The diel migrations and distributions within a Mesopelagic community in the North East Atlantic. 2. Vertical migrations and feeding of Mysids and decapod crustacea

Abstract

1. (i) Vertical migration and feeding patterns are analysed for the mysid Eucopia unguiculata and the decapods Acanthephyra purpurea, A. pelagica, Systellaspis debilis, Gennadas elegans, G. valens, Sergestes arcticus, Sergia ? bisulcatus and S. japonicus. The vertical distribution of juvenile Pasiphaea multidentata is also described. 2. (ii) The analysis is based upon samples taken at four different depths, 600, 450, 250 and 100 m, each of which was repeatedly fished for a period of 48 hr. 3. (iii) It is concluded that net feeding was unimportant in the one hour tows. 4. (iv) At least part of the adult population of E. unguiculata had a diel migration on two successive nights. It is not clear whether this migration involved all or part of the population. Some juveniles also probably migrated but the amplitude of their vertical movements was very low and probably involved only a small part of their population. 5. (v) E. unguiculata had a seasonal vertical migration at 44°N 13°W. 6. (vi) Adult E. unguiculata fed continuously throughout their diel cycles. They apparently fed more often by night than by day and more intensively at the shallow end of their migration route. 7. (vii) Much of their prey was fragmentary, but recognisable food consisted mainly of small copepods and coelenterates. 8. (viii) All the species of decapods carried out vertical migrations. 9. (ix) The migrations of the three most abundant populations are analysed in detail. Adult S. debilis migrated as a compact population, whereas juvenile S. debilis, spread up through the water column at dusk. G. elegans migrated in a similar way to juvenile Systellaspis. 10. (x) The migrations of all three abundant populations were remarkably repetitious; those of Systellaspis remained regular over at least fourteen days and those of Gennadas over nine days. This great repeatability gives considerable support to the validity of considering each individual sample set in relation to the others. 11. (xi) Because of this repeatability, migration rates can be calculated for the abundant populations. 12. (xii) The migrations of Systellaspis were continuous throughout the day and possibly also during the night. The migration cycle of Gennadas was incompletely described since much of its population migrated down to depths below 600 m. 13. (xiii) Possible correlations between the different migration patterns and light are discussed. Probably none of the three abundant populations maintain themselves within a constant light regime throughout a diel cycle although individual adult S. debilis may be able to do so. 14. (xiv) All the decapods in which the diet was analysed fed continuously throughout the diel cycle, and most of them ate different prey at opposite ends of their migration routes. 15. (xv) Large individuals of S. debilis ate more than small. 16. (xvi) The residence time of food in the foregut of S. debilis is discussed. 17. (xvii) The diet of G. elegans included a high proportion of green detritus which probably originated from Radiolaria. 18. (xviii) Thin layer chromatography has indicated the presence of chlorophyll or its breakdown product phaeophytin in the foreguts of G. elegans. 19. (xix) Apart from G. elegans, and possible selection according to size, no evidence of prey selectivity was found. The decapods apparently ate whatever was most abundant at the time and depth. 20. (xx) Different migration patterns and/or different diets separate potentially competing species of decapod. These differences, combined with one of size, make it unlikely that Eucopia competed with any of the decapods for food.