Overview
Forms of polychaete reproduction are highly variable. Mature individuals of generally benthic species (i.e. many Nereididae) undergo morphological, physiological and behavioural modifications that enable them to leave the bottom and to swim and broadcast their gametes in the free water column (epitoky). After that they ususally die or sometimes revert to the atokous stage. In many polychaetes of the familiy Nereididae, life cycle terminates with a single phase of reproduction and the death of the individual. This so called monotelic mode of reproduction is found in other animals as well such as squid and salmon. Related to this "once in a lifetime" reproduction, the gamete biomass reached before spawning can be enormous reaching up to 30% of the body weight in polychaetes.
Other polychaetes (i.e. Myrianida) modify the posterior part of their body into a new individual with a head (stolon). After detachment they release gametes and die while the anterior benthic stock of the worm continues to live for further reproductive activity by multiple stolonisation.
Asexual reproduction is also very common. Sabellids for example are capable of forming a complete individual that then separates from the “parent” stock (paratomy). More widespread is the fragmentation of the body with no prior cephalisation (architomy). Some worms may even continue to fragment into sections until single segment results (Dodecaceria) (Beesley et al. 2000).
The typical polychaete larva is the trochophora which develops into the metatrochophora and finally into the nectochata, a segmented larva with functioning parapodia and prominent bundles of chaetae. Juveniles with parapodia usually drop out of the plankton and settle down starting a benthic life (Larink et al. 2006).
Eleocytes
Platynereis bicanaliculata from the Pacific West Coast (see right column), closely related to Platynereis dumerilii shows a light brown colour of the middle body segments due to the massive proliferation of a specialized type of free coelomic cells, the eleocytes, shown below, indicating the onset of the sexual maturation phase as the final part of the life cycle.
Eleocytes (see right column) are large (20-40 µm) coelomic cells containing numerous lipid droplets and a large vacuole which rapidly stains with the fluorescent dye Acridine orange. The red fluorescence of this pH sensitive dye indicates the acidic nature of this cellular compartment while the nucleus is stained gree-yellow by this dye. Eleocytes are mostly found in Nereidid polychaetes, but are also present in other families such as the Eunicidae, but they have not been studied in these other families.
The vacuole is a multifunctional organelle which stores essential amino acids such as tyrosine, phenylalanine in high concentrations and in the male eleocytes of some species, the vacuole contains extremely high concentrations of adenine nucleotides AMP and ADP. As our research on Nereis virens has shown, these nucleotides are degraded during the time course of sexual maturation of the animal and the purine skeleton is released in the form of inosine into the coelomic fluid. Inosine, in turn, is incorporated by the growing male germ cells and used for nucleic acid synthesis. Recently, we have found that the vacuole serves a s a sink for the degradation product of heme, biliverdin. In Nereis virens, the biliverdin is conjugated with glutathione, possibly to prevent it from diffusion out of the vacuole.
In the female, the main function of the eleocytes is the synthesis and secretion of the yolk protein vitellogenin. Vitellogenin is released into the hemolymph and incorporated by the growing oocytes as the main source of energy for the developing embryo. A second lipoprotein, a large discoidal lipoprotein present in both sexes, is likely to be synthesized by the eleocytes and also incorporated by the oocytes.