Abstract
BackgroundThe decapods are a major group of crustaceans that includes shrimps, prawns, crayfishes, lobsters, and crabs. Several studies focused on the study of the digestive system of the decapods, constituted by the oesophagus, stomach, midgut tract, midgut gland, and hindgut. Nevertheless, in the midgut tract there are associated a set of organs called “midgut caeca”, which are among the most controversial and less studied digestive organs of this group. This work used the common spider crab Maja brachydactyla Balss, 1922 as a model to resolve the origin, development, and potential role of the midgut caeca. Such organs were studied in the larvae (zoea I, zoea II, megalopa), first juveniles, and adult phases, being employed traditional and modern techniques: dissection, micro-computed tomography (Micro-CT), and light and electron microscopical analyses (TEM and SEM).ResultsThe common spider crab has a pair of anterior midgut caeca and a single posterior caecum that originate from the endoderm germ layer: they develop from the midgut tract, and their epithelium is composed by secretory cells while lacking a cuticle lining. The midgut caeca are small buds in the newly hatched larvae, enlarge linearly during the larval development, and then continue growing until became elongated and coiled blind-tubules in adults. The adult midgut caeca are internally folded to increase their inner surface. The electron microscopy observations showed that the midgut caeca are highly active organs with important macroapocrine and microapocrine secretory activity. Our results suggest that the role of the caeca might be related to the digestive enzyme secretion. The secretory activity should increase as the animal grows in size.ConclusionThe present study resolves the embryonic origin of the midgut caeca (endoderm derived organs), development (general lengthening starting from small buds), and role (active secretory organs). The secretory activity of the midgut caeca should be incorporated in the current models of the digestive physiology in different decapod taxa.
Highlights
The decapods are a major group of crustaceans that includes shrimps, prawns, crayfishes, lobsters, and crabs
The morphology of the digestive system of the brachyurans is shared with other decapods [18, 24, 25, 28]: the mouth connects with a short muscular oesophagus lined by a cuticle, it is followed by a stomach with several calcified pieces and a gastric mill composed by cuticular teeth
The adult specimens have a pair of anterior midgut caeca (AC) and a single posterior caecum (PC) arising from the same sites than the described in the larvae and juveniles (Fig. 1F– H)
Summary
The decapods are a major group of crustaceans that includes shrimps, prawns, crayfishes, lobsters, and crabs. This work used the common spider crab Maja brachydactyla Balss, 1922 as a model to resolve the origin, develop‐ ment, and potential role of the midgut caeca Such organs were studied in the larvae (zoea I, zoea II, megalopa), first juveniles, and adult phases, being employed traditional and modern techniques: dissection, micro-computed tomog‐ raphy (Micro-CT), and light and electron microscopical analyses (TEM and SEM). The monophyly of the decapods is supported by several studies [3, 6–8], and the true crabs (Brachyura Latreille, 1802) are probably one of the most diverse and ubiquitous decapod taxa with over 7250 species in 93 families [9, 10] These crabs can be found in marine, estuarine, freshwater, and terrestrial environments, from mountainous landscapes to the deep-sea floor [9]. The terminal portion of the midgut tract continues with the hindgut tract, which is involved in the excretion of solid residuals and osmoregulation
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