Why can nematode-prokaryotic symbioses be associated with deep-sea hotspots?

Nematode-prokaryotic symbioses (NPS) have been documented since the 1930s. Now they are acknowledged as ubiquitous in shallow and deep waters and important in advancing our understanding of how marine ecosystems may function in their presence. Two enigmatic NPS examples we have been studying over the past few years belong to the nematode subfamily Astomonematinae and family Oncholaimidae, hosting obligate endosymbionts and transient or permanent ecto-/endo-symbiotic prokaryotes, respectively. Astomonema is dependent on the prokaryote symbionts as an energy source since they lack a mouth and their pharynx is vestigial, precluding food uptake via the mouth. Astomonema was first discovered in the deep sea in 2012; and has since been observed in six different deep-sea canyon systems. Oncholaimus dyvae, recovered from 1700m water depth at the hydrothermal vent Lucky Strike, exhibits preference for high temperatures and vent emissions, and shows evidence of symbiotic bacteria attached to the cuticle and inside the digestive cavity and intestine. Other oncholaimids and astomonematines (and some other groups like the stilbonematines) have been described with prokaryotic symbionts from deep and shallow waters and under permanent or transient, but often ‘extreme’ conditions such as thiotrophic, methanogenic, and hydrothermal settings. Here we compare results that give clues as to the mutual advantage and origins of NPS, symbiont transfer, and their functional ecology, and try and answer the questions “What are the environmental conditions that can support NPS life styles in the deep sea?”, “How can NPS support or be associated with deep-sea hotspots of biodiversity?”, and “How may increasing oceanic stress influence NPS prevalence?”