Figure 1.4. (a) Schematic diagram of the major endocrine organs in P. monodon. The sinus gland is composed of the terminals from neurons which have their cell bodies in the X-organ and brain. (b) Electron microscopy section of the sinus gland demonstrating hormone filled vesicles (dark circles) which fuse and release their contents into the blood. Magnified 8500x.

Eyestalk ablation results in the excision of one of the paired X-organ-sinus gland complexes (fig 1.4a) that are the source of reproductive inhibitory hormones. The sinus gland (fig 1.4b) contains vesicles filled with hormones which are released into a blood sinus and hence into the general circulation where the reproductive inhibitory substance prevents spawning.