Infraorder Brachyura: Classification, Life-History Traits & Unique Species

Dorsal view: Dungeness crab (Metacarcinus magister). https://invertebase.org/stri/taxa/index.php?tid=60162&clid=0&pid=0&taxauthid=1

Classifying Brachyura

Brachyura, or the true crabs, are an infraorder of decapod crustaceans [54]. They have several defining characteristics, such as having four pairs of walking legs and a single pair of chelae (claws), a reduced pleon (also called an apron), and a fused carapace that gives them that stereotypical crab look [5],[40]. The pleon is one of the more notable and distinguishable characteristics, being that the term Brachyura is actually Greek: it directly translates to “short-tailed” [24].

The reduced pleon is used to differentiate between males and females of this infraorder [14]. The diagram on the left shows the significantly more tapered pleon of the male [28]. The right shows the widened and rounded pleon of the female.

As mentioned above, these are decapod crustaceans. Their classifications are the following [41]:

Kingdom: Animalia
Phylum: Arthropoda
Clade (subgroup): Pancrustacea
Class: Malacastroca
Order: Decapoda
Clade (subgroup): Reptantia
Infraorder: Brachyura

The earliest “crabs” appear about 200 million years ago, during the Jurassic Period [43],[50]. There are well-understood trends in their morphology: an overall shortening of the body, including reductions in the abdomen and pleon [58]. Nonetheless, the extremes of these trends are seen within the “untrue crabs”: infraorder Anomura.

This figure displays a fossil and reconstruction of Eocarcinus praecursor, via Gerhard Scholtz’s research [57]. Within the 2020 research article, the author notes that this species is the earliest “stem-group fossil”, or closest relative to an extant group.

During the Cretaceous, asymmetrical claws started to become a significant feature, expanding across many groups of Brachyurans [50]. Since then, the infraorder has diversified; over 7,200 species comprise Brachyura [30],[58].

Life History I: Ecdysis

All arthropods undergo ecdysis, a physiological process that results in the organism shedding its outer cuticle [45]. It is hormone-mediated, particuarly by ecdysteroids [48]. These hormones are part of a group called polyhydroxylated ketosteroids, which have an influence on reproduction and development [53],[59]. The overall process has been organized under the Drach system [39]. Pierre Drach created the following organization in 1939, describing these morphological and physiological changes[39],[48]:

Stage A: Postecdysis I
Stage B: Postecdysis II
Stage C: Intermolt
Stage D: Proecdysis
Stage E: Ecdysis

There are designated substages to each of the stages above, which are not covered here. To keep things moving forward, below are summaries of each stage.

Drach’s Steps

Stage A: the immediate post-molt period of the arthropod, where its cuticle is soft and susceptible to the elements around the animal [45]. It’s s also called early Metecdysis. In this stage, the arthropod is absorbing water into its tissues, which creates a hydrostatic pressure to stretch the new epicuticle outward [52]. The organism is permeable and cannot support its own weight [45].

Stage B: similar to Stage A, where calcification is starting to kick in [40],[45]. It is referred to as late Metecdysis. Here, the arthropod utilizes its calcium reserves and calcium ions from the water to build up the exocuticle (different from the epicuticle) [52]. The chelipeds and walking legs are hardened enough to use, with the carapace remaining flexible [52]. Calcification continues for all appendages and the carapace, while the endocuticle begins to secrete [45],[52].

Stage C: where calcification continues until the layers reach peak structural rigidity; the organism regains metabolic and hormonal stability, with no more tissue synthesis [45]. The carapace, gastric mill, and other mouthparts are fully calcified [45],[52]. A last membranous layer is secreted between the epidermal and endocuticle [52]. The arthropod feeds to build muscle mass, replacing the internal water volume contained in Stage A [53].

Stage D: entering “pre-molt” and preparing by producing more ecdysteroids [45]. The current exoskeleton is slightly broken down in the process, and excess calcium is retained by the organism [52]. There is a gap (which widens through this stage) between the actual exoskeleton and the tissue beneath it [52]. Apolysis soon happens- the complete separation of living epidermal cells detaching from the membranous layer beneath [45],[52]. In that gap is where the new epicuticle forms [52]. As the old exoskeleton softens, chitinases and proteases are secreted in the reserved space [53]. Simultaneously, the new epicuticle is forming [45].

There’s no more feeding [44]; the stomach lining is molting, too! The brachyuran seeks a safe place to lie, such as a dark crevice.
Intense demineralization occurs where the carapace meets the apron, also called the ecdysial suture lines [52].

Stage E: the actual shedding event [45]. This is the most vulnerable time for the animal. The brachyuran swallows water into the gut, thereby increasing blood volume [52]. In turn, this creates hydrostatic pressure that pushes against the old, broken-down epicuticle [52]. It “ruptures” at the ecdysial suture lines [52]. Using muscular contractions, the crab slowly exits through the opening of the ecydisal suture lines [45],[52]. As soon as all organs and appendages are outside of the shed, Stage A occurs.

This source provides insight into ecdysis for horseshoe crabs, which are NOT brachyurans but have a similar molting process. https://www.biorxiv.org/content/10.64898/2026.02.27.708456v1.full

Life History II: Reproduction

Reproduction in Brachyurans occurs internally, typically after females shed their exoskeleton [59]. Post-ecdysis, the new cuticle is soft, allowing access to open gonopores: the male inserts his spermatophores into them, permitting the sperm to reach her spermathecae [40]. The male will hold onto the female until her exoskeleton hardens [54]. The female can store sperm for years, possessing the ability to fertilize multiple batches of eggs over time [31].

Here is an additional FWC source if you’re interested: https://myfwc.com/research/saltwater/crustaceans/blue-crabs/life-cycle/

Brooding the eggs is the next step, where the female attaches them to the pleon (or apron). This “look” has inspired the name “The Sponge Stage” when referring to brooding [31]. There are several advantages to keeping the eggs here, such as protection, ease of grooming, and the ability to aerate the eggs with her pleopods (swimmerets) [38].

Within females, the number of eggs she produces is often related to the width of her carapace and the number of eggs she carries within her pleon [46]. There is evidence for tradeoffs occurring between egg number and egg size, which are all heavily impacted by the conditions the mother endures [38].

Life History III: Life Stages

Embryonic development continues, tucked into the female’s pleon [60]. This development continues for a varied amount of time, depending on species, water temperature, and timing of the lunar cycle [12]. Eggs are released at nocturnal high tides, notably during new or full moons [38]. Spring tides occur under these conditions and are able to carry eggs out to the pelagic.

Brachyurans, like many other arthropods, go through indirect development. Indirect development describes organisms that hatch and do not morphologically or behaviorally resemble their adult forms (i.e., through diet, locomotion, habitat, etc.). For many brachyurans, eggs that are released become protozoea larvae, which quickly molt into zoea larvae [37]. Below is an image of zoea and its “shrimp-like appearance” [37]. Their swimming capabilities stem from the cephalothorax, which forms the mandibles and antennae once the larvae become adults [60]. Zoea are both filter-feeders and predators, feeding on smaller plankton [60].

Zoeae is the first life stage of many crustaceans, expanding far beyond Brachyura [56]. It is free-swimming, planktonic, and predatorial [46]. You can see the rounded body, long segmented tail, and spiny appendages around the body [47]. These stages have multiple parts, characterized by molts; each stage is named as Zoea I, Zoea II, Zoea III… The period between each molt period is called an instar, where brachyurans can have anywhere from two to eight zoeal instars [46]. Though these may be days apart, every molt shows clearer tagmata and an increase in size: a cephalothorax and an abdomen become apparent [60].

During the final molt of the zoeae stage, the organism metamorphoses into a new larval stage: the megalops stage [60]. Some species skip this stage entirely and immediately enter the first juvenile instar [61]. Here, brachyurans finally begin to resemble themselves.

Megalops are dorsoventrally flattened with a broad carapace and have a tuckable abdomen that fits underneath the cephalothorax; this allows the larvae to sit atop substrate [42]. The abdomen also functions posteriorly (pictured below), and aids the pleopods in swimming action [42].

By now, megalops display full chelipeds and long setae upon their pleopods [47],[51]. They are fully capable of directional swimming, particuarly against currents [60]. Megalops are larger than most plankton, preventing them from occupying the same habitat as before [49]. Though they aren’t settled on the substrate just yet, they are influenced strongly by the benthic boundary layer immediately above the substrate [52]. There is only one megalops instar, where they finally settle on the substrate before metamorphosing into the first juvenile instar [42].

Most species remain benthic as juveniles, like adults. Though these stages are similar, juveniles possess differing appendage proportions and setae arrangements [55]. This stage is marked by growth, stemming towards whether the brachyuran will be an indeterminate or determinate grower. As sexual maturity approaches, the juveniles are placed into these two categories, which are a tell on whether they will enter anecdysis and put all energy into reproduction, or invest their energy into both growth and reproduction. The majority of brachyurans encompass the second group. Indeterminate juveniles have significantly more instars, whereas determinate juveniles have fewer instars.

The beginning of the juvenile stage is marked by several instars, sometimes all occurring within weeks. After most molts, an appendage is added. “The puberty molt” marks the end of the juvenile stage; the proportions of all appendages are allometric, gonads are present and functional, and growth rate ceases [52]. The post-puberty stage of this molt can turn into a terminal molt, marking the end of growth for determinate growers. For indeterminate growers, reproduction and molting can occur in a cyclic pattern.

The adult stage completes the life cycle of Brachyura. The shape and behavior of these animals are typically what we think of when “crabs” are mentioned. Though brachyurans are solitary in nature, they are incredibly active. Diets are mostly omnivorous, but some prefer to feed exclusively on algae. They engage in durophagy- the ability to feed on hard-shelled organisms like clams and mussels.

Fun fact: The Florida Stone Crab (Menippe mercenaria) has claws that can exert a crushing force of 14,000-43,200 PSI (4x stronger than the bite force of an alligator). See it to the left.

Diversity in Brachyura

Though I referred to marine brachyurans within this post, there are roughly 1300 freshwater species and 300 terrestrial species. Marine species encompass the vast majority of this group, extending to over 5,800 species. Brachyurans are found all over the world, in a wide variety of environments: mangroves, cold seeps, mudflats, reefs, and hydrothermal vents. Here are a few examples that exemplify Brachyuran diversity, and some of my favorites…

*Gandalfus yunohana*, a blind, deep-sea crab that thrives around hydrothermal vents [25]. It is native to the Philippine Sea Plate, located near Japan. *G. yunohana* feeds on chemosynthetic mats of bacteria, found near vents [33].

*Geocarcinus quadratus*, also known as the Halloween Crab, is a terrestrial crab that heavily contributes to nutrient cycling in South America’s tropical rainforests. They are popular in the hobbyist community due to the crab’s vibrant colorations. Here is a care-guide with additional information: https://www.indoorecosystem.net/guides/gecarcinus-care-guide

*Stenorhynchus seticornis*, the Yellow Arrow Crab, is yet another popular hobbyist choice [2],[32]. They are known to be incredibly hardy and striking in appearance [4]. They feed on marine invertebrates, such as bristleworms, making them a popular choice for reef-keepers [3].

My favorite species of crab lies within the genus *Calappa*. The species shown above is *Calappa Calappa*, the Smooth-Box Crab [20]. Their carapace is curved down, protecting their legs [20].

*Pilumnus vespertilio*, or the Teddybear Crab, is a species of intertidal brachyuran [16],[21]. They also have acquired the name ‘Common Hairy Crab’ [26]. Their meat is poisonous to humans, and it cannot be consumed; their diet consists of zoanthids [21],[22].

*Zebrida adamsii*– the Zebra Urchin Crab [36]. They live on sea urchins, with specialized appendages that help them hook into their tests [36]. While solitary most of the time, two crabs will fit on an urchin if there is space [36]. The symbiosis works for both the urchin and the crab: the crab picks debris off of the urchin (food), keeping the urchin clean [36].

*Gecarcoidea natalis*, the Christmas Island Red Crab [13],[19]. They are known for their annual mass migrations, covering roads in a carpet of crabs! Movements occur from the beginning of the wet season (October to November) towards the nearby coasts [13]. This occurs over the course of a week, during which roads are blocked to take precautions for the migration [13].

*Ixa inermis*, also known as the Spindle Crab [6],[8]. There is minimal information available for this species, with about 77 observations recorded [10]. The shape of the crab is distinct for this species, particuarly looking at the rhomboid-shaped carapace [15]. The legs are thin, with the chelipeds carrying the same shape [15].

*Lybia tessellata*, or the Pom Pom Crab, is notorious for its symbiosis with the anemone, *Actinia prehensa* [7],[27]. On each claw is an anemone polyp, which the crab uses to ward off predators and to obtain food [27]. In return, the anemone polyps get water flow and food scraps [7].

*Parthenope longimanus*, known as the White Long-Armed Crab [9]. The chelipeds are clearly disproportionate to the rest of the crab’s body [9]. It is thought that the arms are used in sexual selection by males [9]. Females have shorter arms and claws [9].

*Xenocarcinus* is a genus of spider crabs [18],[35]. They seem to be associated with wire corals (*Cirrhipathes*) and gorgonia corals (Octocorallia) [29]. They are native to the Indo-Pacific, within the subtidal zone [29]. Species within the genus have been known to host coral polyps [18],[31].

*Achaeus japonicus,* the Orangutan Crab, is known for its hairy appearance [11]. These hairs are used to collect plankton and detritus for consumption [17]. Sources indicate it lives in association with corals like *Plerogyra sinuosa*, the bladder corals [1],[17].

Diversity & Image References

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[15] IXA INERMIS LEACH (CRUSTACEA, DECAPODA, BRACHYURA) a NEW RECORD from INDIAN WATERS.

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[36] “Zebrida Adamsii.” Critter Republic Dive Center – Philippines Best SCUBA Destination., 7 Nov. 2024, critterrepublic.com/zebrida-adamsii/.

Life History & Fossil Information

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[39] Criel, J, and Hilde. “Molt Staging in Artemia Adapted to Drach’s System.” Journal of Morphology, vol. 199, no. 1, 1 Jan. 1989, pp. 41–52, https://doi.org/10.1002/jmor.1051990105. Accessed 30 May 2026.

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[41] Davie, Peter J.F., et al. “SYSTEMATICS and CLASSIFICATION of BRACHYURA1).” Treatise on Zoology — Anatomy, Taxonomy, Biology. The Crustacea, vol. 9, Aug. 2015, pp. 1049–1130.

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