Myogenesis of Malacostraca - the "egg-nauplius" concept revisited

Cytology, function and dynamics of stem and progenitor cells in decapod crustaceans

Stem cells play key roles in development, tissue homeostasis, regeneration, ageing and diseases. Comprehensive reviews on stem cells are available for the determinately growing mammals and insects and some lower invertebrates like hydra but are rare for larger, indeterminately growing invertebrates that can live for many decades. This paper reviews the cytology, function and dynamics of stem and progenitor cells in the decapod crustaceans, a species-rich and ecologically and economically important animal group that includes mainly indeterminate growers but also some determinate growers. Further advantages of decapods for stem cell research are almost 1000-fold differences in body size and longevity, the regeneration of damaged appendages and the virtual absence of age-related diseases and tumours in the indeterminately growing species. The available data demonstrate that the Decapoda possess a remarkable variety of structurally and functionally different stem cells in embryos and larvae, and in the epidermis, musculature, haematopoietic tissue, heart, brain, hepatopancreas, olfactory sense organs and gonads of adults. Some of these seem to be rather continuously active over a lifetime but others are cyclically activated and silenced in periods of days, weeks and years, depending on the specific organ and function. Stem cell proliferation is triggered by signals related to development, moulting, feeding, reproduction, injury, infection, environmental enrichment and social status. Some regulatory pathways have already been identified, including the evolutionarily conserved GATA-binding and runt-domain transcription factors, the widespread neurotransmitter serotonin, the arthropod-specific hormone 20-hydroxyecdysone and the novel astakine growth factors. Knowledge of stem cells in decapods primarily refines our picture on the development, growth and maintenance of tissues and organs in this animal group. Cultured decapod stem cells have good potential for toxicity testing and virus research with practical relevance for aquaculture. Knowledge of stem cells in decapods also broadens our understanding of the evolution of stem cells and regeneration in the animal kingdom. The stem cells of long-lived, indeterminately growing decapods may hold the key to understanding how stem and progenitor cells function into old age without adverse side effects, possibly evoking new ideas for the development of anti-ageing and anti-cancer treatments in humans.

A network analysis of crab metamorphosis and the hypothesis of development as a process of unfolding of an intensive complexity

Development has intrigued humanity since ancient times. Today, the main paradigm in developmental biology and evolutionary developmental biology (evo-devo) is the genetic program, in which development is explained by the interplay and interaction of genes, that is, by the action of gene regulatory networks (GRNs). However, it is not even clear that a GRN, no matter how complex, can be translated into a form. Therefore, the fundamental enigma of development still remains: how is a complex organism formed from a single cell? This question unfolded the historical drama and the dialectical tension between preformation and epigenesis. In order to shed light on these issues, I studied the development of crabs (infraorder Brachyura), as representative of the subphylum Crustacea, using network theory. The external morphology of the different phases of brachyuran metamorphosis were modeled as networks and their main characteristics analyzed. As one could expect, the parameters usually regarded as indicative of network complexity, such as modularity and hierarchy, increased during development. However, when more sophisticated complexity measures were tested, it was evidenced that whereas a group of complexity measures increased during development, another group decreased. This led to consider that two kinds of complexities were being measured. I called them intensive and extensive complexity. In view of these results, I propose that crab development involves a passage from an intensive to an extensive complexity. In other words, crab development can be interpreted as a process of unfolding of an intensive, preexistent complexity.

Immunolocalization of Neurotransmitters and Neuromodulators in the Developing Crayfish Brain

In the field of neurosciences, the crayfish nervous system is an important model for understanding how arthropods process sensory stimuli and generate specific behaviors. Furthermore, crayfish embryos have been important study objects for well over 200 years. Immunohistochemistry against neurotransmitters, neuromodulators, and neurohormones is widely used to analyze the ontogeny of neurons in the emerging brain of several crustacean species and to date represents one of the most powerful approaches to analyze aspects of brain development in this group of organisms. In recent years, the analysis of brain development in crustaceans has gained new momentum by the establishment of the Marmorkrebs Procambarus virginalis (Marbled Crayfish), a parthenogenetic crayfish, as new model system. The embryonic development of marbled crayfish is well characterized and these animals can be easily cultivated in the lab. This chapter describes protocols for immunolocalization of neuroactive substances in the developing crayfish brain.

A phylogenomic framework, evolutionary timeline and genomic resources for comparative studies of decapod crustaceans

Comprising over 15 000 living species, decapods (crabs, shrimp and lobsters) are the most instantly recognizable crustaceans, representing a considerable global food source. Although decapod systematics have received much study, limitations of morphological and Sanger sequence data have yet to produce a consensus for higher-level relationships. Here, we introduce a new anchored hybrid enrichment kit for decapod phylogenetics designed from genomic and transcriptomic sequences that we used to capture new high-throughput sequence data from 94 species, including 58 of 179 extant decapod families, and 11 of 12 major lineages. The enrichment kit yields 410 loci (greater than 86 000 bp) conserved across all lineages of Decapoda, more clade-specific molecular data than any prior study. Phylogenomic analyses recover a robust decapod tree of life strongly supporting the monophyly of all infraorders, and monophyly of each of the reptant, 'lobster' and 'crab' groups, with some results supporting pleocyemate monophyly. We show that crown decapods diverged in the Late Ordovician and most crown lineages diverged in the Triassic-Jurassic, highlighting a cryptic Palaeozoic history, and post-extinction diversification. New insights into decapod relationships provide a phylogenomic window into morphology and behaviour, and a basis to rapidly and cheaply expand sampling in this economically and ecologically significant invertebrate clade.

An atlas of larval organogenesis in the European shore crab Carcinus maenas L. (Decapoda, Brachyura, Portunidae)

BACKGROUND

The life history stages of brachyuran crustaceans include pelagic larvae of the Zoea type which grow by a series of moults from one instar to the next. Zoeae actively feed and possess a wide range of organ systems necessary for autonomously developing in the plankton. They also display a rich behavioural repertoire that allows for responses to variations in environmental key factors such as light, hydrostatic pressure, tidal currents, and temperature. Brachyuran larvae have served as distinguished models in the field of Ecological Developmental Biology fostering our understanding of diverse ecophysiological aspects such as phenotypic plasticity, carry-over effects on life-history traits, and adaptive mechanisms that enhance tolerance to fluctuations in environmental abiotic factors. In order to link such studies to the level of tissues and organs, this report analyses the internal anatomy of laboratory-reared larvae of the European shore crab Carcinus maenas. This species has a native distribution extending across most European waters and has attracted attention because it has invaded five temperate geographic regions outside of its native range and therefore can serve as a model to analyse thermal tolerance of species affected by rising sea temperatures as an effect of climate change.

RESULTS

Here, we used X-ray micro-computed tomography combined with 3D reconstruction to describe organogenesis in brachyuran larvae. We provide a detailed atlas of the larval internal organization to complement existing descriptions of its external morphology. In a multimethodological approach, we also used cuticular autofluorescence and classical histology to analyse the anatomy of selected organ systems.

CONCLUSIONS

Much of our fascination for the anatomy of brachyuran larvae stems from the opportunity to observe a complex organism on a single microscopic slide and the realization that the entire decapod crustacean bauplan unfolds from organ anlagen compressed into a miniature organism in the sub-millimetre range. The combination of imaging techniques used in the present study provides novel insights into the bewildering diversity of organ systems that brachyuran larvae possess. Our analysis may serve as a basis for future studies bridging the fields of evolutionary developmental biology and ecological developmental biology.

Cuticle morphogenesis in crustacean embryonic and postembryonic stages

The crustacean cuticle is a chitin-based extracellular matrix, produced in general by epidermal cells and ectodermally derived epithelial cells of the digestive tract. Cuticle morphogenesis is an integrative part of embryonic and postembryonic development and it was studied in several groups of crustaceans, but mainly with a focus on one selected aspect of morphogenesis. Early studies were focused mainly on in vivo or histological observations of embryonic or larval molt cycles and more recently, some ultrastructural studies of the cuticle differentiation during development were performed. The aim of this paper is to review data on exoskeletal and gut cuticle formation during embryonic and postembryonic development in crustaceans, obtained in different developmental stages of different species and to bring together and discuss different aspects of cuticle morphogenesis, namely data on the morphology, ultrastructure, composition, connections to muscles and molt cycles in relation to cuticle differentiation. Based on the comparative evaluation of microscopic analyses of cuticle in crustacean embryonic and postembryonic stages, common principles of cuticle morphogenesis during development are discussed. Additional studies are suggested to further clarify this topic and to connect the new knowledge to related fields.

Naupliar and Metanaupliar Development of Thysanoessa raschii (Malacostraca, Euphausiacea) from Godthåbsfjord, Greenland, with a Reinstatement of the Ancestral Status of the Free-Living Nauplius in Malacostracan Evolution

The presence of a characteristic crustacean larval type, the nauplius, in many crustacean taxa has often been considered one of the few uniting characters of the Crustacea. Within Malacostraca, the largest crustacean group, nauplii are only present in two taxa, Euphauciacea (krill) and Decapoda Dendrobranchiata. The presence of nauplii in these two taxa has traditionally been considered a retained primitive characteristic, but free-living nauplii have also been suggested to have reappeared a couple of times from direct developing ancestors during malacostracan evolution. Based on a re-study of Thysanoessa raschii (Euphausiacea) using preserved material collected in Greenland, we readdress this important controversy in crustacean evolution, and, in the process, redescribe the naupliar and metanaupliar development of T. raschii. In contrast to most previous studies of euphausiid development, we recognize three (not two) naupliar (= ortho-naupliar) stages (N1-N3) followed by a metanauplius (MN). While there are many morphological changes between nauplius 1 and 2 (e.g., appearance of long caudal setae), the changes between nauplius 2 and 3 are few but distinct. They involve the size of some caudal spines (largest in N3) and the setation of the antennal endopod (an extra seta in N3). A wider comparison between free-living nauplii of both Malacostraca and non-Malacostraca revealed similarities between nauplii in many taxa both at the general level (e.g., the gradual development and number of appendages) and at the more detailed level (e.g., unclear segmentation of naupliar appendages, caudal setation, presence of frontal filaments). We recognize these similarities as homologies and therefore suggest that free-living nauplii were part of the ancestral malacostracan type of development. The derived morphology (e.g., lack of feeding structures, no fully formed gut, high content of yolk) of both euphausiid and dendrobranchiate nauplii is evidently related to their non-feeding (lecithotrophic) status.

Evolution of eumalacostracan development-new insights into loss and reacquisition of larval stages revealed by heterochrony analysis

BACKGROUND

Within Malacostraca (Crustacea), direct development and development through diverse forms of larvae are found. Recent investigations suggest that larva-related developmental features have undergone heterochronic evolution in Malacostraca. In the light of current phylogenetic hypotheses, the free-swimming nauplius larva was lost in the lineage leading to Malacostraca and evolved convergently in the malacostracan groups Dendrobranchiata and Euphausiacea. Here we reconstruct the evolutionary history of eumalacostracan (Malacostraca without Phyllocarida) development with regard to early appendage morphogenesis, muscle and central nervous system development, and determine the heterochronic transformations involved in changes of ontogenetic mode.

RESULTS

Timing of 33 developmental events from the different tissues was analyzed for six eumalacostracan species (material for Euphausiacea was not available) and one outgroup, using a modified version of Parsimov-based genetic inference (PGi). Our results confirm previous suggestions that the event sequence of nauplius larva development is partly retained in embryogenesis of those species which do not develop such a larva. The ontogenetic mode involving a nauplius larva was likely replaced by direct development in the malacostracan stem lineage. Secondary evolution of the nauplius larva of Dendrobranchiata from this ancestral condition, involved only a very small number of heterochronies, despite the drastic change of life history. In the lineage leading to Peracarida, timing patterns of nauplius-related development were lost. Throughout eumalacostracan evolution, events related to epidermal and neural tissue development were clearly less affected by heterochrony than events related to muscle development.

CONCLUSIONS

Weak integration between mesodermal and ectodermal development may have allowed timing in muscle formation to be altered independently of ectodermal development. We conclude that heterochrony in muscle development played a crucial role in evolutionary loss and secondary evolution of a nauplius larva in Malacostraca.