Freshwater Crab Biology

The following is modified from 'The Freshwater Crabs of West Africa. Family Potamonautidae' By Neil Cumberlidge (1999), IRD, pages 1-382.

Anatomy of a Generalized Freshwater Crab

bg, branchial groove; cg, cardiac groove; ceg, cervical groove; ig, intestinal groove; mg, midgroove; ug, urogastric groove; a1-a6, abdominal segments 1-6; s1-s8, thoracic sternal segments 1-8.

RESPIRATORY SYSTEM
The gills lie in the two branchial chambers formed by the branchiostegites of the carapace. Freshwater crabs typically have nine pairs of gills (the same as marine crabs).

In aquatic freshwater crabs, the majority of the respiratory water enters the gilll chambers ventrally through the inhalant openings (the 'Milne-Edwards openings') situated between the basal joints of the chelipeds between the basal joints of the walking legs and the neighbouring margin of the carapace. Exhalent water leaves the gill chambers via the two efferent canals on either side of the buccal cavity. The respiratory stream is maintained by the beating action of the gill bailers (scaphognathites) which are part of the second maxillae.

A reduction in the number of gills is seen in some air-breathing species such as the terrestrial African freshwater crab Globonautes macropus. Terrestrial species of African freshwater crabs (e.g., Globonautes macropus) regularly carry out aerial respiration by means of a "lung" (called a "pseudolung") that consists of a fleshy vascularized membrane in the dorsal part of the gill chamber, although this species has also retained fully functional gills in the ventral part of the gill chamber.

Photograph of the pseudolung of Globonautes macropus from Liberia. Note the perforated appearance and fleshy structure of this respiratory organ.

REPRODUCTIVE SYSTEM
Male. The reproductive system of male freshwater crabs consists of a pair of testes, a pair of vas deferens (ducts between the testes and the penes), two penises and two gonopods. The testes of male African freshwater crabs lie in the cephalothorax on top of the hepatopancreas. They produce spermatozoa that are carried away in paired ducts (the vasa deferens) which open ventrally. The right and left penises are short, soft, flexible membranous tubes located at the terminal ends of the vasa deferens. The penises are sited on the coxae of pereiopods 5 and do not pass through the sternum in any species of freshwater crab.

Gonopods of Potamonautes dybowskii in natural resting position beneath the abdomen

Copulation. Gonopods 1 and 2 are paired abdominal appendages (pleopods) that are modified to function as copulatory organs that transfer the spermatophores from the male penises to the female sexual openings. The gonopod 1 of freshwater crabs is a three or four-part hollow tubular organ with an apical opening in the terminal part. The long thin, broad-based gonopod 2 fits tightly into the subterminal segment of gonopod 1 and leaves a lateral basal opening into which the penis extends. Male and female freshwater crabs copulate in the normal brachyuran way, by laying head-to-head and sternum-to-sternum, with their abdomens relaxed so that the abdomen of the female overlaps that of the male. This brings the female openings into contact with the gonopods which swing out away from the sternum when the male abdomen is relaxed. The terminal articles of gonopods 1 connect with, and are inserted into, the paired vulvae of the female sited on sternite 5. The spermatozoa (in spermatophores), together with the secretions of the vas deferens, are ejected through the penis into the subterminal gonopod chamber between the bases of the subterminal segments of the two gonopods. There is only a single spermatozoa in each spermatophore (i.e., freshwater crabs exhibit cleistospermy). During copulation the pumping action of gonopod 2 forces the spermatophores up the gonopod chamber inside the two gonopods. The spermatophores are pumped out of the apical opening of the terminal article of gonopod 1 into the spermathecae that lie just deep to the female sexual openings, where they are stored until the eggs are laid.

Female. The reproductive system of female freshwater crabs consists of the paired ovaries, ovarian ducts, spermathecae, vaginas and vulvae. The ovaries of female African freshwater crabs lie in the cephalothorax on top of the hepatopancreas. The eggs are carried away in paired ovarian ducts that open ventrally through the pair of female openings (vulvae) on sternite 5. The vulvae are elongated sideways to receive the first gonopods, and the distal parts of the oviducts just deep near to the openings have spermathecae to receive and store the spermatophores. Female crabs store the spermatophores for long periods, and the eggs do not have to be laid immediately after mating. Sperm in the spermatophores fertilises the eggs as they are laid. The fertilised eggs are attached to the female abdominal pleopods by long sticky threads secreted by the female.

Development. True freshwater crabs spend their entire lives in fresh water and it is a shared defining characteristic to be able to complete their entire life cycle independently of sea water. All true freshwater crabs lay their eggs and rear their young in a freshwater environment, rather than in a brackish or marine habitat like other crabs. The adaptation of true freshwater crabs to freshwater environments has involved a number of modifications of their reproductive system and behaviour. These modifications include the production of relatively few eggs, each of a relatively large diameter, a complete reduction of the larval stages ("direct development") whereby the eggs hatch directly into juvenile crabs ("hatchlings"), and the protection of the hatchlings for several weeks after egg hatching. The number of eggs laid by species of marine crabs varies with the size of the species, and large-bodied species can lay from several thousand to more than a million eggs. In contrast, female freshwater crabs produce far fewer eggs: by the hundreds, rather than by the thousands or tens of thousands. The newly laid eggs of marine crabs are very small (0.25-0.35 mm in diameter), and double in size as they develop. In contrast, the newly laid eggs of freshwater crabs are much larger (about 1 mm in diameter), increase to between 3 and 5 mm in diameter as they develop, and remain attached to the pleopods of the female until hatching. The newly laid eggs of African freshwater crabs are bright orange and change colour slowly to dull brown, dirty grey, and then to black before they finally hatch into small crabs. Young freshwater crabs leave the eggs as small versions of adult crabs rather than as larvae. The hatchlings are retained on the female’s pleopods in the female’s abdominal brood pouch for several weeks after hatching and female freshwater crabs show a degree of maternal care.

The lifecycle of all true freshwater crabs.

A, Marojejy longimerus from Madagascar. B and C, Potamonautes perlatus from South Africa (Barnard, 1950)

Ecology and Distribution

Freshwater crabs were originally known as river crabs, and were given family names such as Thelphusidae and Potamonidae, the latter name being derived from the Greek word root ‘Potamon’ meaning ‘river’. While the river-living habit is an accurate description of the lifestyle of some species of freshwater crabs it by no means applies to all species. In Africa, as in other parts of the tropics, some species of freshwater crabs have moved out of rivers, streams and lakes and have colonised nearby land. In forest and savanna ecosystems, the move onto land favoured (among other things) the development of the ability to breathe air as well as water. Some species of freshwater crabs rival land crabs (Gecarcinidae) in their degree of independence from water.

Other Crabs Found in Freshwater Habitats

Other crabs found in tropical freshwaters may not be true freshwater crabs, but actually may belong to a marine-breeding family that can tolerate dilute conditions for part of its life but which needs seawater at some point in its life cycle. Examples include members of the Sesarmidae, Varunidae, Hymenosomatidae, Portunidae and Xanthidae.