Macrobiotus is a genus of tardigrades consisting of about 100 species within the family Macrobiotidae.[1] Of genera within the phylum Tardigrada, Macrobiotus is one of the most species-abundant.[2]Macrobiotus hufelandi, identified by German researcher Carl August Sigismund Schultze in 1834, is recognized as the first taxonomically named tardigrade species.[3][4] By extension, Macrobiotus was the first ever described tardigrade genus. Macrobiotus shares similarities to other tardigrade genera, particularly in moulting a non-living cuticle over multiple life stages and robust stress tolerance mechanisms.[5] However, Macrobiotus tends to be differentiated from other tardigrades by being one of few genera that freely lay ornamented eggs in reproduction. This genera has a relatively diverse distribution in both terrestrial and aquatic environments.[6]
Macrobiotus belong to the clade Ecdysozoa, characterized for non-living cuticle layers that can be moulted over various life stages.[5] This cuticular exoskeleton layer facilitates gas and water exchange. Protective benefits of the cuticle typically outweigh the energy costs required to moult.
Nervous System
Macrobiotus neurons innervate a brain with inner and outer connectives, the latter connecting the brain to the trunk.[7] Anterior leg nerves, posterior leg nerves, and peripheral nerves branch off of trunk ganglia. Connectives bridge adjacent trunk ganglia, and interpedal commissures bridge adjacent connectives.
Other Physiological Features
Many Macrobiotus species also share a firm buccal tube containing a ventral lamina without a ventral hook, a pharynx with one microplacoid and two macroplacoids, and tend to exhibit 10 peribuccal lamellae.[2]
Habitat and Distribution
Habitat
Macrobiotus tend to inhabit limnoterrestrial habitats primarily consisting of mosses, though some Macrobiotus have also been discovered near lichens or aquatic plants.[6][8] Some species of Macrobiotus are obligatory freshwater invertebrates.[9]
Macrobiotus species exhibit a unique style of ornamented egg morphology and deposition process of freely laying ornamented eggs, contrasting to the ancestral state of freely laying smooth eggs.[5] This is a synapomorphy of two families, Eohypsibioidea and Macrobiotidae, with Macrobiotoidea including the genus Macrobiotus. Ornamented eggs can limit water evaporation due to surface tension between water molecules and the egg's ornamentation.
Free-laying of eggs also differs from other common reproductive tendencies for tardigrades, whereby the old cuticle (or the exuvia) is used as an 'egg keeper' to allow egg development after laying.[5] It is hypothesized that releasing eggs into the environment reduces predation of embryonic Macrobiotus species, providing an evolutionary advantage compared to laying eggs in only one place within the exuvia.
Partner finding behaviour occurs for freely-ovipositing Macrobiotus shonaicus species, where the male touches a female tardigrade's cloaca with their mouth several times, finishing their courtship ritual once the female tardigrade stops moving.[20] Freely-ovipositing species are thought to identify partners using pheromones to accommodate only having simple light detection abilities.
Some free-laying egg species like M. shonaicus utilize chemotaxis for spermatozoa released during ejaculation to identify and swim towards the female's cloaca.[20] Eggs are typically laid shortly after reproduction by the female, ranging from 16 minutes to 2 days after mating.
Macrobiotus exhibits bisexual achiasmy, whereby the process of crossing-over of genetic information between homologous chromosomes in meiosis is absent in both the male and female sex.[21]
Stress Response
Anhydrobiosis
Macrobiotus can withstand desiccation conditions by entering a state of anhydrobiosis.[22][23] Compared to other tardigrades, Macrobiotus displays a moderate degree of desiccation tolerance. Some species have been observed to accumulate trehalose during anhydrobiotic states. Trehalose can induce the anhydrobiotic state, serving as a potential energy source while also protecting cellular membranes and proteins in freezing and desiccation conditions.[24] Survival during anhydrobiosis depends on body size, particularly the size of energy-storing storage cells, though the reproductive state of oocyte maturation has no effect.[22] It is predicted that the exoskeletal cuticle likely facilitates anhydrobiosis.[5]
Some Macrobiotus may also demonstrate an encysted state primarily during periods of rest, but also metabolic depression.[23]
Starvation
One example species, M. sapiens, forms storage cells within their internal cavity in response to reduced food availability.[18] This strategy helps Macrobiotus species survive for several weeks without food, during which storage cell sizes significantly decrease.
Moulting
Macrobiotus, like other tardigrades, requires periodic moutling of their cuticle exoskeleton to grow in size.[5] Cuticle lining the foregut independently moults first and is expelled through the mouth, followed by moutling the cuticles lining the exterior body and hindgut. Moulting may occur in response to poor environmental conditions or starvation as overall body size decreases.
Both male and female partners of M. shonaicus, a freely-ovipositing species, moult immediately before reproduction, moulting during the process of developing mature oocytes.[20]
1 2 3 4 Bertolani, R.; etal. (2023). "The Macrobiotus persimilis-polonicus complex (Eutardigrada, Macrobiotidae), another example of problematic species identification, with the description of four new species". Organisms Diversity & Evolution. 23 (2): 329–368. Bibcode:2023ODivE..23..329B. doi:10.1007/s13127-022-00599-z. hdl:11380/1303147.
↑ Stec, D.; Vecchi, M.; Calhim, S.; Michalczyk, Ł. (2021). "New multilocus phylogeny reorganises the family Macrobiotidae (Eutardigrada) and unveils complex morphological evolution of the Macrobiotus hufelandi group". Molecular Phylogenetics and Evolution. 160 106987. Bibcode:2021MolPE.16006987S. doi:10.1016/j.ympev.2020.106987. PMID33059070.
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