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We report here on the seasonal changes in ultramicroscopic morphology of the eggs of the calanoid copepod Temora longicornis. During a field study between 1996 and 1998 in the Menai Strait, eastern Irish Sea, T. longicornis produced two types of eggs; hatching or subitaneous eggs were spawned during autumn–winter, whereas a high proportion of non-hatching eggs (up to ∼8%) were found during spring–summer. Hatching and non-hatching eggs looked very similar under the light microscope, both being round in shape and with no apparent external ornamentation or spines. However, analysis under the scanning electron microscope showed clear differences in external morphology and in the thickness of the chorion of the two egg types. The surface of the subitaneous egg was smooth overall with circular pits and the egg chorion was thin (∼1 μm). The surface of the non-hatching egg was ornamented with rough ridges and the chorion was thick (∼5 μm). The diameter and the surface of the eggs spawned by T. longicornis were also different from those of Acartia clausi and Centropages hamatus, two calanoid species commonly found in the Menai Strait. It is concluded that the non-hatching eggs produced by T. longicornis are resting egg stages. The massive production of resting eggs by T. longicornis during peak reproduction may have profound implications for the population dynamics of this copepod species and the ecology of the area.

Seasonal variation in egg morphology and hatching success in the calanoid copepods Temora longicornis, Acartia clausi and Centropages hamatus