Freßaktivitäten der Veliger vonNassarius reticulatus undCrepidula fornicata und die Verwendung von Kunstfutter zur Aufrechterhaltung von Kulturen dieser Larven

Abstract

1. Veligers ofCrepidula fornicata (L.) andNassarius reticulatus (L.) were fed on the algaeCricosphaera ap.carterae, Dunaliella primolecta andExuviaella baltica (each at a concentration of 20 × 103 cells/ml) in suspension in columns of seawater (height 115 cm), and their movements observed over a 5 to 8 h period. 2.Nassarius reticulatus larvae were also fed on three artificial foods (cornflour, drinking chocolate and Liquifry — consisting mainly of pulverised diatoms) which settle to form a layer on a substratum and their growth was measured over a 12–14 day period. 3. The time taken from the first mouthful to the first defecation was recorded for larvae feeding on all six foods. 4. There are two patterns of swimming activity associated with feeding on algae in suspension dependent upon the food value of the alga to the particular species of veliger. 5. With a good food(Cricosphaera ap.carterae andDunaliella primolecta forNassarius reticulatus, Exuviaella baltica forCrepidula fornicata) all larvae congregate at the surface to feed, dropping intermittently 26 to 34 cm while digestion continues. The first defecation occurs about 30 min after feeding begins. 6. With a poor food(Exuviaella baltica forNassarius reticulatus) and without food, larvae are distributed more evenly throughout the water column and make random swimming movements. 7. When no food is present in the column two rising rates are evident 20–30 sec/cm and 7–8 sec/cm, and one sinking rate 20–34 sec/cm. This also holds in the presence of bad and good foods except inCricosphaera ap.carterae in which larvae rose more slowly (40 sec/cm). 8. The treatment of the six foods in the gut differs: when the stomach is full, feeding more or less ceases in case of algal foods and faeces are formed after 30 min; feeding is more continuous with artifical foods only some particles being retained for digestion and faeces are produced after 10–15 min. 9. Some growth ofNassarius reticulatus occurs in artificial foods, although it is less than inCricosphaera ap.carterae, and ceases after 8–10 days. Growth over the first seven days is most vigorous in cornflour, good in Liquifry but poor in drinking chocolate owing to bacterial contamination. Growth resumed when larvae fed on Liquifry were givenCricosphaera ap.carterae indicating that they remained healthy. 10. The concentration of cornflour and Liquifry is a critical factor. At 2 × 103 particles/ml growth was poor, probably as a result of starvation, and a concentration of 20 × 103 particles/ml or higher is recommended. 11. The food value of an artificial food affects growth. Cornflour and drinking chocolate have a very high carbohydrate content giving an unbalanced diet. Liquifry has a more balanced food content but it contains large amounts of indigestible material which must reduce the food value. 12. Artifical foods can be used to maintain cultures of larvae over a short period, but not for long term growth as they lack essential micronutrients present in natural algal foods.