Morphological and histological description of the midgut caeca in true crabs (Malacostraca: Decapoda: Brachyura): origin, development and potential role.BACKGROUND
 The 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).


 The 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.


 CONCLUSION
 The 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.
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