Can microbes rely on Elasmobranchs?

Particulate organic matter, in the form of marine snow, is usually the one of very few inputs of carbon into the deep oceans. However, very rare whale falls can support huge diversity of chemoautotrophic organisms as shown by Bennett et al (1994). They found that an intact whale acted as a habitat for colonisation by Beggiatoa, found as white and yellow microbial mats. As well as being able to support two species of hydrothermal vent clams: Calyptogena cf. pacifica  and Vesicomya cf. gigas. But, until Higgs et al (2014) it was unknown whether elasmobranchs or ‘fish-falls’ could support a similar diversity of life.

Higgs et al (2014) analysed photos and videos from four large elasmobranch carcasses over an area of 1.48 km² on the Angola continental margin to assess what diversity a fish fall can support. Three of the carcasses showed signs of scavenger presence only; demonstrating fish falls play a larger role for mobile scavengers than primary fauna, such as microbial mats and hydrothermal clams, which are usually associated with whale falls.

Only one of the rays, Mobula genus, showed signs of further development beyond scavengers. There was a presence of a white microbial mat, assumed to be composed of sulphur oxidising bacteria. The presence of this mat provokes questions which the paper simply can not answer, such as are they similar microbial assemblages to those on whale falls or are they completely unique to Mobula?  Besides this, there were no similarities to whale falls observed; no hydrothermal vent clams were noted as in (Bennett et al, 1994). However, this may have been due to a different stage in ecological succession and colonisation of surrounding sediments. No physical samples were taken from any of the carcasses, thus it is hard to accurately date the fall and speculate the cause.

The study acknowledges many limitations, including: inability to take samples, inaccurate dating values. For me, the largest limitation is that there were no physical samples taken and that repeat visits did not occur. Consequently, only one isolated moment in time, shortly after the fall was surveyed. This may mean key characteristics, such as the presence of microbial mats or hydrothermal vent clams, were missed due to a different stage in succession occurring. Therefore, I do not think this research is conclusive that fish falls cannot support microbial presence.

Although fish falls are rare and surveying is usually opportunistic, I feel this research was perhaps published too early. It would have been better to have a longer time frame to see if microbial and other whale fall characteristic species developed. The whale fall previously mentioned (Bennett et al, 1994) was discovered in 1987 and studied two years later, whereas Higgs et al. dated the fish falls at about two weeks, give or take due to inaccurate dating techniques. In saying this, it is useful information to know that many large fish falls occur off the coast of Angola, especially when this paper predicts these falls to transport 4% of the POC to the seafloor; far more than whale fall predictions.

This paper demonstrates fish falls are still ecologically important as they export energy and carbon to deeper waters but as yet have not been found to support microbial activity.

Reviewed paper: Higgs N.D., Gates A.R., Jones D.O.B (2014) Fish Food in the Deep Sea: Revisiting the Role of Large Food-Falls. PLOS ONE  

Bennett, B. A., Smith, C. R., Glaser, B., & Maybaum, H. L. (1994). Faunal community structure of a chemoautotrophic assemblage on whale bones in the deep northeast Pacific Ocean. Marine Ecology-Progress Series, 108, 205-205.