Number of families 2
Small, worm-like, blind Crustacea with head, thorax, and abdomen. Thorax with seven pairs of walking legs and a reduced eighth one, which in the male is transformed for copulatory purposes. Abdomen with one pair of appendages on the last segment, one pair on the first, and very rarely also one pair on the second segment.
Evolution and systematics
Bathynellcea have no fossil record. Their closest relatives are the Anaspidacea in the Southern Hemisphere. Both groups are confined to fresh water, but have had marine ancestors. This is evidenced by their fossil relatives, the Palaeocaridacea, which, during the Carboniferous and Permian, inhabited the littoral of the then tropical seas in the Northern Hemisphere. Transition into fresh water was achieved independently, first by the Bathynellacea and later also by the Anaspidacea. Unlike the Anaspidacea, the Bathynellacea have disappeared almost completely from surface waters today.
When compared with adults of related Crustacea taxa, Bathynellacea appear very different, but they share a remarkable similarity with the larvae of these other Crustacea. In fact, Bathynellacea have the appearance of larvae, indicating that they have had larval-like ancestors. There is good evidence that the postembryonic development of their surface-living ancestors passed through a series of several larval stages. In the course of adaptation to life in the groundwater, abbreviation of this development caused by precocious sexual maturity at an early stage of larval development marked the beginning of the evolution of subterranean Bathynellacea. Today they reach adulthood at a stage that corresponds to the last larval stage of their ancestor. A morphological consequence of this adaptational process was that their body became continually smaller and its structure increasingly simpler. As a result, they finally fitted into the small spaces between sand particles of the groundwater-bearing strata. Ecologically they progressed from a benthic life through a colonization of coarse substrates with large interstitial spaces to an existence in ever smaller sediments with ever narrower spaces between the particles. The ancestral form of the Bathynellacea had conquered a completely new habitat and marked the beginning of an impressive radiation leading to a worldwide colonization of the groundwater.
The order Bathynellacea comprises two families, 60 genera, and about 200 species. The two families are Bathynellidae and Parabathynellidae.
Bathynellacea range in length between 0.02 in (0.5 mm) and 0.14 in (3.5 mm). As an adaptation to their subterranean existence, they do not have eyes and lack body pigment. The body is divided into head, thorax, and abdomen. The head carries the antennae with chemo- and mechanosensory structures as well as the mouthparts. The mandibles, used for biting, are symmetrical in structure. The thorax has seven pairs of biramous walking legs. The eighth pair is reduced in both sexes and in the male is transformed into a characteristic copulatory organ. The abdomen carries one pair of appendages on the first and last somite (pleotelson). There can also be a pair of appendages on the second abdominal somite. The pair on the last somite (uropods) does not form a tailfan together with the pleotelson, as is the case in several related groups of Crustacea. The basal segment (sympodite) of the uropods carries a characteristic row of spines. Bathynellidae can be identified by the second antennae being directed anteriorly, by the last abdominal somite (pleotelson) carrying two dorsal setae, by the rim of the upper lip (labrum) being smooth and not serrate, by the outer branches (exopodites) of all seven walking legs being one-segmented, by the first abdominal appendages (pleopods) being two-segmented, and by the sympodite of the uropods being relatively short.
In contrast, Parabathynellidae have the second antennae bent backwards and carry lateral setae on the pleotelson. The rim of their labrum is serrate or fringed with fine setae. The exopodites of the walking legs are composed of one or more segments and only rarely do the exopodites of all the legs have the same number of segments. The first pleopods, if present, are one-segmented or represented by only two setae. The sympodite of the uropods is more elongate than in the Bathynellidae.
Bathynellacea are found in the groundwater nearly all around the globe. They are absent in the Antarctic and in the Northern Hemisphere in those areas that had been covered by ice during the last glaciation. Recolonization has been observed at a few places but has not progressed very far beyond the old ice frontier. Bathynellacea are not known from Central America and are also absent from volcanic islands and several islands of continental origin (e.g., New Caledonia, Fiji, Caribbean islands).
Both families have a worldwide distribution, but Bathynellidae are more common in temperate regions of the world than in the tropics. Parabathynellidae have a more even distribution but do not reach as far north as Bathynellidae in the Northern Hemisphere. All recolonizers of previously icecovered areas belong to the Bathynellidae.
There are two hypotheses to explain the distribution of Bathynellacea. Both agree on marine ancestors as the starting point of the adaptational process and on plate tectonics as a major factor. The first hypothesis maintains that the ancestors of Bathynellacea invaded fresh surface waters and that from their larvae the Bathynellacea arose to become inhabitants exclusively of the groundwater. Subsequent spread in the groundwater itself led to today's worldwide occurrence. According to the second hypothesis, as a first step a marine ancestor became adapted to an interstitial life in littoral sands and in a second step had to switch to freshwater conditions as a result of marine shoreline regression. Multiple such invasions of the groundwater caused by repeated sea-level changes at different geological times together with plate tectonics led to the currently observed worldwide distribution.
Bathynellacea are typical inhabitants of groundwater and can be collected in wells, in sandy banks of rivers and sandy shores of lakes, in caves, and in springs. One species is known from a hot spring in Africa at a temperature of 131°F (55°C). Bathynellacea are absent from surface waters. Only two species are known from Lake Baikal, where they occur on sandy patches down to 4,725-ft depth (1,440-m), but not in the open water. A few species have been recorded from near the seashore and tolerate brackish water conditions. Only one species is polyhaline.
Bathynellacea are elegant, persistent crawlers and ungainly, short-winded swimmers. Their crawling movements are a combination of swimming and walking. As an escape reaction or when caught in blind alleys of the interstitial labyrinth, they engage in a specialized turning maneuver that consists of placing the abdomen beneath the rest of the body, sliding it along the ventral side towards the head, and turning at the same time about the longitudinal axis to return to a ventral position.
Feeding ecology and diet
Not much is known about the feeding and food of Bathynellacea. Plant debris (detritus), nematodes, and turbellarian worms have been observed from the guts of various specimens. Among Parabathynellidae, in particular, there is great variation in types of mouthparts, indicating specialization on different types of food. Most Bathynellacea may be omnivorous, but many are certainly specialized, feeding on detritus, protozoans, and/or bacteria. One species has mouthparts with setae suited for scraping off material from sand grains.
In Bathynellacea the sexes are separate. Mating behavior has never been observed. The eggs are shed freely and singly. The egg passes through a nauplius stage. After hatching, postembryonic development passes through two phases, a larval (parazoëal) phase with three stages, and a juvenile (bathynellid) phase with a variable number of stages.
Most species are known only from their type locality. In Europe, from where more is known than elsewhere about areas of distribution of single species, some are of local occurrence while others have a wider distribution. None seem to be threatened, and none are listed by the IUCN Red List.
Significance to humans
Bathynellacea are part of a complex groundwater biocoenosis made up of bacteria, protozoans, fungi, and metazoans (animals). All of these organisms act together to decompose particles washed into the groundwater from outside. These particles would otherwise clog the spaces between the sand grains, preventing groundwater from circulating freely. Bigger particles are broken up by Bathynellacea, and after they are passed through the gut, they are further degraded by protozoans and bacteria. Thus the interstitial spaces are kept open. Humans benefit from this ecological service because it helps to keep drinking water clean.
List of SpeciesAntrobathynella stammeri
No common name
Bathynella natans stammeri Jakobi, 1954, from a well at Möhrendorf, near Erlangen, Germany.
other common names
Can be distinguished from other European bathynellids by the specific structure of the eighth leg and by a hyaline, cone-shaped protrusion at the inner side of the coxa of the seventh leg of the male as well as by the basal segment of the uropods of both sexes carrying a row of four spines of which the anterior one is bigger than the others and separated from them by a distinct gap.
Widely distributed in Europe from Ireland to Romania.
Known from all groundwater habitats.
Agile crawler from the second postembryonic stage onwards; clumsy when just hatched from the egg, and may fall prey to cyclopoid copepods.
feeding ecology and diet
Feeds on detritus and worms of different kind.
Reproduces all year round, deposits eggs singly. Embryonic phase takes two months, parazoëal phase at 48°F (9°C), about three months, bathynellid phase about four months. Becomes adult after nine months of development but not sexually mature. There are further molts, four in the male and five in the female, until sexual maturity is reached. Lives several years; two years are recorded from laboratory cultures.
Not listed by IUCN. Does not appear to be threatened.
significance to humans
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Schminke, Horst Kurt. "Adaptation of Bathynellacea (Crustacea, Syncarida) to Life in the Interstitial ("Zoea Theory")." Internationale Revue der Gesamten Hydrobiologie 66 (1981): 575–637.
Horst Kurt Schminke, PhD