The mandibular organ of the female spider crab,Libinia emarginata, in immature, mature, and ovigerous crabs

Secretory activity of mandibular organ fluctuates in response to reproductive season of the field crab Paratelphusa sp. (Brachyura; Decapoda): an ultrastructural study

Background

Mandibular organ (MO) in decapods is suggested to play regulatory role in reproduction, in few species; however, MO is considered to control growth. The present study addresses this question by an ultrastructural study on the MO of the field crab, Paratelphusa sp. Our sampling for consecutive years (2008 to 2012) revealed that Paratelphusa sp. devotes July to October for reproduction, judged by the occurrence of growing ovaries and the berried females. From November to the succeeding June, the females are in a state of reproductive arrest (non-reproductive period); ovaries during this season would appear as white bands with no signs of yolk deposition.

Results

Morphologically, MO of Paratelphusa sp. is positioned posterior to the mandibles and is in close apposition with the distal end of the mandibular apodeme. MO of Paratelphusa sp. exhibited significant levels (t = 8.097, P < 0.0001, N = 10) of season-dependent size variations. Our electron microscopic observations reveal that the MO is highly secretory during the reproductive period, evidenced by the occurrence of sacculated Golgi bodies having dense inclusions, several mitochondria with tubular cristae, and extensive networks of SER and rough endoplasmic reticulum (RER). During the non-reproductive period, however, the MO is least active; RER, the mitochondria, and the Golgi are only sparsely seen. Interestingly, the plasma membrane exhibits a highly convoluted appearance all the way through the non-reproductive period.

Conclusions

The present study reveals that the secretory activity of MO of Paratelphusa sp. is entrained with reproductive activity. The existence of a high correlation between MO secretory activity and ovarian growth implicates the former’s role in reproduction.

Identification of a juvenile hormone-like compound in a crustacean

Juvenile hormone (JH) has central roles in the regulation of insect development and reproduction but has not previously been identified in other arthropod classes. The hemolymph of a crustacean, Libinia emarginata (Leach), has now been analyzed for JH-like compounds. Samples contained 0.003 to 0.030 nanogram of JH III per milliliter and 10 to 50 nanograms of methyl farnesoate per milliliter; methyl farnesoate is a compound structurally related to JH III that has JH bioactivity. Several tissues were examined for synthesis and secretion of JH-like compounds. Of these tissues, only the mandibular organs produced and secreted JH III and methyl farnesoate. However, microchemical analysis revealed that this JH III was racemic, and thus likely an artifactual oxidation product of methyl farnesoate. Secretion of methyl farnesoate was related to reproduction in females, with the highest rates observed in Libinia near the end of the ovarian cycle when oocyte growth and vitellogenesis are greatest. These results indicate that JH-like compounds such as methyl farnesoate have regulatory roles in crustaceans.