This is likely to be a means of camouflage; ''Cystisoma'', a marine amphipod, is similarly transparent to avoid predation.<ref name="OpenOcean">{{cite journal | vauthors = Cronin TW | title = Camouflage: Being Invisible in the Open Ocean | journal = Current Biology | volume = 26 | issue = 22 | pages = R1179–R1181 | date = November 2016 | pmid = 27875694 | doi = 10.1016/j.cub.2016.09.056 | s2cid = 25085458 | doi-access = free | bibcode = 2016CBio...26R1179C }}</ref> Indeed, Takhteev noted that ''M. branickii'' exhibits some ecological parallels with other marine amphipods, those being the Vitjazianidae and Hyperiopsidae families.<ref name="Parallel">{{cite journal |last1=Takhteev |first1=V. V. |title=Trends in the evolution of Baikal amphipods and evolutionary parallels with some marine malacostracan faunas |journal=Advances in Ecological Research |date=2000 |volume=31 |pages=197–220 |doi=10.1016/S0065-2504(00)31013-3 |bibcode=2000AdER...31..197T |isbn=0-12-013931-6 |url=https://www.sciencedirect.com/science/article/abs/pii/S0065250400310133 |access-date=18 May 2025|url-access=subscription }}</ref>
Male ''M. branickii'' are up to {{Convert|6|mm|abbr=on}} long, immature females are {{Convert|7|-|14|mm|abbr=on}},<ref name="NewDataEco"/> and mature females reach up to {{Convert|37|mm|abbr=on}};<ref name="Isotope"/> they exhibit significant sexual size dimorphism, with some considering the males to be "dwarfed".<ref name="Parallel"/>
The mitochondrial genome of ''M. branickii'' was analyzed in 2021; containing 42,256 base pairs, it is the longest sequence known for amphipods and one of the largest genomes within the animal kingdom.<ref>{{cite journal |author=Elena V Romanova|author2=Yurij S Bukin|author3=Kirill V Mikhailov|author4=Maria D Logacheva|author5=Vladimir V Aleoshin|author6=Dmitry Y Sherbakov |title=The Mitochondrial Genome of a Freshwater Pelagic Amphipod Macrohectopus branickii Is among the Longest in Metazoa |journal=Genes (Basel) |date=20 December 2021 |volume=12 |issue=12 |page=2030 |doi=10.3390/genes12122030 |doi-access=free |pmid=34946978 |pmc=8700879 }}</ref>
==Ecology== ''Macrohectopus branickii'' is endemic to Lake Baikal,<ref name="WoRMS"/> with the "center" of its range being the deepest, central part of the lake.<ref>{{cite journal |last1=Mekhanikova |first1=I. V. |title=The Rare Abyssal Baikal Amphipod Polyacanthisca calceolata (Crustacea, Amphipoda) at the St. Petersburg Methane Seep, Central Baikal |journal=Biology Bulletin |date=22 February 2022 |volume=48 |pages=543–557 |doi=10.1134/S1062359021140119 |url=https://link.springer.com/article/10.1134/S1062359021140119 |access-date=18 May 2025|url-access=subscription }}</ref> It prefers waters colder than {{Convert|4|Celsius|abbr=on}}.<ref>{{cite journal |author=N.G. Melnik|author2=N.A. Bondarenko|author3=O.I. Belykh|author4=V.V. Blinov|author5=V.G. Ivanov|author6=I.V. Korovyakova|author7=T.Ya. Kostornova|author8=M.I. Lazarev|author9=N.F. Logacheva|author10=G.I. Pomazkova|author11=P.P. Sherstyankin|author12=L.M. Sorokovikova|author13=L.I. Tolstikova|author14=E.P. Tereza|title=Distribution of pelagic invertebrates near a thermal bar in Lake Baikal |journal=Hydrobiologia |date=2006 |volume=568(S) |pages=69–76 |doi=10.1007/s10750-006-0331-9 |bibcode=2006HyBio.568...69M }}</ref>
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 into a plethora of morphological forms. Out of all these species, ''M. branickii'' is the only pelagic member of the group;<ref name="NewDataEco"/> molecular and morphological analysis, however, indicates that the genus originates within Micruropodidae, a family of burrowers, though ''Micruropus wahli'' is a capable swimmer and may be caught at the surface through light-fishing;<ref>{{cite journal |author=Kenneth S. Macdonald III |author2=Lev Yampolsky |author3=J. Emmett Duffy|title=Molecular and morphological evolution of the amphipod radiation of Lake Baikal |journal=Molecular Phylogenetics and Evolution |date=29 December 2004 |volume=35 |issue=2 |pages=323–343 |doi=10.1016/j.ympev.2005.01.013 |pmid=15804407|url-status=dead|archive-url=http://web.archive.org/web/20170812024825/https://www.vims.edu/research/units/legacy/marine_biodiversity/publications/_pdf/Macdonald_et_al_2005_MPE.pdf|archive-date=12 August 2017|url=https://www.vims.edu/research/units/legacy/marine_biodiversity/publications/_pdf/Macdonald_et_al_2005_MPE.pdf |access-date=10 March 2026}}</ref> this attraction to light is also observed for the pelagic amphipod.<ref name="Myotis"/> Phototaxis may make them vulnerable to light pollution in littoral zones close to settlements.<ref>{{cite journal |author=Dmitry Karnaukhov|author2=Maria Teplykh|author3=Еkaterina Dolinskaya|author4=Sofya Biritskaya|author5=Yana Ermolaeva|author6=Viktoria Pushnica|author7=Iya Kuznetsova|author8=Anastasia Okholina|author9=Lidia Bukhaeva|author10=Еugene Silow|title=Light pollution affects the coastal zone of Lake Baikal |journal=Limnology Review |date=September 2021 |volume=21 |issue=3 |pages=165–168 |doi=10.2478/limre-2021-0015 |url=https://www.researchgate.net/publication/357247126 |access-date=18 May 2025|doi-access=free}}</ref> [[File:Macrohectopus branickii.png|left|thumb|These pelagic amphipods migrate vertically every day]] The pelagic amphipod migrates throughout the lake all year round, continuing to do so in their peak breeding season.<ref name="NewDataEco"/> This species also undergoes diel vertical migration,<ref>{{cite journal |author=D. Karnaukhov, S. Biritskaya, M. Teplykh, N. Silenko, E. Dolinskaya, E. Silow|title=The abundance and structure of population of pelagic amphipod Macrohectopus branickii in the coastal zone of Lake Baikal |journal=Acta Biologica Sibirica |date=October 2019 |volume=5 |issue=3 |pages=154–158 |doi=10.14258/abs.v5.i3.6574 |url=https://www.researchgate.net/publication/336535421 |access-date=18 May 2025|doi-access=free }}</ref> moving to the surface at night from their daytime depth range of {{Convert|200|-|700|m|abbr=on}}. The larger mature females inhabit the deeper range of this depth compared to smaller individuals.<ref name="Myotis"/> During nighttime, they may be especially abundant in the water column above underwater slopes.<ref name="Myotis"/> 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.<ref name="DielDyna"/><ref>{{cite journal |author=Makarov M.M.|author2=Dzyuba E.V.|author3=Zaydykov I.Yu.|author4=Naumova E.Yu.|title=Hydroacoustic studies of macrozooplankton in Lake Baikal |journal=Limnology & Freshwater Biology |date=25 December 2024 |issue=6 |pages=1491–1502 |doi=10.31951/2658-3518-2024-A-6-1491 |url=http://limnolfwbiol.com/index.php/LFWB/article/view/1274/910 |access-date=18 May 2025|doi-access=free }}</ref><ref>{{cite journal |author=Elena Yu. Naumova|author2=Igor Yu. Zaidykov|author3= Mikhail M. Makarov|title=Recent quantitative values of Macrohectopus branickii (Dyb.) (amphipoda) from Lake Baikal |journal=Journal of Great Lakes Research |date=February 2020 |volume=46 |issue=1 |pages=48–52 |doi=10.1016/j.jglr.2019.10.002 |bibcode=2020JGLR...46...48N |url=https://www.sciencedirect.com/science/article/abs/pii/S038013301930190X |access-date=18 May 2025|url-access=subscription }}</ref> The pelagic amphipods tend to feed more on zooplankton the larger they grow, though the exact ratios vary between individuals;<ref name="Isotope"/> these prey items are consistently located above {{Convert|50|m|abbr=on}} depth throughout the day.<ref name="DielDyna"/>
===Trophic ecology=== {{Expand section|some discussion on nitrogen isotopes (biochemistry is beyond my understanding)|date=May 2025}} [[File:Из жизни байкальской нерпы близ Ушканьих островов 10 (cropped).jpg|thumb|Predators of this species include the Baikal seal]] ''Macrohectopus branickii'' is the main species of macroplankton within the lake,<ref>{{cite journal |last1=Matveyev |first1=A. N. |last2=Samusenok |first2=V. P. |date=22 June 2015 |title=The fishes and fishery in Lake Baikal |journal=Aquatic Ecosystem Health & Management |volume=18 |issue=2 |pages=134–148 |bibcode=2015AqEHM..18..134M |doi=10.1080/14634988.2015.1028868 |issn=1463-4988}}</ref> being an important part of the food chain; the microcrustaceans ''Epischura baikalensis'' and ''Cyclops kolensis'' feed on phytoplankton (predominantly ''Aulacoseira baicalensis''). They then fall prey to the pelagic amphipod,<ref name="NewDataEco"/> which in turn feed various fish such as omul (''Coregonus migratorius''), sculpins (''Comephorus spp.'', ''Cottocomephorus spp.''), and the seal, which feeds on the fish and the amphipods.<ref name="Isotope">{{cite journal |author=Koichi Yoshii |author2=Natalia G. Melnik|author3=Oleg A. Timoshkin|author4=Nina A. Bondarenko|author5=Pavel N. Anoshko|author6=Takahito Yoshioka|author7=Eitaro Wada|title=Stable isotope analyses of the pelagic food web in Lake Baikal |journal= Limnology and Oceanography|date=1999 |volume=44 |issue=3 |pages=502–511 |doi=10.4319/lo.1999.44.3.0502 |doi-access=free|bibcode=1999LimOc..44..502Y |url=https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1999.44.3.0502 |access-date=18 May 2025|url-access=subscription }}</ref><ref>{{Cite journal |last1=Watanabe |first1=Yuuki Y. |last2=Baranov |first2=Eugene A. |last3=Miyazaki |first3=Nobuyuki |date=2020-12-08 |title=Ultrahigh foraging rates of Baikal seals make tiny endemic amphipods profitable in Lake Baikal |journal=Proceedings of the National Academy of Sciences |volume=117 |issue=49 |pages=31242–31248 |doi=10.1073/pnas.2014021117 |pmc=7733859 |pmid=33199633|bibcode=2020PNAS..11731242W |doi-access=free }}</ref> 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 to resubmerge. Before resubmerging, they are vulnerable to the water bat ''Myotis petax'', which "trawl up" the amphipods for an easy meal.<ref name="Myotis"/> On occasion they may also rise up to the surface during daytime, and seagulls are quick to take advantage of this opportunity.<ref name="Myotis"/>
Its role within the lake's ecosystem has been compared to that of Antarctic krill and mysis shrimp,<ref name="DielDyna">{{cite journal |last1=Rudstam |first1=Lars G. |last2=Melnik |first2=Nataly G. |last3=Timoshkin |first3=Oleg A. |last4=Hansson |first4=Sture |last5=Pushkin |first5=Sergei V. |last6=Nemov |first6=Vladimir |date=1992 |title=Diel Dynamics of an Aggregation of ''Macrohectopus branickii'' (Dyb.) (Amphipoda, Gammaridae) in the Barguzin Bay, Lake Baikal, Russia |url=https://www.reabic.net/Publ/Rudstam_et%20al_1992.pdf |journal=Journal of Great Lakes Research |volume=18 |issue=2 |pages=286–297 |bibcode=1992JGLR...18..286R |doi=10.1016/S0380-1330(92)71296-9 |issn=0380-1330 |access-date=18 May 2025}}</ref> being an important lower-level consumer due to its abundance; the density of pelagic amphipods has been estimated at tens of grams per {{Convert|1|m²|abbr=on}}, with total numbers reaching several million animals and constituting 90-99% of the zooplanktonic biomass.<ref name="NewDataEco">{{cite journal |author=Dmitry Yu. Karnaukhov|author2=Ekaterina M. Dolinskaya|author3=Sofya A. Biritskaya|author4=Maria A. Teplykh|author5=Yana K. Ermolaeva|author6=Viktoria A. Pushnica|author7=Iya V. Kuznetsova|author8=Anastasia I. Okholina|author9=Lidia B. Bukhaeva|author10=Olga O. Rusanovskaya|author11=Eugene A. Silow|title=New data regarding ecology of freshwater pelagic amphipod Macrohectopus branickii and other crustaceans of plankton from the southern part of Lake Baikal |journal=Acta Biologica Sibirica |date=12 April 2021 |volume=7 |pages=39–48 |doi=10.3897/abs.7.e65636 |doi-access= free|url=https://www.researchgate.net/publication/350830175 |access-date=18 May 2025}}</ref> Annual biomass production of ''M. branickii'' is calculated to be around {{Convert|330000|tonne|abbr=on}}.<ref name="Myotis">{{cite journal |author=S. I. Didorenko|author2=A. D. Botvinkin|author3=V. V. Takhteev|title=A New Trophic Relationship in the Baikal Ecosystem: The Pelagic Amphipod Macrohectopus branickii (Crustacea, Amphipoda) and the Bat Myotis petax (Mammalia, Chiroptera) |journal=Biology Bulletin|date=17 December 2021 |volume=48 |issue=7 |pages=907–914 |doi=10.1134/S1062359021070116 |bibcode=2021BioBu..48..907D |url=https://link.springer.com/article/10.1134/S1062359021070116 |access-date=18 May 2025|url-access=subscription }}</ref>
==Notes== {{Notelist}}
==References== {{Reflist}}
{{Taxonbar|from1=Q18606058|from2=Q13805074}}
Category:Gammaroidea Category:Endemic fauna of Russia Category:Crustaceans of Russia Category:Crustaceans described in 1874 Category:Fauna of Lake Baikal Category:Monotypic amphipod genera