The feeding and reproductive biology of the red whelk,Neptunea antiqua(L.) (Gastropoda, Prosobranchia)

Abstract

Abstract Neptunea antiqua occurs along the cold-temperate part of the European west coast from beyond the low water mark to at least 300 m's depth. It is common in the northern North Sea, the Skagerrak, the southern and western Kattegat, the Sound (Øresund) and the Danish Belts. The feeding biology: The proboscis of Neptunea can be everted and may be extended to 1½ times or maximally 2¼ times the length of the snail's own shell. While snails like Nassarius reticulatus and Buccinum undatum will quickly respond to the scent of crushed mussels and soon move straight towards the bait, Neptunea shows very little or no response to the same stimulus. When the prey consisted of living, undamaged mussels, Neptunea, even when hungry, was never seen to be attracted. While the marching activity of e. g. Nassarius is strongly stimulated by the smell of mussel tissue, this could never be seen for Neptunea, although Neptunea can crawl at a speed surpassing the top speed of Nassarius. When feeding on artificially opened mussels, Neptunea will consume a quantity of mussel tissue (wet weight) corresponding to maximally 6.4–6.8% and minimally 1.8% of the snail's own living weight per day, the last figure probably being the more realistic one. Neptunea never attacked living specimens of Sabella pavonina, while it consumed Ophelia, Lumbriconereis, Stylarioides and Amphitrite. On a basis of wet weight of tissue consumed Neptunea would eat living Ophelia at a rate corresponding to roughly 1%of its own living weight per day. The worm tissue passed through the digestive tract of Neptunea in about 40 to 70 hours, which must be regarded as slow. - It may be concluded that the food consumption of Neptunea is more modest than that of Buccinum and Nassarius and that Neptunea seems to be a scavenger more than a predator, although it may succesfully attack living polychaetes. No evidence was found that poison from the salivary glands was used by the snail in capturing its prey. The reproductive biology: The sexes are separate, and the ratio between males and females is approximately 50: 50. The sexual activity of the males usually starts when the shells are 5–6 cm long, sometimes earlier, while no females with a length of shell smaller than 7.9 cm have ever been seen spawning in our tanks. Out of a total of 238 specimens from the Sound, the 18 largest specimens were all females, and 7 of the 11 females which spawned in our tanks, were larger than any male of the population. The females, thus, seem to postpone spawning until they have grown up to their full size. Spawning in our tanks took place from early February to the end of April. The average time for an individual female to spawn an average-sized egg-cluster, i.e. 37 capsules, was 21 days, at a spawning rate of 1.8 capsules per day. — The males are attracted by the females even after the spawning has started, and copulation will continue during the egg-laying period of the female. More than one male will usually copulate with each female during this season. Eggclusters from deep muddy bottoms in the Skagerrak are usually deposited on the dorsal side of the shell of another ♂ or ♀ Neptunea; this must mean that both sexes aggregate to spawn and remain in narrow contact often during a month or more until a full cluster has been finished. Usually a female spawns only one cluster rarely leaving this till the last capsule has been laid. Since the volume of an egg cluster will comprise ½ to ⅔ the volume of the female producing it, and since the spawning females are fully or nearly starving during the spawning period, the outspawned females will be extremely weak and exhausted. In our tanks, 8 out 9 females died less than 3 months after the spawning was finished. — The embryonic development until hatching of the non-pelagic young takes about 5–5½ months in our tanks, and probably about half a year on the sea bottom. Thus, the main hatching period in the Sound will be from early August to the end of October. The hatching young are up to 6 mm long in the Sound and up to 12 mm long in the Skagerrak. The seven smallest specimens of a year-old group of Neptunea were 2–3 cm long.