Lisheng Wu

An approach to the study of copepod egg banks based on efficient DNA extraction from individual copepod eggs

Abstract A method of extracting DNA from individual copepod eggs was introduced for this study which included a modified proteinase K procedure and an efficient DNA sedimentation process. DNA was extracted from egg samples including freshly spawned eggs from the three copepod species Apocyclops borneoensis, Centropages tenuiremis and Calanus sinicus, together with 18 resting eggs separated from different sediment layers. A short fragment of the 28S rDNA (~300 bp) sequence that varied between copepod species was amplified and sequenced. These sequences were used to construct a UPGMA tree which helped to assess species composition and the distribution of copepods buried in the sediments. The results showed that C. tenuiremis and Acartia pacifica were closely clustered in the tree with egg samples from deeper sediment layers, whereas A. borneoensis and C. sinicus were grouped with surface egg samples. Species composition in the sediments varied between sediment layers and sampling locations. The DNA extraction method was valid for analysing individual copepod eggs with different egg-spawning types and sizes and the results helped us to reconstruct the copepod egg composition and distribution in the sediments. We believe that the technique has a wide usage in analysing the copepod egg bank in sediments, and possibly even for other zooplankton.

Seasonal variation in diel production pattern of Centropages tenuiremis in Xiamen waters, China

The seasonal variations in diel production pattern, and egg hatching time of the copepod Centropages tenuiremis in Xiamen waters, China, were investigated between January and June of 2003. The results show that in the winter–spring (January to early May) the adult females tended to lay subitaneous eggs at night, while in the beginning of summer (late May–June) they tended to lay diapause eggs as well as subitaneous eggs during the daytime. The egg hatching time negatively correlated with water temperature. These results, combined with the fact that C. tenuiremis migrates vertically in Xiamen waters, may explain the reproductive strategy of this dominant species in winter–spring.

Diel variations in Centropages tenuiremis (Copepoda) feeding, spawning and its relationship with temperature

Diel rhythms in feeding and spawning were investigated in Centropages tenuiremis from Xiamen Bay in March to May, 2006. Circular statistics were used to determine the peak time of spawning. The results showed that the feeding activities of females were stably higher at night-time, and there was a remarkable earlier shift in spawning peak time with warmer seawater. Thus, the lag times between peak times of gut pigment content and spawning were shortened with the increase of temperature. It suggested that there was a direct effect of feeding rhythms on egg production variations in copepods, and the seawater temperature would work on the converting time and then influence the spawning peak time. So the effect of temperature cannot be ignored in the investigation of the effects of feeding on egg production.

Comparative study on the effects of two diatoms as diets on planktonic calanoid and benthic harpacticoid copepods

Two diatoms, Chaetoceros muelleri and Nitzschia closterium f. minutissima, were used for rearing nauplii to adults of Tigriopus japonicus, Acartia pacifica, and Pseudodiaptomus annandalei at different concentrations. The survival rates to adults and activities of superoxide dismutase (SOD) and glutathione-S-transferase (GST) of the adult copepods were measured. The results showed that when fed on N. closterium f. minutissima, A. pacifica was unable to complete naupliar development; while SOD and GST activities of P. annandalei fed N. closterium f. minutissima were significantly higher than those fed the control algae Isochrysis galbana (Prymnesiod), suggesting this diatom species is harmful to these calanoid copepods. The survival rates of T. japonicus were very high when fed N. closterium f. minutissima, indicating that T. japonicus has high acceptance to diatoms. To evaluate whether calanoid copepod had the capacity to acclimatize to diatoms, P. annandalei was cultured for four generations on the two diatoms at 1.7 μgC/ml and survival rates as well as SOD and GST activities were determined for each generation. It was shown that starting at the second generation, P. annandalei exhibited adaption to the diatoms with improved survival. When fed on N. closterium f. minutissima, both SOD and GST activities of P. annandalei showed a trend of decreasing with the progress of generation, whereas no significant difference was found among four generations on C. muelleri. This study demonstrates that whether a diatom species is considered harmful to copepods, which is largely species specific, and diatom concentration is an important influencing factor. Additionally, the capacity of copepod adaption to diatoms over time should not be neglected.

Effects of microalgae as diets on the survival, development and fecundity of a pelagic cyclopoid copepod Apocyclops borneoensis

It has been proposed that the feeding habit of cyclopoids is different from that of calanoid copepods in that they feed mainly on microalgae during early development but become carnivorous later. However, a different view also exists, believing that microalgae are the prime food for some cyclopoid copepods. In the present study, microalgae from various taxonomic groups, including a dinoflagellate (Prorocentrum micans), three diatoms (Chaetoceros muelleri, Skeletonema costatum and Nitzschia closterium f. minutissima), and a prymnesiod (Isochrysis galbana), were offered at different concentrations to the cyclopoid copepod, Apocyclops borneoensis, with survival, development and reproduction of the copepod closely monitored. The results showed that A. borneoensis is capable of utilizing any of the microalgae species tested for development and reproduction, but significant differences in survival, development rates of both nauplii and copepodites, and fecundity were detected among species. The results also showed that within a same algal species, food concentration also significantly affected various biological parameters measured. Overall, C. muelleri and I. galbana were the better diets for A. borneoensis and their optimal food concentration ranged from 8.50 to 17.00 mg C ml21. The optimal food concentration of P. micans was also found to be 8.50–17.00 mg C ml21, however for the other two algae, S. costatum and N. closterium f. minutissima, it was lower at 1.70–8.50 mg Cml21. The present study provides novel information on the feeding habit of A. borneoensis and the effects of both quality and quantity of microalgae diets on a range of biological parameters are described.