Major conditions affecting the distribution of littoral gastropods
Sandison, Eyvor E.
(1) The rates of oxygen consumption of Thais lnpillur. hittorina littorea, L. obtueata. h, saxatilis and possibly of neritoides in air, and when immersed in water, give a good indication of their reactions to various physical and chemical conditions which may be imposed experimentally, and which are designed to simulate conditions which these species may encounter on the shore.(2) Gastropods from higher tidal levels tend to show a progressive ability to utilise atmospheric oxygen, but the rates of respiration of each species either in air or in water display great variability. Some of these respiratory luctuatione may be attributable to the external influences of the time of day, and the effect of tidal rhythm, to which the animal reacts.(3) Factors of primary importance in determining the vertical distribution of the five species of intertidal gastropod under investigation in this thesis, are considered to be the combined effects of the temperature of asphyxiation in air, together with the ability of the animal to withstand desiccation. Above the air asphyxiating temperature an individual becomes immobilised, the operculum remains ope,±, and if desiccation cannot be prevented, the animal may die owing to the latter cause. There is a direct correlation between these two effects, and the vertical distribution of Thais and the hittorina spp. on the shore; obtusata is particularly susceptible tb desiccation, and this further confines the species to a moist, weedy habitat.(4) The effects of the teiaperature of asphyxiation in water and the temperature which is lethal to a species, are of secondary importance. An animal immersed in water will become immobilised above the temperature of asphyxiation, but it is not exposed to any other harmful factor, and when the temperature falls below this critical point the animal can resume normal activity. The lethal temperature is too high to be of practical account to an animal under anything but abnormal conditions.(5) There is some evidence that the rate of water loss is correlated with the tidal level inhabited by a species on the shore, but the behaviour of the species on being subjected to conditions of desiccation is a factor which is closely linked with water loss. On exposure to dessication, the death of an animal is thought to be caused by suffocation due to mucus drying on the respiratory surfaces, rather than by the actual loss of water in the soft tissues.(6) From its respiratory behaviour, oaxatilis indicates that it is less affected by salinity changes than are the other species. But resistance to variations in salinity is considered to be of secondary importance as a factor determining the vertical distribution of these littoral gastropods.(7) Amongst the biological factors, the reproductive period and the type of larva produced are most likely to be effective in determining the distribution of species; for example in the special distribution of L. neritoidee. The availability of food, and type of diet may have a certain influence on habitat, and in special cases, e.g. T, laplllue, on tidal level. Tropisms and shape may influence habitat, but are not considered to be effective in determining the vertical distribution of a species on the shore.