The foregut-ossicle system of Dromia wilsoni, Dromia personata and Lauridromia intermedia (Decapoda, Brachyura, Dromiidae), studied with a new staining method

The complete mitochondrial genome of Calappa bilineata: The first representative from the family Calappidae and its phylogenetic position within Brachyura

In this study, we determined the complete mitogenome sequence of Calappa bilineata, which is the first mitogenome of Calappidae up to now. The total length is 15,606 bp and includes 13 protein-coding genes, 22 transfer RNAs, two ribosomal RNAs and one control region. The genome composition is highly A + T biased (68.7%), and exhibits a negative AT-skew (-0.010) and GC-skew (-0.267). As with other invertebrate mitogenomes, the PCGs start with the standard ATN and stop with the standard TAN codons or incomplete T. Phylogenetic analysis showed that C. bilineata was most closely related to Matuta planipes (Matutidae), and these two species formed a sister clade, constituting a Calappoidea group and forming a sister clade with part of Eriphioidea. The existence of the polyphyletic families raised doubts over the traditional classification system. These results will help to better understand the features of the C. bilineata mitogenome and lay foundation for further evolutionary relationships within Brachyura.

Structure of the stomach cuticle in adult and larvae of the spider crab Maja brachydactyla (Brachyura, Decapoda)

The stomach of decapods is a complex organ with specialized structures that are delimited by a cuticle. The morphology and ontogeny of the stomach are largely described, but few studies have focused on the morphology of its cuticle. This study examined the morphology of the stomach cuticle of cardiac sacs, gastric mill ossicles, cardio-pyloric valve and pyloric filters, and during various stages (zoea I and II, megalopa, first juvenile, and adult) of the common spider crab Maja brachydactyla using dissection, histology and transmission electron microscopy. The results show that cuticle morphology varies among structures (e.g., cardiac sacs, urocardiac ossicle, cardio-pyloric valve, pyloric filters), within a single structure (e.g., different sides of the urocardiac ossicle) and among different life stages. The cuticle during the larval stages is very thin and the different layers (epicuticle, exocuticle, and endocuticle) are infrequently distinguishable by histology. Major changes during larval development regarding cuticle morphology are observed after the molt to megalopa, including the increment in thickness in the gastric mill ossicles and cardio-pyloric valve, and the disappearance of the long thickened setae of the cardio-pyloric valve. The cuticle of all the stomach structures in the adults is thicker than in larval and juvenile stages. The cuticle varies in thickness, differential staining affinity and morphology of the cuticle layers. The structure-function relationship of the cuticle morphology is discussed.

The crustacean cuticle does not record chronological age: New evidence from the gastric mill ossicles

A proposed method to determine chronological age of crustaceans uses putative annual bands in the gastric mill ossicles of the foregut. The interpretation of cuticle bands as growth rings is based on the idea that ossicles are retained through the moult and could accumulate a continuous record of age. However, recent studies presented conflicting findings on the dynamics of gastric mill ossicles during ecdysis. We herein study cuticle bands in ossicles in four species of commercially important decapod crustaceans (Homarus gammarus, Nephrops norvegicus, Cancer pagurus and Necora puber) in different phases of the moult cycle using dissections, light microscopy, micro-computed tomography and cryo-scanning electron microscopy. Our results demonstrate that the gastric mill is moulted and ossicles are not retained but replaced during ecdysis. It is therefore not plausible to conclude that ossicles register a lifetime growth record as annual bands and thereby provide age information. Other mechanisms for the formation of cuticle bands and their correlation to size-based age estimates need to be considered and the effect of moulting on other cuticle structures where 'annual growth bands' have been reported should be investigated urgently. Based on our results, there is no evidence for a causative link between cuticle bands and chronological age, meaning it is unreliable for determining crustacean age.

Functional morphology of comminuting feeding structures of Trichodactylus borellianus (Brachyura, Decapoda, Trichodactylidae), an omnivorous freshwater crab

Crustaceans exhibit great diversity of feeding structures with morphological traits that are useful to infer the general trophic habits of species. In this study, we analyzed the functional morphology of comminuting feeding structures (mandibles, chelipeds, gastric mill) of the freshwater crab Trichodactylus borellianus directly related with the food fragmentation. The heterochely and mechanical advantage (MA) of the chelae were also studied. In both analyses, we considered the relationship between morphology and the natural diet. We expected to find a consistent relation between feeding habits and morphological traits. In general, we found simple structures armed with uniform setal systems and feeding appendages without pronounced teeth or spines. Mandibles have primarily cutting functions, helping with the food anchoring and fragmentation with mandibular palps armed with pappose setae. Chelipeds were covered with spines and simple setae. Adult males exhibited right-handedness with high MA of the major chelae. The ingested, relatively large pieces of food are finally chewed by a gastric mill equipped with sharp cusps characteristic of decapods with low ingestion of crude fiber material. The morphology of the feeding apparatus revealed that it is well adapted to an omnivorous diet, being able to cope with dietary changes.

Calcitic sclerites at base of malacostracan pleopods (Crustacea)--part of a coxa

BACKGROUND

Cuticular specialisations such as joints and membranes play an important role in the function of arthropod limbs. This includes sclerotisations and mineral incrustations of cuticular areas to achieve either more rigidity or flexibility. The anterior eight thoracopods of Malacostraca have limb stems comprising a coxa and a basipod, which carries the two rami. Their pleopods, the limbs of the posterior trunk part, have for long been regarded to lack a coxa. Several calcitic sclerites occur in the area between ventral body and limb stem. This raises the questions: do these elements represent specialisations of the membrane due to functional requirements, and do they originate from an originally larger limb portion, i.e., the coxa, or in fact represent it.

RESULTS

We investigated 16 species of selected malacostracan taxa from all major in-groups. Calcitic sclerites occur in constant numbers and position within a species (no individual variation, and independent of specific modification such as in genital appendages). These are even constant within a supra-specific taxon, which facilitates comparisons. In general the sclerites connect via two pivot joints to the sternite medially and the tergopleura laterally, and two more to the limb stem. Based on this, we reconstructed putative ground-pattern conditions for the sclerites of the examined taxa of Malacostraca.

CONCLUSIONS

The pattern of sclerites is characteristic for each monophyletic malacostracan taxon. The highest number of sclerites most likely represents the plesiomorphic state. Reduction of sclerite numbers occurs in Caridoida and its in-groups. Sclerite arrangement in these taxa provides an important character complex for phylogenetic studies. The presence of pivot joints to the body proximally and basipod distally demonstrates the existence of a coxa, which is just slightly less sclerotised, particularly on its posterior side. This can be explained by enhanced flexibility of the pleopods evolved in the course to their major role as swimming devices. Both the pivot joints and the proximal and distal extension of the calcitic sclerites demarcate the minimum area of the coxa. With this, sclerites appear as very valuable also in shedding more light on the putative relationships between Malacostraca, Myriapoda, Insecta, and Remipedia.

Foregut morphology and ontogeny of the mud crab Dyspanopeus sayi (Smith, 1869) (Decapoda, Brachyura, Panopeidae)

The morphology of the foregut of the Say's mud crab Dyspanopeus sayi was described in adults and larvae. The ossicle system was illustrated based on a staining method with Alizarin-Red. The gastric teeth and cardio-pyloric valve were dissected and examined using optical and scanning electron microscopy. In the adults, the morphology of ossicles and gastric teeth of D. sayi is very similar to the related species Rhithropanopeus harrisii. The foregut of first zoea (ZI) presented a functional cardio-pyloric valve while the filter press was lacking. The filter press was observed in the pyloric chamber from ZII. The most significant changes in morphology take place after metamorphosis from ZIV to megalopa, including the occurrence of the gastric mill. The organization and morphology of many megalopal foregut ossicles are recognizable in the adult phase, although the morphology of the gastric teeth differs from the morphology of adults. A correlation of gastric mill structures with food preferences and their contribution to the phylogeny are briefly discussed.

Development and functional morphology of the larval foregut of two brachyuran species from Northern Brazil

Feeding is an important factor for the successful rearing of larvae of the crab species. Further information on the morphological features of the foregut may to reveal larval feeding behaviour and or/whether there is a lecithotrophy in some or even in all stages of the larval cycle. In the present study, the structural development of the foregut and their digestive functions were examined in larvae of two brachyurans, Uca vocator and Panopeus occidentalis, reared in the laboratory. During larval development, the foreguts of the larvae in the first and last zoeal stages and in the megalopa stage were microscopically examined, described and illustrated. The zoeal foreguts of both species were well developed, showing specialization with a functional cardiopyloric valve and a filter press. The megalopa stage had a complex and specialized gastric mill similar to that found in adult crabs with the appearance of rigidly calcified structures. These results support the hypothesis that the feeding behaviour of each larval stage is directly related to the morphological structure of the foregut. Such facts strongly indicate that all larval stages of both . vocator and P occidentalis need an external food source before completing the larval development in a planktonic environment.

Autofluorescence imaging, an excellent tool for comparative morphology

Here we present a set of methods for documenting (exo-)morphology by applying autofluorescence imaging. For arthropods, but also for other taxa, autofluorescence imaging combined with composite imaging is a fast documentation method with high-resolution capacities. Compared to conventional micro- and macrophotography, the illumination is much more homogenous, and structures are often better contrasted. Applying different wavelengths to the same object can additionally be used to enhance distinct structures. Autofluorescence imaging can be applied to dried and embedded specimens, but also directly on specimens within their storage liquid. This has an enormous potential for the documentation of rare specimens and especially type specimens without the need of preparation. Also for various fossils, autofluorescence can be used to enhance the contrast between the fossil and the matrix significantly, making even smallest details visible. 'Life-colour' fluorescence especially is identified as a technique with great potential. It provides additional information for which otherwise more complex methods would have to be applied. The complete range of differences and variations between fluorescence macrophotography and different types of fluorescence microscopy techniques are here explored and evaluated in detail. Also future improvements are suggested. In summary, autofluorescence imaging is a powerful, easy and fast-to-apply tool for morphological studies.

Characterisation of cellulose and hemicellulose digestion in land crabs with special reference to Gecarcoidea natalis

This article reviews the current knowledge of cellulose and hemicellulose digestion by herbivorous land crabs using the gecarcinid Gecarcoidea natalis as a model species for this group. Cellulose digestion in the gecarcinids is hypothesised to require mechanical fragmentation and enzymatic hydrolysis. Mechanical fragmentation is achieved by the chelae, mandibles and gastric mill, which reduce the material to particles less than 53 µm. The gastric mill shows adaptations towards a plant diet; in particular, there are transverse ridges on the medial and lateral teeth and ventral cusps on the lateral teeth that complement and interlock to provide efficient cutting surfaces. Enzymatic hydrolysis of cellulose and hemicellulose is achieved through cellulase and hemicellulase enzymes. In the gecarcinids, 2–3 endo-β-1,4-glucanases, one β-glucohydrolase and a laminarinase have been identified. The endo-β-1,4-glucanases are multifunctional, with both endo-β-1,4-glucanase and lichenase activity. Complete cellulose hydrolysis is achieved through the synergistic action of the endo-β-1,4-glucanase and β-glucohydrolase. The evidence for the endogenous production of the cellulase and hemicellulase enzymes, their evolutionary origin and possible evolution in invertebrates as they colonised land is also discussed.

Development and functional morphology of the foreguts of larvae and postlarvae of three crustacean decapods

The development of the foregut structure and the digestive function of the decapods Litopenaeus vannamei, Sesarma rectum and Callichirus major larvae and post larvae were examined. The protozoeal foregut of L. vannamei is simple, lacking a cardiopyloric valve and bearing a rudimentary filter press. In mysis, the filter press is more developed. In the juvenile stage, grooves and a small lateral tooth arise. In S. rectum, the foregut has a functional cardiopyloric valve and a filter press. The megalopal and juvenile stages of this species have a gastric mill similar to those in adult crabs. In C. major, the foregut of the zoeae is specialized, with the appearance of some rigid structures, but no gastric mill was found. Calcified structures are observed in the megalopae and they become more developed in the juvenile stage. The results support suppositions, previously reported in other studies, that feeding behavior of each larval and postlarval stage is directly related to the morphological characteristics of the foreguts.

Phylogenetics of the brachyuran crabs (Crustacea: Decapoda): The status of Podotremata based on small subunit nuclear ribosomal RNA

The true crabs, the Brachyura, are generally divided into two major groups: Eubrachyura or 'advanced' crabs, and Podotremata or 'primitive' crabs. The status of Podotremata is one of the most controversial issues in brachyuran systematics. The podotreme crabs, best recognised by the possession of gonopores on the coxae of the pereopods, have variously been regarded as mono-, para- or polyphyletic, or even as non-brachyuran. For the first time, the phylogenetic positions of the podotreme crabs were studied by cladistic analysis of small subunit nuclear ribosomal RNA sequences. Eight of 10 podotreme families were represented along with representatives of 17 eubrachyuran families. Under both maximum parsimony and Bayesian Inference, Podotremata was found to be significantly paraphyletic, comprising three major clades: Dromiacea, Raninoida, and Cyclodorippoida. The most 'basal' is Dromiacea, followed by Raninoida and Cylodorippoida. Notably, Cyclodorippoida was identified as the sister group of the Eubrachyura. Previous hypotheses that the dromiid crab, Hypoconcha, is an anomuran were unsupported, though Dromiidae as presently composed could be paraphyletic. Topologies constrained for podotreme monophyly were found to be significantly worse (P < 0.04) than unconstrained topologies under Templeton and S-H tests. The clear pattern of podotreme paraphyly and robustness of topologies recovered indicates that Podotremata as a formal concept is untenable. Relationships among the eubrachyurans were generally equivocal, though results indicate the majoids or dorippoids were the least derived of the Eubrachyura. A new high level classification of the Brachyura is proposed.