The transition from marine to freshwater habitats is one of the major steps in the evolution of life. In the decapod crustaceans, four groups have colonized fresh water at different geological times since the Triassic, the freshwater shrimps, freshwater crayfish, freshwater crabs and freshwater anomurans. Some families have even colonized terrestrial habitats via the freshwater route or directly via the sea shore. Since none of these taxa has ever reinvaded its environment of origin the Decapoda appear particularly suitable to investigate life-history adaptations to fresh water. Evolutionary comparison of marine, freshwater and terrestrial decapods suggests that the reduction of egg number, abbreviation of larval development, extension of brood care and lecithotrophy of the first posthatching life stages are key adaptations to fresh water. Marine decapods usually have high numbers of small eggs and develop through a prolonged planktonic larval cycle, whereas the production of small numbers of large eggs, direct development and extended brood care until the juvenile stage is the rule in freshwater crayfish, primary freshwater crabs and aeglid anomurans. The amphidromous freshwater shrimp and freshwater crab species and all terrestrial decapods that invaded land via the sea shore have retained ocean-type planktonic development. Abbreviation of larval development and extension of brood care are interpreted as adaptations to the particularly strong variations of hydrodynamic parameters, physico-chemical factors and phytoplankton availability in freshwater habitats. These life-history changes increase fitness of the offspring and are obviously favoured by natural selection, explaining their multiple origins in fresh water. There is no evidence for their early evolution in the marine ancestors of the extant freshwater groups and a preadaptive role for the conquest of fresh water. The costs of the shift from relative r- to K-strategy in freshwater decapods are traded-off against fecundity, future reproduction and growth of females and perhaps against size of species but not against longevity of species. Direct development and extension of brood care is associated with the reduction of dispersal and gene flow among populations, which may explain the high degree of speciation and endemism in directly developing freshwater decapods. Direct development and extended brood care also favour the evolution of social systems, which in freshwater decapods range from simple subsocial organization to eusociality. Hermaphroditism and parthenogenesis, which have evolved in some terrestrial crayfish burrowers and invasive open water crayfish, respectively, may enable populations to adapt to restrictive or new environments by spatio-temporal alteration of their socio-ecological characteristics. Under conditions of rapid habitat loss, environmental pollution and global warming, the reduced dispersal ability of direct developers may turn into a severe disadvantage, posing a higher threat of extinction to freshwater crayfish, primary freshwater crabs, aeglids and landlocked freshwater shrimps as compared to amphidromous freshwater shrimps and secondary freshwater crabs.

Abbreviation of larval development and extension of brood care as key features of the evolution of freshwater Decapoda

Within Anura, direct development involves ontogenetic changes of the biphasic ancestral pattern. The recent partitioning of the genus Eleutherodactylus, along with the proposition of the unranked taxon Terrarana, has renewed an interest to the morphological and ecological diversity among direct-developing frogs. The morphological changes during embryonic development of Oreobates barituensis is similar to those of other Neotropical direct-developing species, including the reduction or absence of several larval and embryonic characters (e.g., external gills and adhesive glands), heterochronic changes (e.g., early developing limbs and late persistence of ciliated epidermal cells), and the appearance of new structures (e.g., egg tooth). The tail achieves an extraordinary peramorphic development (encloses the entire embryo), and the location of its expanded part is interpreted as a heterotopic change resulting in a novel trait. An enveloping tail with apparently non-heterotopic fins, combined with the absence of gills, has been only reported for a species of the related genus Craugastor, and these morphologies suggest an informative perspective for the study of evolution of direct development in terraranans.

Direct-developing frogs: ontogeny of Oreobates barituensis (Anura: Terrarana) and the development of a novel trait

Increasing our knowledge on direct-developing frogs: The ontogeny of Ischnocnema henselii (Anura: Brachycephalidae)

The New World direct-developing frogs (Brachycephaloidea = Terrarana) comprise nearly a thousand species that share direct development among other putative synapomorphies, yet embryonic development in this group has been thoroughly described in only about 20 species. Here we describe the early ontogeny of the craugastorid Haddadus binotatus, making special emphasis on tail structure and development, and its differences and similarities with that of other terraranans. The morphological changes during embryonic development of H. binotatus and those of other Neotropical direct-developing species are alike, with some variation including the absence of external gills, timing of limb differentiation, and tail configuration. The tail with a rotated core axis and lateral and asymmetric fins that cover the posterior half of the embryo represents an outstanding case of developmental repatterning. We present some interpretations of the evolution of the tail and its three major aspects, the rotation of the c...

https://bioone.org/journals/Journal-of-Herpetology/volume-49/issue-3/14-072/A-Tale-of-a-Tail--Variation-during-the-Early/10.1670/14-072.pdf

A Tale of a Tail: Variation during the Early Ontogeny of Haddadus binotatus (Brachycephaloidea: Craugastoridae) as Compared with Other Direct Developers

A new microhylid frog in the genus Oreophryne is described from southern New Guinea. It is a small species (males to 22 mm snout-urostyle length (SUL), females to 27 mm SUL) distinguished from congeners by having a cartilaginous connection between the procoracoid and scapula, no webs between fingers, basal webs mainly between toes 3 and 4 and between toes 4 and 5, and the fifth toe slightly longer than the third. In life, most specimens have a conspicuous lime green (yellowish in preservative) bar between the eyes, a lime green and irregular longitudinal stripe on the flanks, and an irregular spot of the same colour above the insertion of the upper arms. The advertisement call is a pronounced rattle lasting about one second with a note repetition rate of 26–28 notes per second. Males of the new species guard the eggs of their mates on the underside of leaves hanging 30–400 cm above the ground.

A new egg-guarding species of Oreophryne (Amphibia, Anura, Microhylidae) from southern Papua New Guinea

Embryonic development in fifteen Australopapuan microhylid frogs of the genera Austrochaperina, Cophixalus and Oreophryne is described. These frogs have direct development during which the embryo develops to a minute froglet within the jelly capsule. Development of the operculum, presence of external gills, tail structure, gut development and timing of forelimb emergence are described and compared with the direct-developing eleutherodactylid Eleutherodactylus coqui from Puerto Rico and three Australian myobatrachid genera with direct development (Arenophryne, Metacrinia and Myobatrachus). We comment on those differences that likely reflect examples of convergent and divergent evolution and heterochrony.

Direct development in some Australopapuan microhylid frogs of the genera Austrochaperina, Oreophryne (Anuura: Microhylidae) from northern Australia and Papua New Guinea

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Direct development in some Australopapuan microhylid frogs of the genera Austrochaperina, Oreophryne (Anuura: Microhylidae) from northern Australia and Papua New Guinea