The eggs of these crabs are large, and they produce few young when compared to their seawater-living relatives. What a system to explore the ecological and evolutionary questions of. The basic strategy for a successful species; produce young in a manner that increases the probability that some survive to produce young of their own. Strategies straight from the textbook make this sound utterly boring r-selected, or K-selected, which depicts a two-state system, which really is not the case, so let’s abandon this terminology Let’s look at the extreme choices.
Form 1:
Produce lots and lots of young (1 million), exhausting energy by doing so. Then rest, or perhaps die. No care or anything else to my young is provided because energy is exhausted by producing young (or, again, the mom is dead).
Form 100:
Produce a single young. Afterward, carry, protect, feed, and generally take care of this youngster until it is nearly ready to produce a youngster of its own.
Form 1 and form 100 are extremes, with, at least 98 other options in between. Some of the options involved hedging your bets, based on environmental circumstances, for example. If the environment is changing rapidly, I may produce 4 eggs, even though I have the energy to produce 10, so I can survive the changing conditions and hopefully produce more eggs later. This is not a conscious decision in animals, this is an evolutionary outcome. An example of this, birds lay eggs, if the season is longer than normal, they may produce another batch of eggs. If the season turns crappy they will not produce that second batch, instead waiting for the next year’s “good” season. If the adults die before the next season they have lost that opportunity for more young.
This genus of freshwater crab is endemic to Northeastern Brazil, and a few other regions of rain-forested SA, and will spend their lives away from seawater. They are considered freshwater crabs. But there are nearly terrestrial, they have a lung of sorts in their gill chamber. The coconut crab (Birgus latro) is a terrestrial crab, mostly, well a hermit crab, in the group Anomura and not in the “true crab” group Brachyura. These crabs still visit seawater to breed. The Christmas Island crab ( Gecarcoidea natalis) is another species of terrestrial crab tied to the ocean. They are famous for their annual migration to the sea, en masse, to breed. Then males dig burrows close to the shoreline and the females join them. The females stay in the burrows for a few weeks before venturing to the water where they release their 100,000 eggs. They spend several life stages in the sea over about a month before closing in on the shoreline for their transition to tiny crabs and heading inland.
These terrestrial crabs are so attuned to life on land that adults can drown in the water. Coconut crabs produce hundreds of thousands of eggs and the eggs spend time as plankton in seawater, then as juveniles, they still hang out in the ocean crawling around. As adults, they move inland (what an odd life cycle) but take shelter in burrows under the coconut trees. Each of these species must visit the seawater to breed but otherwise lives a terrestrial existence.
Fredius crabs live in and around freshwater, but much less is known about them compared to the Coconut or Christmas Island crab. They appear to be tied to humid environments and take cover in burrows along pond shorelines. As mentioned above, this species has parental care; the female crabs carry eggs and the young also hang onto mom for about 2 weeks after hatching. The young emerge from the egg after all morphological stages have passed, thus they are little tiny crabs (direct development). I could confirm the number of eggs that this genus typically produced, just a rather wide range (10 to a few thousand).
There are some similarities to burrowing freshwater crayfish. Crayfish dig burrows up above the water line, though the burrow reaches down below the waterline. These burrows are used by females carrying eggs and after hatching the young hang onto mom for a while. The young do not spend any time in the plankton and crawl away from mom when ready.
The interesting questions here are evolutionary ones: Why have these freshwater crabs adjusted, over time, their reproductive strategy? Their sea-going relatives produce lots more eggs, and throw them into the plankton, why increase egg size, and start taking care of your young? Does it have a relationship to freshwater/terrestrial living? The ocean provides great dispersion advantages, but rivers do as well. Thus one consequence of direct development and parental care, is that the species does not disperse widely. No surprise that there are many species of endemic crabs, the genus Fredius is endemic to Brazil.
Fredius recently (2020) had a new species recognized and there are more, many more. Some of them were not originally placed into the genus Fredius. Here is an alphabetical list:
Fredius adpressus: Described by Rodríguez & Pereira in 1992.
Fredius beccarii: Described by Coifmann in 1939.
Fredius buritizatilis: Described by Magalhães & Mantellato in 2014. Oddly in an article with more authors: Magalhães et al 2014.
Fredius chaffanjoni: Described by Rathbun in 1905.
Fredius convexa: Described by Rathbun in 1898.
Fredius cuaoensis: Described by Suárez in 2015
Fredius cuyunis: Described by Pretzmann in 1967.
Fredius denticulatus: Described by H. Milne Edwards in 1853.
Fredius estevisi: Described by Rodrigues in 1966.
Fredius fittkaui: Described by Bott in 1967.
Fredius granulatus: Described by Rodríguez & M.R. Campos in 1998.
Fredius ibiapaba: Described in 2020.
Fredius platyacanthus: Described by Rodríguez & Pereira in 1992.
Fredius reflexifrons: Described by Ortman in 1897.
Fredius stenolobus: Described by Rodríguez and Suárez in 1994.
Fredius ykaa: Described by Magalhães in 2009.
Sixteen species in total are listed here: Wikipedia has 12 species listed and has detailed pages on none of them. It is interesting to note how many of these species have been described recently. This is not a well-studied group. It appears Célio Magalhães from the National Institute for Research in the Amazon is starting to set that lack of study record straight.
One question to answer is are all these described species actually different species. A quick Genbank BLAST of an RNA sequence gives a tree where, for example, Fredius estevisi is nested between specimens labeled Fredius platyacanthus (Figure below). In another section of the same BLAST-produced tree, Fredius ibiapaba is nested between specimens of Fredius reflexifrons.
A fascinating group that I knew almost nothing about until I came across one of the papers listed below. I’ve exhausted my brain with this new information so I need to end this.
Ending notes:
The family of these crabs is Pseudothelphusidae, a mouthful. There are other genera, I have only listed those in genus Fredius above, with perhaps 280 species in all. There are thousands of species of freshwater crab on the planet, most appear to be threatened with extinction due to human encroachment taking over their living spaces.
Sources and Further Readings:
Santas LC, Nascimento WM, Matos HS, Pinheiro AP, and Silva JRF. 2020. The distribution of the freshwater crab Fredius reflexifrons (Ortmann, 1897) (Brachyura, Pseudothelphusidae) in an Environmental Protection Area of the Planalto da Ibiapaba, Northeastern Brazil. An Acad Bras Cienc 92: e20180814. DOI 10.1590/0001-3765202020180814.
Santos LC, Tavares M, Silva JéRF, Cervini M, Pinheiro AP, Santana W. 2020. A new species of freshwater crab genus Fredius Pretzmann, 1967 (Crustacea: Brachyura: Pseudothelphusidae) from a naturally isolated orographic forest enclave within the semiarid Caatinga in Ceará, northeastern Brazil. PeerJ 8:e9370 http://doi.org/10.7717/peerj.9370
Vogt G. 2012. Abbreviation of larval development and extension of brood care as key features of the evolution of freshwater Decapoda. Biol Rev 88: 81-116.
Vogt G. 2016. Structural specialties, curiosities, and record breaking features of crustacean reproduction. Journal of Morphology 277: 1399-1422.