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The biodiversity of zooplankton and zoobenthos decreases towards high latitudes, though the two poles have different species compositions despite environmental similarities in temperature, habitat structure, and light cycle. Different geological history and accessibility largely explain the faunal differences between the poles. While some species live close to their environmental tolerance in the cold polar regions, others have adapted to life at low temperatures. This chapter describes the unique cold-water communities of zooplankton and zoobenthos in the two polar regions. It reviews the most important factors that define the zoogeography and diversity of freshwater invertebrates in the Arctic and Antarctic, and provides detail on the ecology and life-history of some key zooplankton and aquatic insects. The final section considers how climate change, especially elevated temperature and increase in UV radiation, are altering high-latitude aquatic invertebrate communities.

This chapter provides an introduction to the Crustacea, one of the most abundant and diverse components of the plankton. Within a single net-haul, the vast diversity within this group, coupled with the large number of species and the morphological similarity both between species and between developmental stages, can often pose a significant identification challenge even to experienced taxonomists. Although all Crustacea originally share a common body plan, their morphology can differ quite markedly due to different degrees of expression of body segmentation patterns and as a result of the loss or morphological modifications of paired appendages. There is also considerable variation between groups in the structure and function of the appendages on different body regions.

The Arthropoda is one of the largest phyla in the animal kingdom, with the insects alone purportedly consisting of several million living species. The other arthropods are estimated to be more than 100,000. The fossil record both of living and extinct groups dates back to the Early Cambrian and provides important insights about the evolution of the phylum. There are two or three subphyla comprised of a number of subgroups: Pycnogonida, Chelicerata (Xiphosura + Arachnida), and Mandibulata (Crustacea + Tracheata (= Hexapoda + Myriapoda)). However, more recent evidence from morphological and molecular studies point to Hexapoda as an ingroup of the Crustacea. The name Pancrustacea may be applied for the clade, while ‘crustaceans’ may be used for the non-hexapod groups. The Myriapoda appears to be the sister group of the Pancrustacea. The Pycnogonida has been interpreted as the sister group of the Euchelicerata (Xiphosura + Arachnida), while the Pentastomida has been placed within the Arthropoda as the sister group of the Branchiura.

The evolution of the Crustacea following their origins in the Cambrian is outlined, with an overview of their paleontological history and global distributions into modern times. Major recent developments in arthropod evolution include recognition that Hexapoda is nested within Crustacea. Perspectives also changed during the last decades of the 20th century on the form of the crustacean ancestor, from being a long-bodied, serially homonomous form (like a remipede or cephalocarid) to a short-bodied, possibly ostracod-like form similar to Cambrian stem and crown group fossil forms. These changes have come through a shift to formal methods of phylogenetic analysis combined with the much larger volume of both morphological and molecular data now available. The most extensive current phylogenies typically recover the short-bodied Oligostraca (containing ostracods and a few minor groups) as basal crustaceans; Malacostraca and Maxillopoda are high in the tree; and Cephalocarida and Remipedia are derived forms as sister to Branchiopoda and Hexapoda, respectively. Each of these major groups can be understood through variations in tagmatization (differentiation of body segments into regions). The early crustacean fossil record (especially the Ordovician) is dominated by ostracods. Malacostracans, although having Cambrian origins, did not significantly radiate until the Mesozoic. Eumalacostraca continued to actively radiate in the Cenozoic and are now the most ubiquitous and morphologically disparate crustaceans. The processes driving crustacean evolution remain to be fully evaluated. Contingency and external factors are undoubtedly important, but most deep lineages of the Crustacea show pervasive macroevolutionary trends toward increasing tagmatization. These trends are apparently driven, meaning the formation of new body plans is not merely a contingent outcome—intrinsic factors may contribute to increasing tagmatization. Further data are required from ontogeny and developmental genetics, paleontology, and phylogenetics in order to better understand how crustaceans have evolved.

Polar Crustacea show high taxonomic and functional diversity and hold crucial roles within regional food webs. Despite the differences in the evolutionary history of the two Polar regions, present data suggest rather similar species richness, with over 2,250 taxa recorded in the Antarctic and over 1,930 noted in the Arctic. A longer duration of isolated evolution resulted in a high percentage of endemic species in the Antarctic, while the relatively young Arctic ecosystem, subjected to advection from adjacent seas, shows a very low level of endemism. Low temperatures and seasonal changes of food availability have a strong impact on polar crustacean life histories, resulting in their slow growth and development, extended life cycles, and reproduction well synchronized with annual peaks of primary production. Many species, Antarctic amphipods in particular, exhibit a clear tendency to attain large size. In both regions, abundant populations of pelagic grazers play a pivotal role in the transport of energy and nutrients to higher trophic levels. The sea-ice habitat unique to polar seas supports a wide range of species, with euphausiids and amphipods being the most important in terms of biomass in the Antarctic and Arctic, respectively. Deep sea fauna remains poorly studied, with new species being collected on a regular basis. Ongoing processes, namely a decline of sea-ice cover, increasing levels of ultraviolet radiation, and invasions of sub-polar species, are likely to reshape crustacean communities in both Polar regions.

The unifying features of the subphylum Crustacea are outlined, and concise descriptions are provided of the morphological characteristics of the major taxonomic groups included. For Ostracoda and Copepoda, keys allow identification of common rock pool species; for Copepoda, common commensal and parasitic species are listed and illustrated. Revised identification keys and descriptions are provided for Cirripedia. For Malacostraca, revised and updated accounts are given for Tanaidacea, Isopoda, and Amphipoda, and a revised classification is provided. The recent systematic revision of the reptant decapods is noted, and nomenclature and species accounts revised where necessary. Species distributions have been reviewed, and several recent additions to the region’s fauna noted.

Crustacea (or Pancrustacea) have explored virtually all possible milieus in different parts of their life cycle, including freshwater, marine, and terrestrial habitats, and even the air (pterygote insects). Many crustacean taxa display complex life cycles that involve prominent shifts in environment, lifestyle, or both. In this chapter, the overwhelming diversity of crustacean life cycles will be explored by focusing on changes in the life cycles, and on how different phases in a life cycle are adapted to their environment. Shifts in crustacean life cycles may be dramatic such as those seen in numerous decapods and barnacles where the development involves a change from a pelagic larval phase to an adult benthic phase. Also, taxa remaining in the same environment during development, such as holoplanktonic Copepoda, Euphausiacea, and Dendrobranchiata, undergo many profound changes in feeding and swimming strategies. Numerous taxa shift from an early larval naupliar (anterior limbs) feeding/swimming system using only cephalic appendages to a juvenile/adult system relying almost exclusively on more posterior appendages. The chapter focuses mainly on nondecapods and is structured around a number of developmental concepts such as anamorphosis, metamorphosis, and epimorphosis. It is argued that few crustacean taxa can be characterized as entirely anamorphic and none as entirely metamorphic. Many taxa show a combination of the two, even sometimes with two distinct metamorphoses (e.g., in barnacles), or being essentially anamorphic but with several distinct jumps in morphology during development (e.g., Euphausiacea and Dendrobranchiata). Within the Metazoa the Crustacea are practically unrivalled in diversity of lifestyles involving, in many taxa, significant changes in milieu (pelagic versus benthic, marine versus terrestrial) or in feeding mode. Probably such complex life cycles are among the key factors in the evolutionary success of Crustacea.

Diversity in reproduction schedules is a central component of life history variability, with life span and age at maturity as key traits. Closely linked is the number of reproductive attempts and if organisms reproduce only once followed by death (semelparity) or spread reproduction over multiple and separated episodes during the reproductive lifespan (iteroparity). Amphipoda and Isopoda are two crustacean groups with many semelparous species, but semelparity is also part of other groups such as Decapoda, Copepoda, and Lepostraca. We briefly review theories posited for the evolution of semelparity and iteroparity, covering models on demography in both deterministic and fluctuating environments, and examine models on optimal resource allocation. We provide predictions of these theories, a guide on how to test them in crustaceans, and illustrate how theory can help us understand the diversity within this major taxon. We also point out a few shortcomings of these theories. One is that immediate recruitment is usually assumed in studies of semelparity, which is a poor assumption for the many crustaceans that form egg banks with prolonged recruitment. Another is the lack of models where iteroparity versus semelparity emerge as a consequence of life history trade-offs, rather than the more common approach that assumes demographic parameters. Furthermore, we argue that treating semelparity and iteroparity as a dichotomy is sometimes problematic and that viewing these strategies as a continuum can be useful. We discuss life history correlates and the particularly relevant links between the semelparity-iteroparity axis and capital breeding and seasonality, parental care, and terminal molts. We also discuss some of the indirect methods used to conclude if a crustacean is semelparous or not, such as a rapid drop in adult abundance after reproduction or signs of growth or storage after reproduction. A central message in the chapter is the high value of life history theory as a guide when formulating explanations and projecting evolutionary changes in reproductive lifespan of crustaceans.

Patterns of voltinism are well documented in many crustaceans and other invetebrates, and these studies provide diverse insights into species biology, population ecology, and drivers of the evolution of voltinism. This chapter examines voltinism across crustacean taxa, with a focus on mysids as an informative model taxon that exhibits a broad range of life pattern diversity. Voltinism, which describes the number of generations per year for population or species, can be measured as generation time and is shaped by multiple environmental factors, including temperature, latitude, salinity, and depth. Generation time also varies with important biological traits, such as body size, life span, and maturation size and age. I discuss the relationships between voltinism and life history strategies, and the influence of voltinism on adaptative plasticity of species and their populations. Many factors shape evolution of voltinism, including fitness components such as survival, reproduction, and dispersal, as well as tradeoffs among age and size at maturity, reproductive investment, and lifespan. I highlight the importance of voltinism for population modeling in crustaceans, and for understanding regional differences in voltinism. Studies comparing and contrasting voltinism will be critical to better understand how climate change, strong habitat modifications, pollution, and invasive species will impact crustacean populations and their dependent communities.

Many crustaceans are highly adapted for digging (submerging into sand or mud) or burrowing (excavating a structure in sand or mud). Some of the mechanisms that crustaceans use for digging include shovels, fans, wedges, and pile drivers. Burrowing often requires some part of the body to act as a basket to hold sand while it is moved. The modular crustacean body plan has resulted in many different appendages being modified for digging and burrowing. Oxygen levels in sand and mud are low, requiring modifications for ventilation of the gills and respiration. Sand- or mud-dwelling crustaceans are often filter feeders or sediment feeders, and setae on or around the mouthparts play a critical role in trapping or filtering particles during feeding. The rigid exoskeleton of crustaceans stands in sharp contrast to other common digging species that have portions of the body that are soft and flexible, such as molluscs and worms.

In swimming crustaceans, a cascade of forces at different Reynolds numbers affect bodies, propulsive limbs, and their setae and setules. This chapter reviews major examples of crustaceans swimming at moderate to high Reynolds numbers by defining the three major modes of locomotion used by crustaceans that either have the size or the velocity to swim at higher Reynolds numbers. The modes they use to achieve fast and furious swimming include (1) drag-based swimming with setose flagellar or paddle-shaped limbs (thoracic exopodites, modified fifth pereopods, pleopods) in many crustacean classes, (2) lift-based sculling in portunid crabs, and (3) jet propulsion using strong abdominal flexion and morphological modifications of the tail fan and anterior limbs in a wide variety of malacostracans. This chapter describes the modes of swimming that lead to motion at higher Reynolds numbers and analyzes the functional morphology of propulsive limbs and the body design modifications that enhance streamlining.

Circulation and respiration are vital functions in all animals, including crustaceans. This chapter deals with the morphology of these coupled systems and their functional aspects. Crustaceans possess an open circulatory system or, more precisely, an open vascular system. This means that the hemolymph leaves the vascular structures, that is, the heart and directly connected arteries, and flows through either lacunae (spaces between organs) or sinuses (spaces whose function is to channel hemolymph). Respiration, that is, the exchange of the respiratory gases oxygen and carbon dioxide, is based on two main features: circulation and ventilation. Externally, the surfaces of the respiratory structures must be ventilated with a current of the medium, and internally, the circulation of hemolymph is a prerequisite for the exchange of respiratory gases. In this chapter we present for all main crustacean groups an overview of the major circulatory and respiratory structures and their functions.

This chapter briefly describes the diversity in the functional morphology of the crustacean reproductive organs from both macroscopic and microscopic approaches. The genital ducts in both females and males show large morphological variability. This anatomical diversity is proposed to be partially driven by the environment since some similar morphofunctional patterns are found in similar habitats. Specially, different patterns of sperm storage are discussed within the framework of sperm competition. Reproductive morphology in hermaphroditic and intersex species is also included and compared, highlighting the significance of these reproductive models. Finally, morphological comparative studies are proposed to address questions related to the evolution of general and particular designs and primitive and advanced patterns, as well as the study of the embryological development of the reproductive system as a key to understand the differences between and within taxa and neurohormonal pathways of sexual differentiation and endocrine disruption.

The myriad behaviors characteristic of crustaceans reflect the diverse lifestyles (pelagic, benthic, terrestrial, parasitic, sessile) of this large taxon. These behaviors are controlled by nervous systems whose basic structure is conserved across the Crustacea. Crustacean nervous systems are also characterized by extensive taxon-specific adaptations, including marked reductions (e.g., sessile barnacles) and increases (e.g., territorial stomatopods) in complexity. This chapter outlines the organization of the sensory and central nervous systems of the principal crustacean taxa, highlighting both conserved and specialized features.

The science of natural history is built on twin pillars: cataloging the species found in nature, and reflecting on the variety and function of body plans into which these species fit. We often use two terms, diversity and disparity, in this connection, but these terms are frequently used interchangeably and thus repeatedly confused in contemporary discourse about issues of function and form. Nevertheless, diversity and disparity are distinct issues and must be treated as such; each influences our views of the evolution and morphology of crustaceans

The evolutionary history of the postantennular appendages of Crustacea is reviewed, including information on limb development early in the evolutionary lineage of this taxon. Further changes of the appendages and the effects on the feeding and locomotory system are followed along the evolutionary lineage of the crustaceans into the crown group, Eucrustacea. Acknowledging these changes is crucial to understand the high degree of variation of modern crustacean limb morphology and to overcome difficulties in recognizing their common features in terms of homology and relationships. Other strategies that evolved subsequently in eucrustacean ingroups include the arrangement of limbs into functional units and consequent changes in their morphologies, and the high modification of limbs for very specific purposes, for example, reproduction/copulation. Lastly, limbs are also lost repeatedly in various taxa. Reconstructing the evolutionary history of limbs along different crustacean lineages is still a major task for future research.

A carapace (a shield extending from the head region and enveloping a smaller or larger part of the body) is a characteristic feature of many crustaceans. This chapter reviews functional, ontogenetic, and evolutionary aspects of the crustacean carapace. Carapace morphology in Crustacea shows much variation, which is reflected in the many functions present in the various subgroups. The influential textbook by Calman 1909 on crustacean morphology and systematics suggested that a carapace was present primitively in both Malacostraca and Crustacea. This assumption was long unchallenged, but attempts have been made to invalidate/reject Calman's carapace hypothesis. Here it is argued that the best starting point may still be to assume homology, at least within Malacostraca. Whether a carapace is homologous between major crustacean taxa is more uncertain due to a general large morphological disparity.

The dorsobranchial exoskeleton of decapods has served as the archetype for studies of the structure and formation of crustacean cuticle. This cuticle consists of four layers: the epi-, exo-, and endocuticles, which are mineralized with calcium carbonate, and the inner membranous layer. The inner three layers are formed from chitin-protein fibrils arranged in parallel lamellae that have a constantly changing orientation from layer to layer. The exoskeleton is thus a composite structure with remarkable biomechanical resistance to fracture propagation. In different species and within different regions of the body of individuals, the cuticle shows many variations on this basic pattern. The cuticle is cyclically shed and reformed to permit growth. The cyclical nature of cuticle formation and the temporal and spatial separation of the events of matrix deposition and calcification render the crustacean cuticle an excellent model for the study of the control of biomineralization.

The cuticle plays an important role in many aspects of crustacean biology, since it is the interface to the surrounding world. Thus, the cuticle displays many structural specializations all over the body. The structures considered here are setae, setules, denticles, and spines. Seven types of setae are recognised based on their detailed external morphology: plumose, pappose, composite, serrate, papposerrate, simple, and cuspidate. In support of the categorization of these setae, each seems to correlate with a specific functional outcome such as feeding, grooming, and locomotion. Little can be learned about the sensory functions from the external morphology of setae, but their ultrastructure seems to provide better cues. In particular, mechanoreceptors display structures related to transduction mechanisms, with the scolopale as a good example. Still, too few data are available outside malacostracans to draw general conclusions for all crustaceans, underlining the need for multidisciplinary and broad intertaxon studies.