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Appendicularian houses fate and role in carbon sedimentation and nutrition of zooplankton

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Your house is my food!

Appendicularians are transparent, filter feeders that live in the pelagic (open sea) zone. This is any body of water not close to the bottom of a lake or ocean, or near the shore.

Climate Change and Environment

Appendicularians are major producers of sinking particles through a very efficient means of food intake and 'disposal'. They are surrounded by a bubble, known as a 'house' of protein and cellulose; it has numerous filters that concentrate the food before it is consumed. As the species grow, the houses are continuously discarded and replaced. The discarded larvacean houses contribute to a large source of organic material descending to the ocean bed, where the sinking particles are taken up by zooplankton. Although not much is known about how zooplankton groups locate sinking particles, researchers believe the process plays a critical role in reducing downward carbon flux in the marine environment. It also represents an important source of food for zooplankton organisms and fish larvae. The 'Appendicularian houses fate and role in carbon sedimentation and nutrition of zooplankton' (Houses fate) project received EU funding to investigate how appendicularian houses are fed on and degraded. Researchers used copepods, eel larvae and microbial activity in their studies to better understand the appendicularian houses and their nutritious importance, as well as their impact on the carbon cycle. One set of experiments showed that the houses shrink drastically after being discarded: after 1 hour, they lose as much as 60\;% of their size. In this first hour, the house releases small, visible particles that can alert potential house consumers to its presence. Following this, all that remains visible is the effect of bacterial degradation. In terms of house consumption, other Houses fate experiments revealed that appendicularian houses are effectively consumed and that the quality of the resulting faecal pellets depends on house density. This highlighted the importance of discarded houses in providing a major source of nutrition for zooplankton. Team members also observed the behaviour of copepods as related to sinking appendicularian houses so as to determine and characterise the different detection mechanisms used. Copepods were discovered to use chemical clues that can detect sinking houses more than 4\;cm away. Having accomplished all objectives, the project has contributed to furthering research on the role of zooplankton in degradation processes in the ocean. This has implications for further research on toxicity and nutrition processes in the marine environment.

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