A mating aggregation of a Mediterranean polychete worm (Hermodice carunculata).
External fertilization is one of the early ancestral ways to produce offspring. Here an underwater explorer based in Fethiye has recorded a spawning aggregation of the Mediterranean polychete worm (Hermodice carunculata).
External fertilization is rather costly. Internal fertilization has evolved numerous times as a result of this. One of the most striking example is the Coelecanth with many specimen containing embryos. Egg production requires more resources compared to sperm. Females therefore need more time to recover after mating. Every year when the mating time comes females that are reproductively ready will always be fewer in number compared to males. Scarcity of females creates competition among males. Variation of the same theme is observable in many externally reproducing species including fish, fireworms and frogs.
In externally fertilizing species usually multiple males copulate with a few females. Thus, sperm from different males will compete to fertilize the eggs. This a phenomenon known as sperm competition. In most cases it is like a raffle: the more tickets you buy the more likely you are to win. The male who provides the larger number of sperm will have a better chance of fertilizing her eggs. Physical position of the male is also a very crucial factor. That is why males of most frogs will assume the amplexus position to secure their sexual success. Similarly, male horseshoe crabs will get into intense mating competition in order to secure the best position in the mating crowd. The following recording shows another spawning aggregation in a Mediterranean fish:
Spawning aggregations due to their inherent inefficiencies attract many other animals who would like to take advantage of this protein-rich “aquatic party”. Spawning aggregations can even attract the largest fish in the World, the whale sharks. Here in this observation we see many fish circling around the fire worms possibly due to nutritious gametes leaking out of the reproducing group of fireworms.
Polychete worms have quite interesting biology and some species have been used in the laboratory as a model organism. For instance, one species of bristle worm Platynereis dumerilii is the study organism for eye evolution. This marine worm has one of the simplest eyes shedding light on how eyes might have evolved in early aquatic habitats.
The white bristles are filled with venom and can easily penetrate through skin. Handling them is not advised. When disturbed, the worm releases the bristles into the water column.
Fireworms near Bermuda have the capacity of bioluminescence. These marine worms were observed to come to the surface to glow and mate on moonless nights. The female comes to the surface of the water and releases two chemicals that produce light when react with each other. This display attracts males who approach with rapid flickering light. When males arrive the female releases glowing chemicals again together with her eggs. The male then fertilizes. Not only is the light show is spectacular but also is very effective for attracting males.