Polychaete Biology

Overview

Polychaetes belong to the phylum of the Annelida that comprise the segmented worms. Currently there about 10000 known valid species. Their habitats are marine environments reaching from coastal sands to the deep-sea.

Being a part of the marine zooplankton (mainly larval stages) they play an important role in the nutrition of many aquatic animals. Benthic adult stages are a natural food item of many flatfish and the cod in the North Sea (Beesley et al. 2000). They occur in high densities and species like Nereis diversicolor, Nereis virens and Arenicola marina are also prayed upon by migrating birds in intertidal mudflats. They also play an important role as decomposers in the ecosystem. Deposit feeders like Capitella sp. I live within the surface layer of the sediment and burrow tubes promoting the decomposition of organic matter and the oxidation of reduced sulphides in the sediment (Chareonpanich et al. 1994). Polychaetes have proven to be useful for human purposes. Besides the use as fish bait the high content of polyunsatturated fatty acids (PUFAs) within their body tissues makes them an ideal food item in the nutrition of fish and shellfish. Further the ability of inducing sexual maturation is well known and widely used in the aquaculture industry.

The highly diverse life styles of Polychaeta have lead to the evolution of variable morphological body forms. As an example, predatory nereids that crawl actively over the substrate have well-developed sensory appendages as well as parapodia. Serpulids that are sedentary and build calcarous tubes in contrast have their appendages transformed into a tentacular crown and only bear reduced parapodia towards the posterior end.

The gross part of polychaetes lives on top or within the benthos. Holopelagic species within the genus Vanadis have large well-developed eyes, expanded foliaceous parapodial lobes and almost transparent bodies. They prey on chaetognaths, tunicates and fish larvae in the free water column.

Deep sea

by G. Bock

Life in great depths requires special adaptations due to the fact that there is no light and it is approximately 2-4°C cold. But there are exceptions, oases of life where animals depend on primary producers different to the phytoplankton at the surface. These ecosystems are constantly evolving and perishing due to continental driftings and the resulting fluid movement through the earth's crust.

Hydrothermal vents (hot vents, black smokers) are chimneys with a large plume which can usually be found at ridges where the seafloor is spreading. Up to 350°C hot and aggressive fluids loaded with zinc, copper and iron sulphides are discharging into the 2°C cold, oxygen rich water. Thereby the transported ores within the fluids from deep inside the oceanic crust percipitate and form dense chimney-like structures and a large black plume.

On the other hand there are areas on the seafloor called cold seeps where fluids leak the earth`s crust with much slower outflow and lower temperature than at hot vents. These fluids usually consist of water that was burried within the sediment over millions of years and is now beeing squeezed out due to compression of the oceanic crust. Along with the water soluble salts and gases like methane are transported towards the ground floor.

In both ecosystems aggregations of methane oxydizing archaea and sulfate reducing bacteria are living deep inside the ocean's crust in anoxic layers. They produce the energy source for the life in these extreme environments – hydrogen sulfide. On top of the ground other bacteria use the energy in this reduced compound together with oxygen to build up carbohydrates. There are various forms of symbiontic bacteria that use hydrogen sulfide. Endosymbionts live inside the giant tube worm Riftia pachyptila. Episymbionts live on top of the shrimp Rimicaris exoculata. Other forms like Beggiatoa are not associated with any other organism and are also prayed upon by the latter.

There are main differences in spreading rates and faunal compositions of ocean ridges. At the Pacific-Antarctic Ridge (PAR) the oceanic plates are moving apart with the fastest detected velocity of 20cm per year. The Mid-Atlantic Ridge (MAR) has only a spreading of 5cm per year. This ridge system is inhabited by the vent shrimp Rimicaris exoculata that is not present on the East-Pacific Rise (EPR) while the opposite is the case for the giant tube worm Riftia pachyptila.