Differences in the responses between tissues of the body wall and the internal organs of Phascolosoma arcuatum (Sipuncula) to changes in salinity

It has been suggested that the body wall (BW) (inclusive of the introvert cum contractor muscles) and the internal organs (IO) of Phascolosoma arcuatum have developed different responses to cope with salinity stress. Increased proteolysis to release free amino acids (FAA) when confronted with high salinity stress was more likely to have occurred in the IO than in the BW as reflected by a lowered water-soluble protein content in the IO of worms exposed to 100% seawater (SW) compared to those of worms exposed to 30% SW. Some of the FAA released through increased proteolysis in the IO of P. arcuatum exposed to 100% SW might undergo partial catabolism and be converted into alanine and glycine, leading to a lower level of NH₄⁺ in the IO of these worms. Glutamate formation from NH₄⁺ and α-ketoglutarate (αKG) in the IO of worms exposed to 100% SW would be enhanced by an apparent increase in the affinity of glutamate dehydrogenase (GDH) to αKG in the animating direction. It is possible that the higher total FAA content in the BW tissues of P. arcuatum exposed to 100% SW compared to that of worms exposed to 30% SW was partially due to the uptake of FAA, which were indirectly supplied by proteolysis in the IO, from the coelomic fluid.