By: Ellie Casement
Figure 1: Dory from “Finding Nemo” suffering from short-term memory loss.
An ocean of Dorys: warming temperatures cause memory loss in reef fish
Introduction
Having the “ memory of a goldfish” is generally not taken as a compliment. Though research has repeatedly shown that fish memories can last for weeks, months, and even years, humans rarely give the cognitive abilities of our aquatic pets much credit (Laland, Brown & Krause, 2003). While the ditzy nature of characters such as Finding Nemo’s Dory supports a narrative of fish forgetfulness, in reality, fish rely on learning and memory as they navigate complex habitats to find food, evade enemies, and seek mates.
A team of researchers from Brazil, Australia, and New Zealand joined forces to explore the effects of a warming ocean on reef fish cognition. They found that fish learning and memory were significantly impaired by exposure to high temperatures, demonstrating how climate change may make coral reefs confusing and perilous environments for increasingly forgetful fish.
Background
Fish are the most abundant group of ectotherms on earth, meaning their body temperature is directly affected by the temperature of their surrounding environment. When the surrounding environment changes, ectothermic species cannot regulate their internal temperature and must respond by adjusting their behavior. Examples include a lizard sunning itself on a cold day and honey bees huddling together to retain heat. However, there are limited behavioral options for organisms to escape or adjust to widespread and long-lasting thermal stressors, such as global warming.
Methods
This research centered on the spiny chromis damselfish (Acanthochromis polyacanthus), a common coral reef inhabitant in the Indo-Australian archipelago. These fish thrive at approximately 28 °C. When exposed to slightly higher temperatures (30-32.5 °C), they experience detrimental effects on performance, growth, and reproduction, whereas prolonged exposure to temperatures exceeding 34 °C typically leads to mortality (Zarco-Perello, Pratchett & Liao, 2012). Due to their territorial nature and tendency to inhabit the same reef patches for extended periods, they are less inclined to migrate and escape marine heatwaves (Pankhurst, Hilder & Pankhurst, 1999).
Thirty adult fish were collected from reefs at the northern end of the Great Barrier Reef and acclimated in laboratory aquaria held at three different temperatures representing current and projected summer conditions:
- 28-28.5 °C = control, or the current normal temperatures on the GBR
- 30-30.5 °C = moderate warming, or current summer maximums on the GBR
- 31.5-32 °C = high warming, or projected future maximums on the GBR
Following acclimation, the fish were introduced to a “learning environment” – a tank with a center area surrounded by four branching halls, each leading to a small, transparent tank. Three of the small tanks were empty, and one contained a “positive stimulus,” which was another live fish. The hall with the positive stimulus was marked with a blue tag as a cue.
Figure 2, A and B: The learning environment, with a center area connected to four branching halls and tanks. The blue tag marks the hall with the reward, i.e. the stimulus fish.
There were four stages to the experiment:
- Habituation: On days 1-4, the animals were released in the center of the tank and allowed to explore for 5-15 minutes without a stimulus fish or cue card present.
- Associative learning training: On days 5-9, the stimulus fish was added to the tank at the end of one hall, marked with the blue cue card. The experimental fish was placed in the center of the tank and released to explore for five minutes. This was repeated four times daily for a total of 20 “training runs” for each fish.
- Learning test: On day 10, the blue tag was displayed, but the stimulus fish was removed to determine if the experimental fish had learned its position. The experimental fish was placed in the center of the tank and released to explore for 10 minutes. After this test, the fish were returned to their housing for 5 days.
- Memory test: On day 15, the same learning test was given: the blue tag was displayed, but no stimulus fish was present. The experimental fish was allowed 10 minutes to explore the tank to determine if it remembered the position of the stimulus fish.
The researchers evaluated the behaviors of the fish to determine their learning and memory capabilities, including the time spent in different areas, the amount of time to find the stimulus fish, and the correctness of the first choice (choosing the hall with the stimulus fish),
Findings
The study found that higher temperatures significantly decreased the spiny chromis damselfish’s ability to learn and remember a task.
At normal temperatures, the fish learned to associate the blue cue card with the reward – they found the stimulus fish faster and had more correct “first choices” for the tagged hall throughout the trials. They also retained this information and quickly entered the hall with the other fish even after five days.
At moderate warming, fish were able to learn but their memory was compromised. While they learned (albeit slower) to associate the blue cue card with the stimulus fish in the training and learning test, they did not remember this information five days later.
No learning was observed after the same number of training trials at high warming.
The observed memory under normal conditions is logical given that adult spiny chromis damselfish tend to explore and defend 6-7 m2 sized territories, and spatial awareness is an important aspect of life in complex reef habitats (Cowlishaw, 2014). For example, being able to navigate around the reef, return to a territory, and avoid areas with unfriendly or predatory species is likely crucial to the survival of these fish. Additionally, the ability of these fish to quickly learn and adapt to a new environment allows them to survive in the face of random and rapid changes on reefs.
This study indicates that thermal stress may reduce brain processing, as well as disrupt attention and exploration, which are important aspects of learning. Over time, organisms may need to direct energy towards adjusting to and surviving an environmental stressor, which decreases the energy available for higher-level functions, such as cognition.
The strong effects of this study are striking given that the time frame and temperatures tested are not unimaginable extremes; these temperatures have already been observed on the Great Barrier Reef during marine heatwaves, and are likely to become more frequent. The researchers of this study also emphasize that these conditions could become common summer temperatures under future warming scenarios, and the detrimental cognitive impact of these conditions are only expected to worsen with increasing temperatures. Therefore, this study highlights the importance of future research on animal cognition and adaptation to warming alongside other stressors, and the urgent need to mitigate rising ocean temperatures.
Read the article here: Silveira, M. M., Donelson, J. M., McCormick, M. I., Araujo-Silva, H., & Luchiari, A. C. (2023). Impact of ocean warming on a coral reef fish learning and memory. PeerJ, 11, e15729.
References
Cowlishaw, M. (2014). Determinants of home range and territory size in coral reef fishes (Doctoral dissertation, James Cook University).
Laland, K. N., Brown, C., & Krause, J. (2003). Learning in fishes: from three-second memory to culture. Fish and fisheries, 4(3), 199-202.
Pankhurst, N. W., Hilder, P. I., & Pankhurst, P. M. (1999). Reproductive condition and behavior in relation to plasma levels of gonadal steroids in the spiny damselfish Acanthochromis polyacanthus. General and Comparative Endocrinology, 115(1), 53-69.
Unkrich, L., & Stanton, A. (2003). Finding Nemo. Buena Vista Pictures.
Zarco-Perello, S., Pratchett, M., & Liao, V. (2012). Temperature-growth performance curves for a coral reef fish, Acanthochromis polyacanthus. Galaxea, Journal of Coral Reef Studies, 14(1), 97-103.
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