BLOOD-GLUCOSE LEVELS IN THE SEA HARE APLYSIA-DACTYLOMELA - INTERRELATIONSHIPS OF ACTIVITY, DIET CHOICE AND FOOD QUALITY

Blood-glucose levels were investigated in the nocturnal sea hare Aplysia dactylomela Rang in the field and laboratory, and related to behaviour, quality and amount of seaweed food eaten, and feeding preferences. General predictions were that increased blood-glucose levels would correlate with activity and digestion of food, and that foods selected by animals at dusk (the start of the daily 12-h active period) would be light in weight, easily digestible, and give a sharp increase in blood-glucose level. Two daily peaks in blood-glucose concentration were exhibited, one at 1100-1300 resulting from food digestion, the other at 1900-2100 representing energy provision for the day's greatest period of activity. Force-feeding experiments using seaweeds and non- or only poorly digestible "bulk" materials (agar and cellulose), and several control treatments, showed that handling and repetitive blood withdrawal had no obvious or consistent effects on blood-glucose titre. Furthermore, extent of blood-glucose elevation varied between seaweeds, but did not accord with the tested degree of preference exhibited by the sea hares in the field or laboratory. The green seaweed Chaetomorpha linum (Muller) Kutzing, an alga not eaten in the field despite its relative abundance and one least preferred in laboratory preference tests, gave the greatest glucose "boost" per unit weight of force-fed alga. No relationship of blood-glucose level to body weight was found. Starving animals maintained a constant blood-glucose level of 25-28-mu-g.ml-1 for almost-equal-to 6 days, after which levels declined significantly. This observation, combined with the absence of weight-effect on blood-glucose level in A. dactylomela and general levels of blood-glucose of 29-34-mu-g.ml-1 found in this and three other Aplysia spp., suggests a regulatory mechanism for blood-glucose concentration in sea hares. While the prediction that rising blood-glucose levels would correlate with increasing food digestion and activity was upheld, little supporting evidence was found for the other prediction, that animals at dusk, if they fed at all, would select easily digestible foods to give a quick sharp glucose "boost" at a time when energy needs were paramount.