Experimental validation of the “relative volume of the pharyngeal lumen (RVPL)” of free-living nematodes as a biomonitoring index using sediment-associated metals and/or Diesel Fuel in microcosms

Since 1981, many numerical indices have been developed for meiobenthic nematodes but later discarded. To overcome problems and drawbacks in this type of indices, the “relative volume of the pharyngeal lumen (RVPL)” was first introduced in biological assessments in 2006 and it focused on the assumption that pollution is commonly expressed in terms of reduced food intake. This index, evaluated by using a morphometric approach, expresses the sucking potential of the pharynx. A series of microcosm experiments was performed in order to test the “relative volume of the pharyngeal lumen (RVPL)” as a biomonitoring index in the cases of 8 free-living nematode species (the deposit-feeders, Daptonema normandicum, Desmodora longiseta, and Leptonemella aphanothecae; the epistrate feeder, Xyala striata; the facultative predators, Oncholaimus campylocercoides and Oncholaimellus mediterraneus; and the predators Bathylaimus capacosus and Mesacanthion hirsutum). Each species was exposed to various concentrations of toxicants (nickel, copper, chromium and Diesel Fuel) used separately or in combinations. Our results showed that RVPL increased with increasing amount of toxicants, visibly caused by a systematic decrease in body volume with toxicants. Responses of all examined species suggested an additive toxicological action. Predators (M. hirsutum and B. capacosus) showed the highest potential of adaptation because of their higher means of RVPL. Observations during this experiment highlight the importance of pharyngeal pumping and cuticular structure to tolerating elevated catabolic water loss under stress conditions. Thus the ability to osmoregulate is an important factor in overcoming food sedimentary toxicants. Based on all these findings, RVPL was found to be an accurate and precise index of monitoring the effect of metals and Diesel Fuel in marine ecosystems.

Research Highlights
► RVPL increased with increasing amount of toxicants.
► Responses of all examined species suggested an additive toxicological action.
► Predators showed the highest means of RVPL.
► The ability to osmoregulate is an important factor in overcoming sedimentary toxicants.