Desiccation Tolerance of the Barnacle Semibalanus Balanoides (L.) in Relation to Shore Height (Cirripedia, Thoracica)

DESICCATION TOLERANCE OF THE BARNACLE SEMIBALANUS BALANOIDES (L.) IN RELATION TO SHORE HEIGHT (CIRRIPEDIA, THORACICA)

FIONA J. WARE and RICHARD G. HARTNOLL Port Erin Marine Laboratory, University of Liverpool, Port Erin, Isle of Man IM9 6JA, British Isles

ABSTRACT

The desiccation tolerance of the barnacle Semibalanus balanoides was investigated in the laboratory. Barnacles were collected from three levels (approximately MLWN, MTL and MHWN) on a shore in the Isle of Man, and the median lethal time under conditions of 20°C and 15% RH determined. For barnacles of the same opercular diameter the median lethal time was significantly longer at higher shore levels. Compared with low-shore barnacles the median lethal time was approximately 40% longer for mid-shore and 70% longer for high-shore barnacles. The relationship between water content and opercular diameter was examined, and barnacles from the three shore levels showed no differences. Therefore it is thought probable that the difference in desiccation tolerance is due either to differences in behaviour affecting rate of water loss, or differences in tolerance of water loss.

RÉSUMÉ La tolérance à la dessication de la balane Semibalanus balanoides a été étudiée en laboratoire. Les balanes ont été recueilles à trois niveaux (approximativement B.M.M.M.E., N.M. et P.M.M.E.) sur une côte de l'île de Man, et le temps léthal a été déterminé à 20°C et à 15% HR. Pour les balanes à même diamètre operculaire le temps léthal moyen a été significativement plus long aux niveaux côtiers les plus élevés. Comparé avec celui des individus aux niveaux côtiers inférieurs, le temps léthal moyen a été plus long d'environ 40% que pour les balanes à niveau moyen et de 70% que pour celles aux niveaux supérieurs. La relation entre le contenu en eau et le diamètre operculaire a été examiné, et les balanes aux trois niveaux côtiers n'ont pas montré de différences à cet égard. Par conséquent, on pense que la différence dans la tolérance à la dessication est due, soit à des différences de comportement affectant le taux de perte d'eau, soit à des différences dans la tolérance à le perte d'eau.

INTRODUCTION .

For intertidal organisms desiccation is a major stress, especially for species which occupy the higher levels on the shore. The ability to withstand desiccation 322 is an important factor in enabling species to extend up the shore, and a number of studies have demonstrated that species of barnacles which extend to higher levels possess increased capacity to resist desiccation. Bames et al. (1963) showed that the essentially subtidal Balanus crenatus Bruguière, 1789, had poor resistance, the intertidal Semibalanus balanoides (L., 1767) was more resistant, and the high shore Chthamalus stellatus (Poli, 1791 ) most resistant of the three. Similar trends, linked to the tidal level occupied, have been reported by Newman (1967) for three American species, and by Foster (1971) for four British barnacles. Foster (1971) demonstrated that this improved resistance to desiccation does not result from an increased tolerance of water loss, since the four species he studied died after losing much the same proportion of their water. It is rather that high-shore species can better control the rate at which they lose water during emersion. In intertidal species the opercular valves are normally closed out of water, leaving only a small median micropylar opening between the adpressed mantle folds, whereas subtidal species allow the opercular valves to gape (Bames & Bames, 1957; Bames et al., 1963). This micropyle is not open permanently, and opens more frequently under higher relative humidity both on the shore and in the laboratory (Grainger & Newell, 1965). Since most of the water loss occurs via the opercular opening, rather than through the shell itself (Achituv & Borut, 1975), there is considerable scope for controlling water loss through appropriate behaviour patterns. Thus the difference in desiccation resistance between species occupying different shore levels has been well documented. However, there is a further pos- sibility, that individual species which occupy wide vertical ranges within the intertidal have different desiccation resistance at different levels on the shore. There are precedents for this concept. Davies (1969) found in the limpet Patella vulgata L., 1758, that populations from the low and high shore differed, in that latter lost water more slowly, and tolerated a greater percentage water loss before dying. The differences in rate of water loss could be attributed, in part at least, to differences in shell shape. However, the difference in tolerance of water loss must be a physiological adaptation. The only intraspecific study in barnacles is of Tetraclita squamosa rufotincta Pilsbry, 1916 (cf. Achituv & Borut, 1975). They determined that the evaporative water loss was proportional to the area of the opercular opening, and that the relative size of this opening decreased with height on the shore. However, they did not compare the desiccation tolerance of barnacles from different shore levels. A promising subject for further studies on intraspecific differences in desiccation resistance in barnacles is Semibalanus balanoides. This is a common intertidal barnacle on the shores of north-western Europe, with a wide vertical