General Features of the External Morphology of Marine Planktonic Copepods
Egg-laying characteristics
In most marine Calanoida eggs are shed individually (e.g. Calanus), while in certain forms (e.g. Euchaeta, Paraeuchaeta) a ventral egg sac (ovisac) is present.
As a general rule, when the female genital apparatus has only one or two closely adjacent external openings, only a single ventral egg sac is formed. When the two openings are widely separated, two lateral egg sacs develop (e.g. Cyclops) or lateral-dorsal ones (e.g. Corycaeus).
Egg sacs equipped with an attachment device to the "genital segment" may either be released as a whole or release nauplii directly.
Eggs may be laid from a few hours to a few days after copulation.
Their number is highly variable among groups (from one to several hundred) and even within the same individual. In general, both for females carrying an egg sac and for those with free spawning, egg numbers are never very high (suggesting a low mortality rate at hatching). In both cases the limiting factor lies in the capacity for dilation of the oviducts under the effect of oocyte growth (Razouls, 1974, 1975). However, Marshall and Orr (1972) demonstrated experimentally that, for a given female, it is possible to obtain a succession of spawning events separated by several days, corresponding to the time needed for the filling and growth of a new series of oocytes in the oviducts. These authors analysed the role of various environmental factors on egg-laying and demonstrated the primary importance of the quantity and quality of available food and the animal's lipid reserves.
Other factors may also modulate the number and size of eggs, the interval between spawning events, and their hatching time. Among these factors may be cited:
- chromosome number (Margalef, 1953);
- cephalothorax size;
- quality and quantity of available food (Runge, 1987; Razouls et al., 1991);
- external conditions inducing diapause or dormancy.
An inverse relationship also exists between egg number and egg size (Hutchinson, 1951).
It is worth recalling that the eggs of many freshwater forms can withstand more or less prolonged desiccation, as can other developmental stages (Elmork, 1980). In Diaptomus castor (a freshwater calanoid) the eggs survive complete desiccation. Roen (1957) showed that this species can produce two types of eggs: some undergo immediate development and are enclosed in a thin egg sac, while others are contained within a thick sac capable of withstanding several months of adverse conditions. The hypothesis of diapause eggs in marine planktonic copepods is now well established since Zillioux and Gonzales (1970, 1972), followed by N.H. Marcus (1979, 1987, 1990). Eggs of Labidocera spp. (marine calanoids) survive prolonged deposition on the seabed; ingested by polychaete annelids, these eggs are not attacked by digestive fluids and are recovered intact in faeces. Johnson (1967) showed that eggs of Tortanus discaudatus (a calanoid in coastal waters) possess a hard chitinous coat resistant to bacterial degradation.
The triggers for hatching remain unknown, except that hatching appears to result from the resuspension of eggs into the overlying water column.
Grice and Gibson (1976, 1981) described the existence of two types of eggs in Pontella mediterranea (a marine calanoid) that differ in their morphology and hatching characteristics. Smooth-shelled eggs laid in summer hatch two days after spawning, whereas spiny-shelled eggs laid in autumn enter diapause and hatch only one hundred to two hundred days later.
Kasahara et al. (1974, 1975) described the eggs of various neritic species collected abundantly from sediments, which subsequently give rise to a new generation. Eggs are naturally denser than water.
Gross and Raymont (1942) calculated a sinking rate of 2 cm per minute at 13°C for the egg of Calanus finmarchicus, but hatching generally occurs rapidly after spawning, within 25 to 30 hours at 10°C.
Although the egg is generally spherical with a smooth coat, it may bear small spines, or even flexible and elongated ones. Tortanus forcipatus (a neritic calanoid) displays highly characteristic discoidal eggs.
In all cases the eggs are laden with reserves, often coloured orange-red or blue-green. These reserves appear as lipid globules, vitelline spherules containing glycogen, and carotenoid-proteins.