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

Abstract

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).

Results: 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.

Keywords: Anterior midgut caeca; Endoderm germ layer; Larvae; Macroapocrine secretion; Microapocrine secretion; Posterior midgut caecum.

Fig. 1

Maja brachydactyla. Gross morphology and development of the midgut caeca. Anterior caeca (A–B): newly hatched zoea I (A) and juvenile 12 days post-hatching (B). Posterior caecum (C–D): newly hatched zoea I (C) and juvenile 12 days post-hatching (D). Isolated midgut tract and associated caeca, newly hatched zoea I (E). Midgut caeca, adult (F–G): anterior caeca (F), and posterior caecum (G). Isolated midgut tract and associated caeca, adult (H). Midgut tract and associated caeca, megalopa 6 days post-hatching, micro-CT rendered image reconstructions, dorsal section (I–J). Anterior caeca, megalopa 6 days post-hatching, micro-CT rendered image reconstruction, transversal section (K). Length growth of the midgut caeca from hatching to the first juvenile stage (L–N): right anterior caecum (L), left anterior caecum (M), and posterior caecum (N). Scale bars (μm): 25 (A–D), 50 (E), 100 (K), 250 (I–J); (mm): 2 (F–G), 10 (H). Abbreviations: AC, anterior caeca; arrow, midgut-hindgut junction; arrowhead, junction between the posterior midgut caecum and the midgut tract; asterisk, stomach-midgut junction; CS, cardiac stomach; HGT, hindgut tract; MGG, midgut gland; MGT, midgut tract; PC, posterior caecum; PS, pyloric stomach

Fig. 2

Maja brachydactyla. Larval stages (zoea I, zoea II, and megalopa). Tissue organization of the midgut caeca. General diagram of the larval midgut caeca (A). Anterior midgut caeca (B–C): zoea II 6 days post-hatching, PAS-Alcian Blue contrasted with Haematoxylin, longitudinal cross section (B); and megalopa 6 days post-hatching, Mallory’s Trichrome, transversal cross section (C). Transition between the posterior midgut caecum and the midgut tract, Haematoxylin-Eosin, longitudinal cross section of the specimen (D–E): megalopa 6 days post–hatching (D), and newly hatched zoea I (E). Posterior midgut caecum, zoea II 6 post-hatching old, Mallory’s Trichrome, transversal cross section of the specimen (F). Anterior midgut caeca, megalopa 10 days post-hatching, TEM (G–H): general view (G), and detail of the myofibrils-like filaments (H). Posterior midgut caeca, megalopa 10 days post-hatching, TEM (I–J): general view (I), and detail of the myofibrils-like filaments (J). Scale bars (nm): 500 (H, J); (μm): 2 (I), 5 (G), 20 (B–F). Abbreviations: AC, anterior midgut caecum; arrow, midgut- hindgut junction; asterisk, midgut-stomach junction; BF, basal folds; BL, basal lamina; CE, midgut caeca epithelium; CT, connective tissue; HE, hindgut tract epithelium; ME, midgut tract epithelium; MF, muscle fibres; MGG, midgut gland; Mt, mitochondria; Mv, microvilli; My, myofibrils-like filaments; RER, rough endoplasmic reticulum; SE, stomach epithelium; PC, posterior midgut caecum

Fig. 3

Maja brachydactyla. Adults. Tissue organization of the midgut caeca. General diagram of the adult midgut caeca (A). Anterior midgut caeca, general view, Haematoxylin-Eosin (B). Posterior midgut caecum, general view, Haematoxylin-Eosin (C). Anterior midgut caeca, detailed view of a caeca fold, PAS-Alcian Blue contrasted with Haematoxylin (D). Anterior midgut caeca, epithelial folds, SEM (E). Anterior midgut caeca, detailed view of the muscle fibres and connective, Mallory’s Trichrome (F). Posterior midgut caecum, detailed view of the muscle fibres and connective, TEM (G). Scale bars (μm): 2 (G), 20 (D, F), 50 (B–C), 100 (E). Abbreviations: arrow, cellular division; asterisk, inner layer of connective tissue; BC, basal cells; BL, basal lamina; CE, midgut caeca epithelium; CT, external layer of connective tissue; EF, epithelial folds; MF, muscle fibres; Mv; microvilli; My, myofibrils

Fig. 4

Maja brachydactyla. Megalopae 10 days post–hatching. Epithelium of the midgut caeca. TEM. General diagram (A). Anterior midgut caeca (B–C): supranuclear region (B), and infranuclear region (C). Posterior midgut caecum (D–E): supranuclear region (D), and infranuclear region (E). Detailed view of the mitochondria and Golgi bodies (F–G): anterior midgut caeca (F), and posterior midgut caecum (G). Anterior midgut caeca, detailed view of the basal folds of the epithelial cells (H). Scale bars (nm): 200 (H), 500 (F–G); (μm): 1 (B–E). Abbreviations: asterisk, cell-to-cell junction; BF, basal folds; BL, basal lamina; EV, electron-dense vesicles (cytoplasm); GB, Golgi bodies; LV, lucent vesicles (cytoplasm); MB, multivesicular body; ML, multilamellar body; Mt, mitochondria; Mv, microvilli; My, myofibrils; N, nucleus; RER, rough endoplasmic reticulum

Fig. 5

Maja brachydactyla. Adults. Epithelium of the midgut caeca: epithelial cells. Epithelial cell, general diagram (A). Anterior midgut caecum, epithelial cell (B–C): supranuclear region (B), and infranuclear region (C). Posterior midgut caeca, epithelial cell, close view of different structures (D–F): cell-to-cell junction (D), rough endoplasmic reticulum, mitochondria, and Golgi complexes (E), and smooth endoplasmic reticulum, and mitochondria (F). Scale bars (nm): 500 (F); (μm): 1 (D–E), 2 (B–C). Abbreviations: asterisk, cell-to-cell junction; BC, basal cell; BF, basal folds; BL, basal lamina; EV, electron dense-vesicles (cytoplasm); GB, Golgi bodies; Mt, mitochondria; Mv, microvilli; N, nucleus; RER, rough endoplasmic reticulum; TS, tubular structures

Fig. 6

Maja brachydactyla. Adults. Epithelium of the midgut caeca: basal cells and cell division. Basal cell, general diagram (A). Anterior midgut caecum, basal cell, general view (B). Anterior midgut caecum, basal cell, close view of the electron-dense vesicles and mitochondria (C). Anterior midgut caecum, cellular division (D). Scale bars (nm): 500 (C); (μm): 2 (B, D). Abbreviations: BF, basal folds; EV, electron dense-vesicles (cytoplasm); GB, Golgi bodies; Mt, mitochondria; N, nucleus; RER, rough endoplasmic reticulum; TS, tubular structures

Fig. 7

Maja brachydactyla. Secretory activity of the midgut caeca. Anterior midgut caeca, zoea II 6 days post-hatching, cell apex and lumen, TEM (A). Anterior midgut caeca, megalopa 10 days post-hatching, fusion between vesicles and apical cell membrane, TEM (B). Anterior midgut caeca, zoea II 6 days post-hatching, microvillus distended, TEM (C). Anterior midgut caeca, adult, epithelial fold and brush border surface, SEM (D). Posterior midgut caecum, adult, epithelial folds and brush border surface, SEM (E). Posterior midgut caecum, adult, detailed view of macro-apocrine secretion (secretory vesicles), SEM (F). Anterior midgut caeca, adult, detailed view of macro-apocrine secretion including cell apex and lumen, TEM (G). Anterior midgut caeca, adult, microvillus distended, TEM (H–I). Posterior midgut caecum, adult, microapocrine secretion (detailed view of the tiny secretion vesicles of the microvilli), SEM (J). Scale bars (nm): 500 (B–C, J); (μm): 1 (A, H–I), 2 (G), 5 (F), 20 (D–E). Abbreviations: arrowhead, vesicle-like distension of the microvillus; asterisk, cell-to-cell junction; EV, electron-dense vesicles (cytoplasm); FV, potential fusion between vesicle and apical cell membrane; HV, holes left after the release of the vesicles; LV, lucent vesicles (cytoplasm); Mv, microvilli; SV, secretory vesicles; VL, vesicle of the lumen