May 10, 2021 Last Updated 2:42 AM, May 10, 2021

Scaring Nemo

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How do clownfish on coral reefs react to encounters with humans?

Human intervention is putting marine ecosystems under increasing pressure. On coral   reefs,   rising   water   temperatures, ocean  acidification, pollution  from  plastic waste or overfishing threaten the organisms living there - be they fish, sea cucumbers or corals. But how does the direct physical presence of humans under water change the behaviour and ecology of animals on coral reefs?

Researchers from AUT’s School of Science and the Leibniz Centre for Tropical Marine  Research  (ZMT)  in  Bremen studied the behaviour of anemone fish during  encounters  with  humans  off  the coast of Vanuatu in the South Pacific. The team, consisting of marine biologist Dr Armagan Sabetian (AUT), AUT marine biology student Lena Trnski and fish ecologist Dr Julian Lilkendey (ZMT, came to some surprising results which have been published in the Journal of Fish Biology.

Anemonefish, also  known  as  clownfish, are  not  only  the  cute  protagonists  in  the hit movie “Finding Nemo”, they are also model  organisms  for  behavioral  studies on  fish. Clownfish live  in  symbiosis  with sea anemones; the anemone offers the fish protection from attackers in its tentacles while the clownfish defends it from invaders and   provide   it   with   nutrition   through its food remains and excrements. When guarding their host anemone, the fish show easily distinguishable behaviours. The close connection to their fixed-in-place anemone makes clownfish particularly susceptible to human presence because they can only avoid an encounter with humans underwater for a limited time due to the small size and localization of their anemone.

During  their investigations  in Vanuatu, the researchers focused on two species of clownfish -  the  Clark’s  anemonefish  and the dusky anemonefish. They wanted to know whether these species changed their behavior  when  encountering  humans. Lead  author  Lena  Trnski  snorkeled  out to  the  reefs  off  Efate  island  and  videoed the  reactions  of  clownfish.  To  simulate the  presence  of  an  observer,  she  hovered one to three meters above the anemone. However, she was not present during the video recordings recorded as control for comparison. She noted the behavior of each fish in the anemone at 15-second intervals. A total of 60 behavioural events were recorded for each anemone fish, both in Lena’s presence and in her absence.

When confronted with human presence, the two species showed very different behavioural patterns. “While Clark’s anemonefish were frightened by encounters with  humans  and  often  hid  deep  within the tentacles of the anemone, the dusky anemonefish reacted less to the presence of a human,” reports study leader Dr. Julian Lilkendey. “We actually expected that the dusky anemone fish would also hide from Lena or even attempt to chase her away, but they usually did not let her disturb them. Individuals were often up to a metre outside the tentacles of their anemone.

“We suspect that dusky anemonefish display a rather bolder behaviour in the presence of humans, as they are a highly specialised species that can only inhabit a few species of anemone,” Trnski explains. “In order to find a suitable host anemone, the dusky anemonefish can benefit from fearlessly   searching   the   reef   -   even   if this behaviour also makes them more susceptible to predators. But it is precisely this fearlessness that is also revealed by the fact that they do not flee from humans.

Courage pays off

Their observations led the researchers to conclude that in regions with a high level of tourism, ‘courageous’clownfish species such as  the  dusky  anemonefish could  displace more fearful species. “Species that retreat into  their anemones  for protection  under stress spend a lot of energy and time on this one escape behaviour and are therefore less able to engage in foraging or reproduction,” says Lilkendey. “Intrepid species have an advantage over them.”

“The resulting displacement process would ultimately lead to a loss of species diversity,” Trnski adds.

“The ecological consequences of behavioural changes caused by human presence are still largely unexplored,” says Lilkendey.

“We suspect that differences in the behaviour of individual animals could have an impact on interactions between species, such as predator-prey relationships or symbioses, which in turn affect community structures and the functioning of the entire ecosystem”.

Trnski, L., Sabetian, A., Lilkendey, J. (2020) Scaring Nemo – Contrasting effects of observer presence  on  two  anemonefish  species.  Journal  of Fish Biology. https://onlinelibrary.wiley.com/doi/ full/10.1111/jfb.14492?af=R

Last modified on Friday, 13 November 2020 07:58

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