Quantitative and experimental studies of the interstitial fauna in four Swedish sandy beaches

Abstract

The distribution of the interstitial metazoans was studied quantitatively in four Swedish beaches, three in the Baltic and one in the Skagerrak. As many as 1.5 × 10⁶ individuals per m2 were found (Askö beach, south of Stockholm). Temperature, salinity, oxygen concentration and availability, grain size, water contents and organic material were measured in connection with the sampling. The oligochaetes, turbellarians and harpacticoids showed different degrees of submergence landwards in the beaches with the oligochaetes near the sand surface, followed by the turbellarians and - deepest down - the harpacticoids. This may be explained by a difference in dependence on water-saturation: the oligochaetes are more or less terrestrial, and the turbellarians can move in a thinner water film than can the harpacticoids. A correlation between the distribution of oxygen and certain species was found in several cases. The microbial film on the sand grains is pointed out as an important food-source and the utilization of dissolved (or precipitated) organic material in the interstitial water is discussed. Of the other abiotic factors temperature and salinity had the greatest influence on the major part of the fauna. Tolerance and preference experiments with these parameters were carried out with the most common species in the beaches, 2 turbellarians, 4 oligochaetes and 6 harpacticoids. Tolerance to high temperature was tested at 30° C. Most tolerant was the harpacticoid Parastenocaris phyllura with an LD 50 of 5–6 days, while the turbellarian Haplovejdovskya subterranea was very sensitive (LD 50 = 16 h). These animals also inhabit the two margins of the beach - the former the backshore and the latter the water's edge. Winter samples of totally frozen sand contained live specimens of several species, e. g. Coronhelmis lutheri, the gastrotrichs Turbanella hyalina and T. cornuta, many nematodes, the oligochaetes Marionina southerni and Aktedrilus monospermatecus, Parastenocaris phyllura, and the halacarid Halacarellus capuzinus. The body fluids of these animals may not have been frozen as the temperature in the frozen layers seldom falls more than a few degrees below zero. According to their salinity tolerances the experimental animals were divided into three classes. The first, most euryhaline, class comprises the turbellarian Coronhelmis lutheri and the harpacticoids Schizopera baltica and Nitochra fallaciosa f baltica. The second class is represented by the oligochaetes Marionina southerni and Aktedrilus monospermatecus and the harpacticoid Nitochra fallaciosa. The most stenohaline class comprises Haplovejdovskya subterranea, Marionina preclitellochaeta and the harpacticoids Paraleptastacus spinicauda, Parastenocaris vicesima and P. phyllura. The salinity tolerance tests clearly showed the importance of mucus secretion as a protective mechanism in the first phase of adjusting to a higher salinity. The optimum zones found through the preference tests with the different species generally appeared to agree well with the field distributions.