Some factors affecting the growth of prosobranch veligers

Extrait

Le vélum des véligères de Prosobranches recueille toutes les particules que ses cils composés peuvent maitriser. Chacune des dix espèces d'algues unicellulaires offertes aux larves deCrepidula fornicata et deNassarius reticulatus subit l'action mécanique de l'estomac. Sauf pourChlamydomonas parkeae, que les larves ne réussissent pas à détruire, et pourPhaeodactylum tricornutum dont les frustules sont perforées, les parois cellulaires sont ramollies ou fragmentées. Les cellules de la glande digestive ingèrent le contenu cellulaire y compris le pigment, mais non les débris des parois. Les pigments sont rejectés ultérieurement; ils retardent le cycle de l'activité glandulaire. La croissance est relativement bonne avec des cellules deMonochrysis lutheri et dePyramimonas grossii, qui se rompent facilement, et avecPhaeodactylum tricornutum; mais il se forme des boules fécales volumineuses qui peuvent obturer la cavité palléale; les grandes frustules dePhaeodactylum tricornutum entrainent un effet purgatif.Chlamydomonas parkeae, Brachiomonas submarina, Exuviaella pusilla etOlisthodiscus sp. sont de mauvais aliments; les deux dernières sont toxiques pour les larves, spécialementOlisthodiscus sp. Nourries deCricosphaera ap.carterae les deux espèces présentent une bonne croissance et un développement jusqu'à la métamorphose; il en est de même pourCrepidula fornicata nourrie deExuviella baltica et deNassarius reticulatus alimenté avecDunaliella primolecta. Un aliment convenable détermine un aspect particulier du comportement alimentaire. Il ne semble pas que les larves sélectionnent leur aliment lorsqu'elles reçoivent simultanément deux ou trois espèces d'algues, mais de petites cellules facilement maitrisées par le vélum sont absorbées plus fréquement que des cellules volumineuses. L'apport occasionnel d'aliments bactériens ne semble causer aucun dommage. Un aliment artificiel (par exemple la farine de blé) permet pendant 5 à 6 jours de conserver les larves et d'obtenir leur croissance. Dans toutes les expériences la température et l'éclairage, dont les fluctuations retentissent sur la croissance des larves, furent maintenus constants. La manipulation des larves se fit à l'aide de pipettes. Dans la mer les détritus organiques doivent constituer un apport alimentaire important.

Summary

1. Larvae ofCrepidula fornicata (L.) andNassarius reticulatus (L.) were used for experiments. The conditions under which they thrived proved successful for veligers of other species.

2. They were kept in glass-filtered sea water (pore size 3.0µ) in acid-clean glass containers, provided with algal foods and handled carefully by means of a pipette. Trapping larvae in a coarse filter as a means of transferring them from one vessel to another was injurious.

3. Shell length was used to estimate growth.

4. The growth of 20 veligers in 30 ml sea water (depth 8 cm) was compared with that of 333 veligers in 500 ml (depth 9 cm). Growth in the larger volume was better, but in all cases the differences were not significant at the 10% level.

5. Growth rate is influenced by light intensity. For comparing the value of different foods experiments were carried out under a constant intensity of 155 lux at the water surface. In the dark, mortality of algal cells stimulates growth of bacteria.

6. The water temperature was maintained at 12° C. Low temperatures, even 8° C, reduce the activity of veligers ofCrepidula fornicata andNassarius reticulatus; high temperatures favour bacterial growth.

7. The growth of recently hatched veligers feeding on one of 10 species of unicellular algae and on some mixtures of these was recorded for 2(C. fornicata) or 3(N. reticulatus) breeding seasons. Food was given at different concentrations (2 × 10³, 20 × 10³, 40 × 10³ cells/ml) which were calculated from haemocytometer counts of the stock cultures, though this gives only an approximate value. Experiments lasted up to 4.5 weeks and a few for a longer period.

8.Cricosphaera ap.carterae andExuviaella baltica were the best foods forCrepidula fornicata, especially at higher concentrations, and larvae were ready to metamorphose in 40 days or less.C. ap.carterae andDunaliella primolecta were good forNassarius reticulatus, especially the former, andE. baltica consistently poorer.

9.Monochrysis lutheri andPyramimonas grossii were moderately good foods, but with these none of the larvae metamorphosed. When fed toCrepidula fornicata at high concentrations (80 × 10³, 120 × 10³ cells/ml) growth of the former approached that withCricosphaera ap.carterae whilst with the latter growth decreased with increased concentration. The food value ofPhaeodactylum tricornutum is lower; the large frustules irritate the gut and act as a purgative.

10.Chlamydomonas parkeae, Brachiomonas submarina, Exuviaella pusilla andOlisthodiscus sp. are poor foods; the last two are toxic.

11. When the food was 2 species of alga mixed in equal proportions the good value ofCricosphaera ap.carterae was still evident. Examination of the stomach contents showed that the larvae were not feeding selectively on this alga. When the second alga was one with smaller cells(Monochrysis lutheri orPyramimonas grossii) these were ingested in greater numbers in accordance with the ease with which the velar cilia manipulated them.

12. Algal cells are subjected to mechanical treatment in the stomach; their walls may be shattered but no fragment has been seen in the cells of the digestive gland. The resistance of cells with complete cellulose walls suggests the absence of a cellulase.

13. With some algal foods (species ofMonochrysis, Pyramimonas, Phaeodactylum) the walls produce a high proportion of faecal waste which, in less vigorous larvae, may clog the exhalant passage of the mantle cavity.

14. Plant pigments are egested by the veliger. They are intimately linked with food, taken into ingesting cells of the digestive gland and later excreted. They thus delay the cycle of events in the gland.

15. Differences in food value of the various algae may be due to differences in micronutrients and vitamins essential to growth. The fact that one species of alga may produce good growth in one species of veliger and not another must reflect either differences in requirements or in assimilation of the food.

16. Prosobranch veligers are found in numbers well below the compensation depth as well as in other areas where the density of algal cells is low. It is suggested that organic detritus in the sea is an important item of food.