Platynereis dumerilii

Platynereis dumerilii is a species of annelid. It was originally placed into the genus Nereis and later reassigned to Platynereis. Platynereis dumerilii lives in coastal marine waters from temperate to tropical zones. It can be found in a wide range from the Azores, the Mediterranean, in the North Sea, the English Channel, and the Atlantic down to the Cape of Good Hope, in the Black Sea, the Red Sea, the Persian Gulf, the Sea of Japan, the Pacific, and the Kerguelen Islands. Platynereis dumerilii is today an important lab animal, it is considered as a living fossil, and it is used in many phylogenetic studies as a model organism. Platynereis dumerilii reaches an age of 3 to 18 month and males reach a length of 2 to 3 cm, while females reach a length of 3 to 4 cm. Platynereis dumerilii builds tubes on its substrate. The substrate may be algae covered hard bottoms, sea grass, pelagic Sargassum rafts in the Sargasso Sea, or even rotting plant debris. Platynereis dumerilii commonly lives in 0 to 5 m depth, and so is typical for shallow bright infra-littoral environments. But it has been also found on a buoy in 50 m and on rotting seaweed in 100 m. It may also live in less favorable environments, like at thermal vents or polluted areas near sewer outfall pipes. It dominates polluted areas and acidic areas with pH values around 6.5 fitting the preferred pH value of a subpopulation of late Platynereis dumerilii nectochaete larvae. Platynereis dumerilii is dioecious, that means it has two separate sexes: During mating, the male swims around the female while the female is swimming in small circles. Both release eggs and sperm into the water. This is triggered by sexual pheromones. The eggs are then fertilized outside of the body in the water. Platynereis dumerilii has like other Nereidids no segmental gonades, the oocytes mature freely swimming in the body cavity (coelom), and stain the body of the mature female epitoke yellow. Platynereis dumerilii develops very stereotypically between batches and therefore time can be used to stage Platynereis dumerilii larvae. However, the temperature influences the speed of development greatly. Therefore, the following developmental times are given with 18 °C as reference temperature: After 24 hours, a fertilized egg gives rise to a trochophore larva. At 48 hours, the trochophore larva becomes a metatrochophore larva. Both trochophore and metatrochophore swim with a ring of cilia in the water and are positively phototactic. The metatrochophore has, beside the larval eyes, already the anlagen for the more complex adult eyes of the adult worm. A day later, at 72 hours after fertilization, the metatrochophore larva becomes a nectochaete larva. The nectochaete larva already has three segments, each with a pair of parapodia bearing chaetae, which serve for locomotion. The nectochaete larva can switch from positive to negative phototaxis. After five to seven days, the larvae start feeding and develop on their own speed, depending on food supply. After three to four weeks, when six segments have formed, the head is formed. Platynereis dumerilii larvae possess two kinds of photoreceptor cells: Rhabdomeric and ciliary photoreceptor cells. The ciliary photoreceptor cells are located in the deep brain of the larva. They are not shaded by pigment and thus perceive non-directional light. The ciliary photoreceptor cells resemble molecularly and morphologically the rods and cones of the human eye. Additional, they express an ciliary opsin that is more similar to the visual ciliary opsins of vertebrate rods and cones than to the visual rhabdomeric opsins of invertebrates. Therefore, it is thought that the urbilaterian, the last common ancestor of mollusks, arthropods, and vertebrates already had ciliary photoreceptor cells. The ciliary opsin is UV-sensitive (λmax = 383 nm), and the ciliary photoreceptor cells react on non-directional UV-light by making the larvae swimming down. This forms a ratio-chromatic depth-gauge with phototaxis of the rhabdomeric photoreceptor cells of the eyes. A rhabdomeric photoreceptor cell forms with a pigment cell a simple eye. A pair of these eyes mediate phototaxis in the early Platynereis dumerilii trochophore larva. In the later nectochaete larva, phototaxis is mediated by the more complex adult eyes. The adult eyes express at least three opsins: Two rhabdomeric opsins and a Go-opsin. The three opsins there mediate phototaxis all the same way via depolarization, even so a scallop Go-opsin is known to hyperpolarize.