Copepod Embryonic Dormancy: “An Egg Is Not Just an Egg”

Abstract

Long-lasting embryonic dormancy in invertebrates defies our understanding of what constitutes life because, for example, eggs of some copepods can delay hatching for decades or even centuries. Copepods, often millimeter-sized crustaceans, are some of the most numerous multicellular organisms on earth and are key organisms in most aquatic food webs. Some important free-living marine and estuarine species overwinter or oversummer by arrested embryogenesis in dormancy. The present contribution discusses the complex mechanisms behind embryonic dormancy by compiling knowledge from the 42 calanoid copepods from the superfamily Centropagoidea with well-described embryonic dormancy, which has been of scientific interest for decades. However, the determination of categories of copepod resting eggs-that is, diapause and quiescence, transitions between categories, the mechanisms controlling arrested development by the embryos, and how they interact with their surroundings-is not fully understood. Moreover, a clear link between the presence of the free-swimming population and their resting eggs in sediments is still not convincingly demonstrated. Here I evaluate the relative significance of potential cues driving the production of and the phase shift between egg categories. Understanding the initiation and termination of embryonic dormancy is of great importance for fundamental science-that is, population and food web ecology as well as climate science, aquaculture live feed, and ballast water research. Molecular techniques are developing rapidly, especially within health sciences, thus providing relevant tools applicable for plankton research. Here I suggest that applying molecular methods in addition to traditional physiological approaches in future research will lead to greater understanding of copepod embryonic dormancy, one of nature's wonders.

Thermotolerance experiments on active and desiccated states of Ramazzottius varieornatus emphasize that tardigrades are sensitive to high temperatures

“…Various groups of small, planktonic crustaceans such as Copepoda possess at least one dormant form during their ontogenesis, which could be the egg, a larval stage or even the adults (reviewed in 62 ). It should be noted that dormancy as found among copepods is more similar to diapause and encystement in tardigrades than to cryptobiosis, while other organisms (such as bdelloid Rotifera) share tardigrades' ability to enter anhydrobiosis at any ontogenetic stage 13,63,64 .…”

Possible metazoan egg fossils from the Darriwilian (Middle Ordovician) of Baltoscandia

“…However, arthropods include other plausible candidates for the parent organism of Vikisphaera. For instance, copepod crustaceans produce diapause eggs in the same size range as Vikisphaera, sometimes with a thick chitinous casing to ensure surviving desiccation and passage through the guts of predators (Hairston et al 1995;Winding Hansen 2019). The fossil record of copepods is extremely scarce due to their low fossilization potential, but molecular data suggest the emergence of copepods in the Cambrian and separation of major lineages at ca 460 Ma ago (Selden et al 2010).…”

Persistent effects of historical sea levels on the population structure of a temporary wetland copepod

“…They inhabit various marine, brackish and freshwater environments (Boxshall & Defaye, 2008), and form integral food‐web conduits between primary producers and higher trophic levels. Most copepod species occur in permanent waterbodies and have not developed the ability to produce propagules that can withstand desiccation (Hansen, 2019), but members of the subfamily Paradiaptominae are freshwater specialists that produce large quantities of desiccation‐resistant propagules under dry environmental phases in their life cycle (Suárez‐Morales et al, 2015). In this way, these copepods have evolved similar adaptations to those of large branchiopod crustaceans (Brendonck et al, 2008), and it has been suggested that mechanisms which govern dispersal and demographic dynamics in both groups are likely to be similar (Brendonck et al, 2017; Vanschoenwinkel et al, 2008).…”