The Function of Spider Coxal Organs: Effects of Feeding and Salt-Loading on Porrhothele Antipodiana (Mygalomorpha: Dipluridae)

The contributions to ionic regulation of two putative excretory systems, the gutderived anal system and the coelomoduct-derived coxal organs, were investigated in Porrhothele antipodiana. Coxal fluid was produced continuously during feeding. The fluid was transported anteriorly from the openings of the coxal organs on the coxae of walking legs 1 and 3, via a cuticular groove on the ventral surface of the cephalothorax, to the pre-oral region. The rate of production 70 μl g−1 h−1) and normal composition of the fluid (Na+ 125, K+ 36mmol−1) could account for the observed loss of Na+ into the prey remains during feeding and their final enrichment with Na+ relative to K+. In normally feeding spiders, coxal fluid was hypo-osmotic to the haemolymph. Salt-loading of the prey induced compensatory changes in both systems. Elevation of the Na+ or K+ content of the prey increased anal fluid excretion, the concentration of the corresponding ion in the stercoral fluid and thus the anal output of that ion. Likewise, dietary Na+-loading increased coxal fluid production and the [Na+] of the coxal fluid. K+-loading of the prey did not induce a regulatory response by the coxal organs. Spiders normally released coxal fluid only during feeding. Spiders that had been fed Na+-loaded prey produced coxal fluid intermittently for several days after the meal. Elevation of haemolymph [Na+] by injection of Na+ into the haemocoel also induced coxal excretion in non-feeding spiders, while having little effect on the volume and composition of the anal urine. Clearance of inulin by the coxal organs was consistent with fluid production by ultrafiltration. It is concluded that in P. antipodiana both systems contribute importantly to ion and water balance and both contribute to the regulatory response to ion loading. It is postulated that the coxal organs of P. antipodiana have an additional (perhaps primary) role in the mechanics of prey ingestion. Delivery of coxal fluid to the pre-oral region may facilitate the supply of predigested prey to the sucking stomach and prevent a rise in viscosity of the food by evaporative concentration. An auxiliary role for the anal urine is as a vehicle for nitrogenous excretion.