Observations of gonads and oocyte development stages (OS) have been achieved in Calanus helgolandicus females fed different algal diets and starved in filtered sea water under laboratory conditions during 8 days. The effects of 20 diets on egg production rates (EPR), hatching success (HS) and proportion of abnormal larvae (AL, development stages N1-2) were examined. With the control diet Prorocentrum minimum EPR and HS values were high, while AL was very low, coinciding with intact cell structures in oogonia (OO) and normal OS (OS1-OS4). With the other diets, oocyte maturation, EPR, HS and AL patterns were partially or totally impaired. Decrease of EPR coincided with the arrest of OS3 maturation and oocyte degradations, characterised by cell fragmentation, presence of apoptotic bodies in the OS3, degradation of cytoplasm in OS3 and OS4 and consequently the arrest of OS4 formation. These degradations were reversible when females were fed a favourable diet. Results reflect the presence of two distinct inhibitory mechanisms. Inhibition mechanism (1) impairs egg production. It was induced by starvation or by several species belonging to Bacillariophyceae (Chaetoceros calcitrans, Guinardia delicatula, Guinardia striata, Odontella regia, Rhizosolenia setigera, Stephanopyxis turris, Thalassiosira pseudonana) and mixed-diatom assemblages (collected in the field) and to the prymnesiophyte Pavlova lutherii. Remarkably other diatoms like Navicula sp., Nitzschia sp., Skeletonema costatum and Thalassiosira rotula did not induce mechanism (1) Inhibitory mechanism (2) affected exclusively HS and AL and was triggered by species independent of the production of polyunsaturated aldehydes (PUA), which are supposed to have adverse impacts on HS and larval development. |
