Close to 50 species of marine Calanoid copepods have been reported to produce diapause eggs (Engel and Hirche in J Plankton Res 26:1083-1093, 2004); eggs that are viable but require a refractory phase before they hatch, sometimes after months. Diapause eggs are often described as morphologically different with respect to egg membrane ultrastructure and having a thicker egg shell with surface ornamentation as opposed to the smooth shell found in subitaneous eggs that hatch within days (Belmonte in J Mar Syst 15:35-39, 1998; Chen and Marcus in Mar Biol 127:587-597, 1997; Castro-Longoria in Crustaceana 74:225-236, 2001). Egg production rates, egg surface ornamentation, and hatching success were monitored in large aquaculture fish enclosures during winter with close to zero water temperatures (N57°). Surprisingly, all female copepods (Acartia spp.-presumably A. tonsa, and Centropages hamatus) produced eggs all through the winter with no obvious pattern with respect to light, temperature and food availability, and no diapause eggs were observed. However, individual females produced several categories of eggs with or without surface spines even within the same egg batch as evidenced by scanning electron microscopy (SEM). Four egg categories were distinguishable: ‘no spines', smooth eggs; ‘short spines', 5-15 μm long; ‘truncated spines', with the spine tips cut-off <10 μm long; and ‘long spines', up to 30 μm long. All egg categories remained unchanged with respect to surface structures from when we took them out of the incubation bottles until they hatched. In general, the frequency of ‘no spines' was 10-40%, and most eggs were ornamented with ‘short-' or ‘long spines'. Further, a given egg can be ornamented with all types of surface spines simultaneously, which might even be a fifth egg category. The different egg categories were all able to hatch within days when exposed to normoxic conditions suggesting that they were subitaneous.
Hansen, B. W., Drillet, G., Møbjerg Kristensen, R., Sørensen, T. F., & Them Tøttrup, M. (2010). Production, hatching success and surface ornamentation of eggs of calanoid copepods during a winter at 57°N. Marine Biology, 157(1), 59-68. https://doi.org/10.1007/s00227-009-1295-x