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Multicrustacea

Multicrustacea is a superclass within the arthropod subphylum Crustacea, encompassing the classes Copepoda, Malacostraca, Thecostraca, and Tantulocarida, and comprising the largest and most species-rich clade of non-hexapod crustaceans.^[1]^[2] This clade was established through phylogenomic analyses of nuclear protein-coding sequences, which provided strong support for its monophyly using likelihood, Bayesian, and parsimony methods across 62 genes from 75 arthropod taxa.^[2] Multicrustacea forms part of the broader Altocrustacea within Pancrustacea—the clade uniting crustaceans and hexapods—and is positioned as the sister group to Oligostraca, with recent studies confirming this topology through expanded transcriptomic datasets.^[3]^[4] The diversity of Multicrustacea is extraordinary, accounting for over 80% of the approximately 67,000 described crustacean species, with key groups including the ecologically dominant Malacostraca (such as crabs, shrimps, and lobsters, exceeding 40,000 species) and the numerically abundant Copepoda (around 14,000 species, vital to marine planktonic food webs).^[5]^[6] These organisms exhibit remarkable morphological variation, from the free-living or parasitic copepods and filter-feeding barnacles (Thecostraca) to the larger, often predatory malacostracans, and the rare, minute Tantulocarida.^[1] Habitats span marine, freshwater, brackish, and terrestrial environments, underscoring their pivotal roles in aquatic ecosystems as primary consumers, predators, and decomposers.^[6] Phylogenetic revisions continue to refine Multicrustacea's boundaries, with ongoing genomic efforts highlighting its ancient origins in the Cambrian and adaptive radiations that parallel the evolution of pancrustacean lineages.^[4]^[7]

Taxonomy

Definition and scope

Multicrustacea is a monophyletic clade within the larger subphylum Pancrustacea, defined through phylogenomic analyses of nuclear protein-coding sequences from 62 single-copy genes across 75 arthropod taxa, which provided strong support for its unity based on both molecular and morphological evidence. This clade was formally established in 2010, representing a major restructuring of crustacean taxonomy by grouping morphologically diverse lineages previously classified separately. The scope of Multicrustacea encompasses approximately 80% of all described non-hexapod crustacean species, excluding more basal lineages such as Remipedia, Cephalocarida, and Branchiopoda, which form distinct clades like Xenocarida and Allotriocarida. This vast diversity highlights its dominance within modern Crustacea, comprising tens of thousands of species adapted to marine, freshwater, and terrestrial environments.^[1] Diagnostic traits of Multicrustacea include the presence of a naupliar larval stage in most members, biramous (two-branched) appendages characteristic of their locomotory and feeding structures, compound eyes in many taxa for enhanced visual acuity, and molecular synapomorphies such as conserved sequences in nuclear protein-coding genes that confirm monophyly, building on earlier support from 18S rRNA analyses. These features, while shared with other pancrustaceans to varying degrees, collectively distinguish the clade in phylogenetic reconstructions. The composition of Multicrustacea includes major classes such as Malacostraca (e.g., crabs, shrimps, and lobsters), Copepoda (free-living and parasitic copepods), and Thecostraca (barnacles and related forms), along with minor groups like Tantulocarida, forming a core of ecologically pivotal crustaceans.^[1]

Phylogenetic position

Multicrustacea is the largest clade within Altocrustacea and positioned as the sister group to Oligostraca (which includes Ostracoda) within Pancrustacea, forming one of the primary divisions of non-hexapod Pancrustacea alongside the basal Xenocarida. This placement situates Multicrustacea as part of the monophyletic Pancrustacea, which encompasses all crustaceans and hexapods (insects and their relatives), with Altocrustacea (including Multicrustacea) emerging as sister to Oligostraca, and the whole sister to Hexapoda. The Oligostraca clade itself diverges early within Pancrustacea from the lineage leading to Hexapoda.^[8] Phylogenomic analyses have robustly supported this structure, with a seminal study by Regier et al. analyzing sequences from 62 single-copy nuclear protein-coding genes across 75 arthropod species, confirming Pancrustacea monophyly and the internal divisions including Oligostraca and Vericrustacea (later refined as Altocrustacea containing Multicrustacea) with high bootstrap support (≥95% in maximum likelihood analyses). Subsequent studies using expanded datasets, such as transcriptomes from over 80 pancrustacean species, have reinforced these relationships, though with refinements to internal nodes within Multicrustacea.^[8]^[9] Key evidence for the phylogenetic position of Multicrustacea derives from molecular data, including nuclear genes and complete mitogenomes, which indicate a divergence of Pancrustacea around 500–550 million years ago during the Cambrian period. Mitogenomic analyses of conserved gene arrangements and protein sequences across multicrustacean orders further corroborate the clade's monophyly and its separation from other crustacean groups. Morphologically, Multicrustacea is characterized by synapomorphies such as the hexanaupliar ground pattern, featuring a larval stage with three pairs of cephalic appendages and a biramous swimming antenna, distinguishing it from the simpler naupliar patterns in basal crustaceans.^[10]^[11] Ongoing debates highlight potential paraphyly in related groupings, notably the rejection of Hexanauplia (encompassing Copepoda and Thecostraca) as a valid subclass due to its non-monophyletic status in recent phylogenomic reconstructions. For instance, 2023 analyses using 393 orthologs from 97 pancrustacean transcriptomes rejected Multicrustacea monophyly altogether, proposing instead that Copepoda nests within Malacostraca and attributing previous support to long-branch attraction artifacts. A 2024 study further emphasized that incomplete lineage sorting and long-branch attraction confound inferences, with Copepoda potentially sister to or nested within Allotriocarida. These findings underscore the need for denser taxon sampling and advanced models to resolve conflicts in pancrustacean phylogeny.^[10]^[12]

Morphology

Body structure

The body plan of Multicrustacea is fundamentally segmented, reflecting the ancestral arthropod condition with approximately 19 somites organized into a tripartite structure: a head (derived from the acron plus five segments), a thorax, and an abdomen, terminating in a telson.^[9] In many lineages, such as malacostracans, the head fuses with some thoracic segments to form a cephalothorax, often shielded by a dorsal carapace that provides protection and support.^[13] This regional specialization, or tagmosis, varies across the clade; for instance, while malacostracans typically retain distinct thoracic (pereon) and abdominal (pleon) regions with 5 cephalic, 8 thoracic, and 6 abdominal somites, other groups like copepods exhibit more compact fusion, with the thorax partially integrated into the cephalothorax and a reduced abdomen (urosome) comprising about 5 cephalic, 5 thoracic, and 5 abdominal somites.^[14] Segmentation enables flexibility in locomotion and environmental adaptation, though somite number and visibility differ among extant taxa due to evolutionary reductions or fusions.^[13] Tagmosis patterns emphasize functional grouping, such as the cephalothorax for sensory and feeding roles, contrasting with the more mobile abdominal segments in free-swimming forms. Internally, Multicrustacea feature an open circulatory system, in which hemolymph is drawn into a dorsal heart through paired ostia (valved openings in the pericardial sinus) and distributed via arteries to tissues before returning freely to the hemocoel.^[15] The digestive tract is divided into a foregut (with chitinous grinding structures like the gastric mill in some decapods), a midgut for nutrient absorption, and a hindgut for water regulation and waste expulsion.^[16] The nervous system centers on a supraesophageal ganglion (brain), fusing the ganglia of the acron and cephalic segments, connected ventrally to a subesophageal ganglion and a chain of segmental ganglia along the ventral nerve cord.^[17] Body size spans several orders of magnitude, from microscopic forms like tantulocarids and copepods measuring about 0.07–0.1 mm to large malacostracans such as spider crabs with leg spans reaching up to 3.8 m.^[14]

Appendages and adaptations

The appendages of Multicrustacea exhibit remarkable diversity in structure and function, reflecting their adaptation to a wide range of aquatic and semi-terrestrial lifestyles. The head region typically bears biramous antennules, which serve primarily as sensory organs for detecting chemical and mechanical stimuli, and biramous antennae that aid in both sensory perception and locomotion in some taxa.^[18] Adjacent to these are the mandibles, robust crushing structures for initial food processing, and the maxillae, which assist in shredding and manipulating food particles through coordinated movements.^[18] Postoral appendages, such as the maxillipeds in many malacostracans or pereopods in decapods, are modified for feeding, grooming, and walking, often featuring segmented rami that enhance dexterity.^[18] In the abdominal region, pleopods function in swimming and respiration by generating water currents, while uropods form part of a tail fan for steering and rapid escape responses.^[18] Functional adaptations of these appendages are specialized across Multicrustacea groups to optimize survival and resource acquisition. In copepods, the swimming legs bear dense arrays of long, fine setae that increase surface area for propulsion and capture of microscopic prey, enabling efficient filter-feeding in planktonic environments.^[19] Decapod crustaceans, such as crabs and lobsters, often possess chelae—pincer-like claws formed by modified pereopods—for predation, defense, and mate competition, with robust dactylus and propodus segments providing powerful grasping force.^[20] Barnacles (Thecostraca) employ cement glands located at the base of the antennules to secrete a proteinaceous adhesive, permanently attaching the cyprid larva to substrates and allowing the adult to forgo locomotor appendages in favor of sessile filter-feeding via extensible cirri.^[21] Sensory adaptations enhance environmental navigation and foraging in Multicrustacea. Compound eyes, composed of numerous ommatidia each with a corneal lens and photoreceptor cells, provide wide-angle vision for detecting movement and light polarization, particularly in active swimmers like decapods.^[22] Statocysts, sac-like organs in the antennular base containing statoliths, detect gravity and angular acceleration to maintain balance during locomotion in many mobile forms such as malacostracans.^[23] Chemoreceptors, including aesthetasc sensilla on the antennules and antennae, house ionotropic receptors that bind odorants, facilitating food location and mate detection through active flicking behaviors.^[24] Variations in appendage morphology occur in response to lifestyle constraints within Multicrustacea. Sessile barnacles lack dedicated locomotion appendages, relying instead on thoracic cirri for feeding while the body remains cemented in place.^[25] Parasitic forms, such as rhizocephalan cirripedes, show extreme reduction of appendages, including cirri and mouthparts, as the adult interna absorbs nutrients directly from the host, eliminating the need for external feeding or mobility structures.^[26]

Life history

Reproduction

Multicrustacea predominantly exhibit dioecious reproduction, with distinct male and female sexes determined genetically and morphologically differentiated gonads.^[27] However, hermaphroditism occurs in certain groups, including simultaneous hermaphroditism in many thecostracans such as barnacles, where individuals possess both ovarian and testicular tissues.^[28] Parthenogenesis is also present in Tantulocarida, where sac-like females produce infective tantulus larvae asexually, alongside a sexual cycle in some stages.^[29] This diversity in sexual systems reflects adaptations to varied ecological niches within the clade. Mating behaviors are diverse across Multicrustacea. In malacostracans like decapods, males transfer spermatophores—packets of sperm—directly to the female's gonopores or external receptacles during copulation, often facilitated by specialized appendages such as the first pair of pleopods.^[30] Copepods typically involve external fertilization, with males releasing spermatophores that attach to the female's body, allowing sperm to fertilize eggs as they are spawned.^[31] In contrast, barnacles (thecostracans) achieve internal fertilization through pseudocopulation, where a hermaphroditic individual extends a long, muscular penis to deposit sperm into a neighbor's mantle cavity.^[32] Gamete production involves paired ovaries in females and testes in males, with oogenesis yielding large, nutrient-provisioned oocytes. In malacostracans, eggs are notably yolk-rich, containing substantial lipid and protein reserves derived from vitellogenin-like precursors to support embryonic development.^[33] Spermatogenesis produces non-motile, aflagellate sperm typical of crustaceans, packaged into spermatophores for transfer in many species. Parental care varies, with free-spawning common in groups like copepods and many decapods, where fertilized eggs are released into the water column. In peracarid malacostracans such as isopods and amphipods, females provide extended brooding by retaining eggs and embryos in a ventral marsupium—a specialized pouch formed by oostegites—offering protection and oxygenation until juveniles hatch.^[34] Following brooding or spawning, larvae often emerge in early developmental stages, such as nauplii in non-malacostracans.

Developmental stages

The developmental stages of Multicrustacea typically begin with the nauplius larva, an ancestral first stage characterized by an unsegmented head, three pairs of biramous appendages used for swimming and feeding, and a single median eye.^[35] This larval form hatches from the egg and represents a synapomorphy of the broader Pancrustacea clade, enabling a planktonic lifestyle in many species within Multicrustacea groups such as copepods and thecostracans.^[35] In Tantulocarida, however, the first stage is the infective tantulus larva, without a free-living nauplius, leading to parthenogenetic sac-like females or sexual adults.^[29] Metamorphosis in Multicrustacea involves the sequential addition of body segments and appendages through a series of molts, transforming the simple naupliar form into more complex juvenile stages. For example, in copepods, development proceeds through six naupliar stages (N1–N6), followed by five copepodid stages (CI–CV), with the final copepodid molt yielding the adult; this pattern allows progressive differentiation of thoracic segments and reproductive structures.^[36] The process emphasizes anamorphic growth, where somites are added posteriorly during each instar, contrasting with the more abrupt changes seen in some other arthropods.^[35] Multicrustacea exhibit both direct and indirect development, reflecting adaptations to diverse environments. Direct development, lacking free-living larval stages, occurs in some malacostracans, such as certain isopods, where embryos develop internally until juveniles resembling miniature adults are released after hatching, minimizing dispersal risks in stable habitats.^[37] In contrast, indirect development predominates in decapods, featuring planktonic larvae such as zoea and megalopa stages that facilitate wide oceanic dispersal before settlement and metamorphosis to the benthic juvenile form.^[38] Growth across all stages relies on ecdysis, the molting process where the old exoskeleton is shed to accommodate expansion, driven by the synthesis of a new chitin-based cuticle in the underlying epidermis.^[39] Chitin synthesis peaks during premolt and postmolt phases, involving enzymes like chitin synthase to form the polysaccharide framework mineralized with calcium carbonate.^[39] During the vulnerable intermolt and immediate postmolt periods, the soft new exoskeleton leaves individuals susceptible to predation and physical damage until hardening occurs.^[40]

Diversity

Major classes and orders

The major classes within Multicrustacea encompass the bulk of crustacean diversity, with Malacostraca standing out as the most speciose group. This clade, defined through phylogenomic analyses of nuclear protein-coding sequences, unites several lineages characterized by advanced appendage specializations and varied life modes, excluding basal crustacean groups like Branchiopoda and Remipedia.^[41]^[42] The class Malacostraca is the largest within Multicrustacea, comprising approximately 40% of all known crustacean species and featuring a highly segmented body plan with a prominent carapace in many forms. It includes diverse orders such as Decapoda, which encompasses economically important taxa like shrimps, crabs, and lobsters noted for their ten walking legs and chelipeds; Isopoda, represented by dorsoventrally flattened species including terrestrial woodlice; and Amphipoda, laterally compressed forms like gammarids adapted to interstitial and pelagic habitats. These orders highlight the class's ecological versatility, from benthic predators to scavengers.^[41]^[42] The class Copepoda consists primarily of small, planktonic forms with a median naupliar eye and biramous swimming antennae, divided into 10 orders that reflect adaptations to free-living, parasitic, or commensal lifestyles. Prominent examples include the order Calanoida, dominant in marine and freshwater plankton with elongated bodies and powerful antennules for filter-feeding, and Harpacticoida, often benthic or interstitial with more robust forms suited to crawling on substrates. This class's high abundance in aquatic food webs underscores its role as a foundational trophic link.^[43]^[44] The class Thecostraca groups sessile and parasitic crustaceans, distinguished by a unique mantle cavity and cirral feeding structures in many members, with key subclasses Cirripedia (barnacles, featuring calcareous plates and thoracic cirri for suspension feeding) and Ascothoracida (small, endoparasitic forms lacking a shell but with specialized brood chambers). These groups exhibit extreme morphological divergence from mobile crustaceans, often cementing to substrates or infesting hosts like mollusks.^[45] The class Tantulocarida comprises a small group of highly specialized parasitic crustaceans, primarily infesting other crustaceans such as copepods and peracarids. These minute forms (often less than 1 mm) exhibit a unique life cycle with a free-swimming infective stage called the tantulus larva, and are known from deep-sea and interstitial marine environments, with about 35 described species across five families.^[46]

Species distribution and counts

The Multicrustacea represent one of the most species-rich clades within Crustacea, with over 56,000 described species across its major constituent classes as of 2025. Malacostraca dominates this diversity, encompassing approximately 40,000 species, including prominent orders such as Decapoda (over 17,000 species) and Isopoda (around 10,000 species). Copepoda contributes significantly with approximately 14,000 valid described species, many of which are minute planktonic or parasitic forms, while Thecostraca includes about 2,100 species, primarily barnacles and their relatives, and Tantulocarida adds roughly 35 highly specialized parasitic species.^[47]^[25]^[48] Species richness in Multicrustacea is overwhelmingly concentrated in marine environments, where over 90% of described species occur, reflecting the clade's evolutionary origins and adaptations to oceanic habitats. Patterns of diversity show pronounced tropical peaks, particularly for malacostracans like decapods, with the Indo-West Pacific region harboring the majority of species due to high productivity and habitat complexity. In contrast, freshwater systems support notable richness among copepods and certain malacostracans, such as amphipods and isopods, though these represent a smaller proportion overall.^[49]^[50] Endemism is elevated in isolated habitats, including anchialine caves, where specialized forms exhibit high levels of local adaptation; for instance, remipedes (now placed outside Multicrustacea in sister clade Allotriocarida) demonstrate extreme endemism in such systems, with each cave often hosting unique species. Undescribed diversity remains substantial, especially in deep-sea environments, where exploration has revealed numerous novel taxa among peracarids and copepods.^[51] Recent advances in molecular techniques, particularly DNA barcoding, have accelerated species discovery and delineation within Multicrustacea, uncovering cryptic diversity and leading to an estimated 10-20% increase in recognized species counts since 2010 through the identification of hidden taxa in planktonic and parasitic groups.^[52]^[53]

Ecology

Habitats and distribution

Multicrustacea dominate marine environments, with approximately 85% of their species occurring in oceanic habitats ranging from intertidal zones to abyssal depths exceeding 6,000 meters.^[54] For instance, copepods, a major group within Multicrustacea, are ubiquitous across all ocean layers, from epipelagic plankton communities to hadal trenches, contributing to their role as key components of global marine biodiversity.^[55] This marine prevalence reflects the superclass's evolutionary success in aquatic systems, where they occupy diverse niches including pelagic, benthic, and symbiotic associations.^[56] In contrast, freshwater habitats host around 5,000 Multicrustacea species, primarily amphipods, decapods such as crayfish, and certain copepods, which thrive in rivers, lakes, and groundwater systems worldwide.^[57] Terrestrial environments are limited to about 4,900 species, mainly oniscidean isopods (woodlice and pillbugs) that inhabit moist soils, leaf litter, and under rocks in temperate and tropical regions, relying on humidity for respiration and osmoregulation.^[58] These non-marine representatives underscore the superclass's adaptability beyond oceans, though they constitute a minor fraction of overall diversity.^[55] Multicrustacea exhibit a cosmopolitan biogeographic distribution, with high species richness in tropical and subtropical regions, particularly the Indo-Pacific, where decapod diversity peaks due to coral reef complexity and historical geological factors.^[59] Notable concentrations include massive krill (Euphausiacea) swarms in polar waters, such as Antarctic populations of Euphausia superba that form aggregations visible from space and sustain Southern Ocean ecosystems.^[60] Adaptations to extreme conditions further expand their range, with thermophilic decapods like Rimicaris exoculata inhabiting hydrothermal vents at temperatures up to 40°C via bioluminescent detection and metal-tolerant exoskeletons, and psychrophilic copepods such as Boeckella poppei enduring subzero Antarctic lakes through lipid accumulation and slow metabolic rates.^[56]

Trophic roles and interactions

Multicrustacea exhibit diverse feeding strategies that position them across multiple trophic levels in aquatic ecosystems. Many species function as herbivores or suspension feeders, such as barnacles (Cirripedia), which filter phytoplankton from the water column using their cirral appendages to capture microscopic algae, thereby grazing on primary producers.^[61] Detritivores are prominent among peracarid groups like amphipods (Amphipoda), which primarily consume decaying organic matter and scavenged debris on the seafloor, facilitating the breakdown of dead plant and animal remains.^[62] Carnivorous taxa include various decapod crabs (Decapoda), such as the blue crab (Callinectes sapidus), which actively hunt mollusks, smaller crustaceans, and polychaetes using their chelae for predation.^[63] Omnivorous behavior is common in shrimps (Caridea), exemplified by species like the common prawn (Palaemon serratus), which opportunistically feed on algae, detritus, small invertebrates, and carrion depending on availability.^[64] In terms of predation dynamics, Multicrustacea serve both as predators and prey, influencing food web structure. Certain decapods, including portunid crabs, act as apex or intermediate predators in coastal and estuarine systems, exerting top-down control by preying on bivalves and gastropods, which can alter benthic community composition.^[65] Conversely, they form critical prey bases for higher trophic levels; for instance, Antarctic krill (Euphausia superba) functions as a keystone species in Southern Ocean food webs, supporting populations of whales, seals, penguins, and fish by comprising up to 90% of the diet for some predators.^[66] This dual role underscores their importance in energy transfer, with planktonic and larval forms particularly vulnerable to predation by planktivorous fish and gelatinous zooplankton. Symbiotic interactions among Multicrustacea highlight complex interspecies relationships. Parasitic copepods (Copepoda), such as those in the family Caligidae, attach to fish hosts via modified appendages, feeding on mucus, skin, and tissues, which can impair host respiration and increase mortality rates.^[67] Commensal isopods, including certain sphaeromatids, inhabit sponge tissues without apparent harm to the host, gaining shelter and access to food particles while potentially aiding in sponge cleaning through incidental detritivory.^[68] Mutualistic associations occur in wood-boring isopods like Limnoria species, where gut microbiomes, including bacteria such as Teredinibacter, produce enzymes that degrade lignocellulose, enabling the isopods to digest wood as a primary food source.^[69] Multicrustacea play vital ecosystem roles, particularly in nutrient cycling and environmental monitoring. Detritivorous and scavenging species, such as amphipods and isopods, accelerate decomposition of organic matter, releasing nutrients like nitrogen and phosphorus back into the water column and sediments, which supports primary productivity in marine and freshwater systems.^[70] Planktonic forms, including copepods and euphausiids, contribute to vertical carbon flux by grazing phytoplankton and producing fecal pellets that sink, indirectly aiding in the maintenance of oxygenated surface waters through enhanced nutrient upwelling.^[71] Additionally, various crustaceans serve as bioindicators of pollution; for example, amphipods exhibit reduced abundance and altered community structure in response to heavy metal contamination, providing early warnings of anthropogenic impacts on aquatic health.^[72]

Evolutionary history

Origins and phylogeny

Multicrustacea, the largest clade within Crustacea, emerged during the early Cambrian period approximately 540 million years ago (Ma), arising from stem-group ancestors shared with the sister clade Oligostraca as part of the broader arthropod radiation following the Ediacaran biota.^[73] This origin aligns with the Cambrian Explosion, when diverse arthropod body plans, including primitive crustacean-like forms with biramous appendages and specialized feeding structures, rapidly diversified in marine environments.^[73] Fossil evidence from lagerstätten such as the Maotianshan Shale in China reveals early representatives with untagmatized trunks and setose antennae, indicating meiobenthic lifestyles akin to modern multicrustaceans.^[73] A key early event was the divergence of Multicrustacea from its sister groups, including Oligostraca (encompassing ostracods and branchiurans), around 520 Ma, as estimated by fossil-calibrated molecular phylogenies.^[74] This basal split, supported by analyses of nuclear protein-coding genes and expressed sequence tags, marks the establishment of Multicrustacea as a monophyletic lineage comprising Malacostraca, Copepoda, Thecostraca, and related groups.^[74] Within Multicrustacea, the radiation of Malacostraca began in the Ordovician but accelerated significantly in the Mesozoic, with eumalacostracans diversifying into over 24 orders and dominating marine and freshwater ecosystems by the Cenozoic.^[75] Decapod crustaceans first appear in the fossil record in the Late Devonian (~372 Ma), with brachyuran crabs diversifying from the Early Jurassic onward.^[76]^[77] Similarly, copepods underwent a major expansion in Mesozoic oceans, contributing up to 80% of zooplankton biomass and colonizing planktonic niches amid the phytoplankton revolution.^[78] Phylogenetic milestones include the evolution of the naupliar larva as a synapomorphy for Pancrustacea, retained in many multicrustacean lineages such as copepods and thecostracans, featuring three pairs of appendages and a median eye for early planktonic dispersal.^[35] Multicrustaceans also achieved multiple independent colonizations of freshwater and terrestrial habitats, with copepods repeatedly invading inland waters and malacostracans like isopods and amphipods transitioning to land multiple times since the Paleozoic.^[79] Molecular clock estimates from fossil-calibrated phylogenies place the basal Multicrustacea split at approximately 495 Ma (95% CI: 520–469 Ma) according to a 2013 study, with a 2023 pancrustacean analysis using 13 vetted fossils supporting a Cambrian divergence before ~521 Ma and underscoring sensitivity to taxon sampling in deep-time reconstructions.^[74]^[10] These timelines highlight Multicrustacea's role in the post-Cambrian arthropod diversification, with ongoing refinements from phylogenomic data.^[10]

Fossil record

The fossil record of Multicrustacea dates back to the Cambrian period, with the clade sharing roots in this era alongside other major pancrustacean lineages such as Allotriocarida and Oligostraca.^[80] The earliest potential representatives appear in Middle Cambrian deposits, including the Chengjiang biota (approximately 518–510 million years ago), where arthropods with biramous limbs and other crustacean-like features suggest possible stem-group multicrustaceans.^[81] Similar exceptional preservation in the slightly younger Burgess Shale (around 508 million years ago) has revealed fossils like Priscansermarinus barnetti, an early stalked barnacle (Thecostraca) with a pedunculate body and biramous appendages, providing evidence for the clade's ancient origins. These Cambrian finds are often debated, with uncertainties in assigning them definitively to crown-group Multicrustacea versus more basal stem lineages due to the primitive morphology and limited soft-tissue preservation.^[74] Throughout the Paleozoic, Multicrustacea are represented by rare but significant groups, including the extinct Cyclida, shark-like crustaceans with specialized predatory morphology known from Carboniferous and Permian deposits in regions like the Urals and Donets Basin (approximately 359–252 million years ago).^[82] Although ostracods (now placed in the sister clade Oligostraca) dominate Paleozoic crustacean fossils with abundant calcified carapaces from Ordovician to Devonian strata, Multicrustacea contributions are sparser, often limited to trace fossils such as branching burrows (e.g., Spongeliomorpha) attributed to early malacostracans in Lower Devonian marine sediments (around 419–393 million years ago).^[83] The Mesozoic era marks a diversification of Multicrustacea, particularly within Malacostraca, with families like Prosoponidae, primitive brachyuran crabs with broad carapaces and reduced abdomens, well-documented from Middle Jurassic to Cretaceous marine deposits in Europe and beyond, exemplifying early brachyuran radiation.^[77] Thecostracans, including barnacles, have a more continuous record starting from Paleozoic stalked forms but with acorn barnacles (Balanomorpha) emerging prominently in the Cenozoic, where they become abundant in shallow marine and intertidal fossils from Eocene to Recent (approximately 56 million years ago to present), reflecting adaptations to sessile lifestyles on substrates like rocks and shells.^[25] Preservation of Multicrustacea fossils is highly variable, with exceptional soft-part details in Cambrian lagerstätten like Chengjiang and Burgess Shale revealing appendages, guts, and larval stages otherwise lost to taphonomic biases.^[81] However, significant gaps exist, particularly for soft-bodied groups like copepods (Copepoda), whose small size and planktonic habits result in few direct body fossils, including rare specimens from the late Carboniferous (~303 Ma) as well as Miocene amber inclusions and Cretaceous parasites.^[84] Trace fossils, including burrows and trackways, provide indirect evidence of multicrustacean activity from the Devonian onward, compensating for the incomplete body fossil record in non-lagerstätte deposits.^[85]

Human significance

Economic uses

Multicrustacea, particularly decapods such as shrimp and lobsters, form a cornerstone of global fisheries, with wild-caught and farmed production contributing significantly to the seafood economy. In 2022, the global aquaculture production of decapod crustaceans reached 12.7 million tonnes, valued at USD 75.7 billion, underscoring their economic importance as a protein source.^[86] Wild decapod fisheries contribute significantly to global seafood production, with marine crustaceans accounting for approximately 8% of total supplies.^[87] Krill harvesting, primarily from Antarctic stocks, adds to this sector, with the Antarctic krill products market valued at USD 1.63 billion in 2023, much of which is directed toward aquaculture feed production.^[88] Aquaculture further amplifies the economic role of multicrustaceans, especially through penaeid shrimp farming, with production reaching approximately 8 million tonnes in 2024, valued at over USD 40 billion as of 2023 and positioning it as one of the most lucrative segments of global seafood farming.^[89]^[90] Penaeid species like Litopenaeus vannamei dominate intensive pond systems in Asia and Latin America, where they are cultured for export markets due to high demand in processed foods. Copepods, smaller multicrustaceans, serve as essential live feeds in marine fish hatcheries, enhancing larval survival and growth rates compared to traditional rotifers; their use in species like groupers has led to innovations in centralized production for commercial scalability.^[91]^[92] Beyond direct food production, multicrustacean byproducts yield valuable materials for industrial applications. Chitin, extracted from the shells of crabs, shrimp, and lobsters via chemical demineralization or biological enzymatic processes, finds extensive use in biomedical fields such as wound dressings, drug delivery systems, and tissue engineering scaffolds due to its biocompatibility and antimicrobial properties.^[93]^[94] Barnacles, as model organisms in biofouling studies, inform the development of eco-friendly antifouling coatings for ships and marine structures; research on their adhesive mechanisms has advanced non-toxic alternatives to copper-based paints, reducing environmental impacts while improving vessel efficiency.^[95]^[96] Historically, multicrustacean fisheries date back to ancient Mediterranean civilizations, with evidence of crab and lobster exploitation by Romans as indicated by archaeological finds in amphorae containing crustacean remains and depictions in mosaics and writings from the period.^[97]^[98] These early practices, centered around coastal harvesting for local consumption and trade, laid foundational patterns for the region's enduring seafood economies.

Conservation and threats

Multicrustacea face significant threats from human activities, with overfishing leading to declining populations in commercially important species such as lobsters, where American lobster stocks in the Gulf of Maine and Georges Bank have decreased by 34% since 2018 due to excessive harvesting.^[99] Habitat loss from coastal development and wetland drainage has severely impacted freshwater and estuarine crustaceans, including prairie wetland species where up to 85% of habitats have been lost, correlating with reduced biodiversity.^[100] Climate change exacerbates these pressures through ocean acidification, which hinders calcification in barnacles and other calcifying multicrustaceans by increasing seawater acidity and reducing shell formation.^[101] Invasive species, such as the Chinese mitten crab (Eriocheir sinensis), disrupt native ecosystems by competing for resources and preying on local decapods, altering community structures in invaded regions like North American rivers.^[102] Emerging threats include microplastic ingestion, which has been documented in copepods and other small multicrustaceans, leading to reduced feeding efficiency and increased mortality rates, with studies showing up to 29% of deep-sea crustaceans containing microplastics in their gastrointestinal tracts.^[103] Ocean warming is shifting polar distributions, particularly affecting Antarctic krill (Euphausia superba), as rising temperatures alter migration patterns and reproductive success in high-latitude habitats, with recent assessments indicating southward contractions in suitable habitat.^[104]^[105] Conservation efforts for Multicrustacea include assessments by the International Union for Conservation of Nature (IUCN), which lists approximately 28% of evaluated crustacean species—encompassing lobsters, freshwater crabs, crayfishes, and shrimps—as threatened with extinction (Critically Endangered, Endangered, or Vulnerable) as of 2025.^[106] Protected areas, such as marine protected areas (MPAs) in the Southern Ocean, safeguard krill populations by restricting fishing in key breeding grounds, with proposals for expanded networks to cover sensitive Antarctic Peninsula regions.^[107] Genetic monitoring programs address risks from aquaculture escapes, using tools like the Offshore Aquaculture Escapes Genetics Assessment (OMEGA) model to detect hybridization between farmed and wild decapods, thereby preventing genetic pollution in native stocks.^[108]

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