Lights out! Clownfish can only hatch in the dark – which light pollution is taking away



Some 22% of the worlds’ coastlines are exposed to artificial light at night.
Emily Fobert, Author provided

Emily Fobert, Flinders University

Clownfish achieved worldwide fame following Finding Nemo, but it turns out these fish don’t do so well in the spotlight.

Our research, published in Biology Letters, found when clownfish eggs were exposed to low levels of light at night – as they would be if laid near a coastal town – not a single egg hatched.

This finding adds to the growing body of research on the health affects of light pollution, a rapidly spreading ecological problem.




Read more:
Light pollution: the dark side of keeping the lights on


What is light pollution?

Light pollution occurs when artificial light interferes with ecological systems or processes, usually at night.

Natural light at night, produced by the moon, stars, and other celestial bodies, is minimal. A full moon creates only 0.05-0.1 lux, which pales in comparison to the artificial light produced by humans, which can range from around 10 lux from an LED or low-pressure sodium streetlight, up to 2,000 lux from something like stadium lighting.

Clownfish were exposed to artificial light to see what effect it would have on their reproduction.
Emily Fobert, Author provided

Because nearly all organisms on Earth have evolved with a stable day-night, light and dark cycle, many biological events are now highly attuned to the daily, lunar, and seasonal changes in light produced by the reliable movements of the Earth and Moon around the Sun.

But artificial light can mask these natural light rhythms and interfere with the behaviour and physiology of individual creatures, and ecosystems as a whole.

The ocean is not exempt from these problems. Light pollution is spreading to marine habitats through urbanised coastlines and increasing marine infrastructure such as piers, harbours, cruise ships, and tropical island resorts where bungalows extend out into the lagoon, directly above coral reefs.

Why are clownfish at risk?

Clownfish, like many reef fish, are particularly vulnerable to light pollution because they don’t move around much in their adult stage. Clownfish can travel long distances in the first 2 weeks after hatching, but at the end of this period the young fish will settle in a suitable sea anemone that becomes their forever-home.

Once clownfish find a suitable anemone they stay put forever.
Emily Fobert, Author provided

This means that if a fish chooses an anemone on a shallow reef in an area that is heavily lit at night, they will experience chronic exposure to light pollution throughout their life; they won’t just move away.

Clownfish also lay their eggs attached to rock or other hard surfaces, so in areas exposed to light pollution the eggs will experience continuous artificial light (as opposed to many fish that lay and fertilise eggs in open water, so they are immediately carried away by ocean currents).

What we found

To test how artificial light affects clownfish reproduction, we examined the common clownfish (Amphiprion ocellaris) in a lab experiment.

Five breeding pairs of fish experienced a normal 12-hour daylight, 12-hour dark cycle, while another five pairs of fish had their “night” period replaced with 12 hours of light at 26.5 lux, mimicking light pollution from an average coastal town.

For 60 days, we monitored how often the fish spawned, how many eggs were fertilised, and how many eggs hatched. While we saw no difference in spawning frequency or fertilisation rates between the two groups of fish, the impact of the artificial light treatment on hatch rate was staggering. None of the eggs hatched, compared with an average of 86% in the control group.

Clownfish attach their eggs to rocks or other hard surfaces, leaving them at the mercy of their immediate environmental conditions.
Emily Frobert, Author provided



Read more:
Why does Nemo the clownfish have three white stripes? The riddle solved at last


At the end of the experiment we removed the artificial light and monitored the fish for another 60 days to see how they would recover. As soon as the light at night was removed, eggs resumed hatching at normal rates.

Clownfish, like many reef fish, have evolved to hatch after dusk to avoid the threat of being eaten. Newly hatched baby clownfish, like most coral reef fish, are small (about 5mm long) and transparent. Hatching in darkness likely means they are less visible to predators as they emerge from their eggs.

Our findings show that the presence of artificial light, even at relatively low levels, can disrupt this crucial process, by masking the environmental cue – darkness – that triggers hatching. As many reef fish share similar reproductive behaviours to clownfish, it is likely artificial light will similarly interfere with the ability of other fish species to produce viable offspring.

Healthy, fertilised clownfish eggs did not hatch in the presence of artificial light.
Emily Frobert, Author provided

The larger problem

Light pollution is one of the most pervasive forms of environmental change. An estimated 23% of land surface (excluding the poles) and 22% of coastal regions are exposed to light pollution.

And the problem is only growing. The reach of light pollution across all land and sea is expanding at an estimated rate of 2.2% per year, and this will only increase with the rising global human population.




Read more:
Saving Nemo: how climate change threatens anemonefish and their homes


Although research on the ecological impacts of light pollution is arguably only in its infancy, the evidence for negative consequences for a range of insects, birds, amphibians, reptiles, and mammals, including humans, is stacking up.

Our new research adds another species to the list, and highlights the importance of finding ways to manage or reduce artificial light, on land and below the waves.The Conversation

Emily Fobert, Research Associate, Flinders University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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Saving Nemo: how climate change threatens anemonefish and their homes


Jean-Paul Hobbs, Curtin University and Ashley J Frisch, James Cook University

Anemonefish, or clownfish, were made famous by the 2003 Disney-Pixar film Finding Nemo, and are about to play a starring role in the sequel, Finding Dory. They are well known for their special relationship with anemones, which provide a safe place to call home.

But anemonefish face a number of threats. Some researchers have warned of an increase in the wild-caught anemonefish trade, as happened following Finding Nemo.

Anemones, on which anemonefish depend, are threatened by warming seas in a similar way to corals. In fact anemones were affected by the recent coral bleaching on the Great Barrier Reef, which recent updates show has left a third of coral colonies dead or dying in the north and central parts of the reef.

So will Nemo be left homeless?

A healthy (left) and bleached (right) bubble-tip anemone (Entacmaea quadricolor) on the Great Barrier Reef.
Ashley Frisch

Nemo and his 27 cousins

There are 28 species of anemonefish. Although some people call this group “clownfish”, technically this name is only used for one species, Amphiprion percula. “Nemo” (A. ocellaris) looks similar, but is actually known as the “false clownfish”.

Anemonefish are famous for their special relationship with anemones. Although they can survive in aquariums without anemones, in nature they rely on anemones for protection from predators.

The pink anemonefish (Amphiprion perideraion) in a bleached anemone (Heteractis magnifica) at Christmas Island.
JP Hobbs.

In return for providing a safe home, the resident anemonefish will provide nutrients and defend the anemone from predators such as butterflyfish. Both the number and size of anemonefish is linked to the size and number of anemones – and vice versa. Therefore, any decrease in one partner affects the other.

The collection of anemones and anemonefish for the aquarium trade has to be managed properly to ensure the future of anemonefishes. Anemonefish can be easily bred in captivity and this provides a reliable source for aquarium enthusiasts without impacting wild populations.

Cinnamon anemonefish (Amphiprion melanopus) in a bleached anemone (Entacmaea quadricolor) on the Great Barrier Reef.
Ashley Frisch

Ten species of anemones are inhabited by anemonefish. The highest diversity of anemonefish occurs in Indonesia, where anemonefish species outnumber anemones. As a result, different species of anemonefish have learnt to share the same anemone.

In most other locations, anemonefish aggressively prevent other species from entering their anemone. Anemonefish species differ in the number of anemone species they associate with.

Clark’s anemonefish (Amphiprion clarkii) in a bleached anemone (Cryptodendrum adhaesivum) at Christmas Island.
JP Hobbs.

Clark’s anemonefish (A. clarkii) can live in all ten anemone species and is widely distributed throughout the Indian and Pacific Oceans. In contrast, McCulloch’s anemonefish (A. mccullochi) inhabits only one species of anemone and occurs only on reefs around Lord Howe Island.

After hatching, anemonefish larvae use their keen sense of smell to find their preferred anemone species and avoid unhealthy (bleached) anemones.

Anemones in hot water

Anemones are closely related to corals and get their colour from microscopic algae (zooxanthellae) that live symbiotically within the tissue of the anemone. Like corals, anemones expel their algae and turn white when they become stressed.

This process – termed “bleaching” – is usually in response to periods of elevated seawater temperatures. All ten species of anemones are susceptible to bleaching, which can result in a decrease in the size and number of anemonefishes and reduced reproduction.

McCulloch’s anemonefish (Amphiprion mccullochi) in a bleached anemone (Entacmaea quadricolor) at Lord Howe Island.
Justin Gilligan.

If seawater temperatures remain high for too long, then bleached anemones will die. In 1998, a prolonged period of elevated water temperatures in Japan resulted in mass mortality of bleached anemones and local extinction of anemonefish.

In March 2016, the Great Barrier Reef experienced a severe bleaching event due to elevated water temperatures associated with a strong El Niño event. There was mass bleaching of both corals and anemones.

Marine biologist Jean-Paul Hobbs studying anemonefish (Amphiprion mccullochi) and their host anemones (Entacmaea quadricolor) at Lord Howe Island.
Justin Gilligan.

In April 2016, elevated water temperatures also caused mass bleaching of corals and anemones off north-west Australia, including Christmas Island. Bleached anemones have also recently been reported elsewhere in the Pacific Ocean, Indian Ocean and in the Red Sea.

The future of the bleached anemones and their resident anemonefish will depend on how quickly the water temperature returns to normal. If the temperature decreases swiftly, bleached anemones can regain their colour (reabsorb zooxanthellae) and survive.

However, the frequency and intensity of bleaching events are predicted to increase as the climate changes. Consequently, there are serious concerns about the ability of anemones and anemonefish to cope with rising water temperatures.

Reducing global greenhouse gas emissions will limit subsequent bleaching events and help ensure the future of Nemo and its relatives.

The Conversation

Jean-Paul Hobbs, Research Fellow, Department of Environment and Agriculture, Curtin University and Ashley J Frisch, Postdoctoral Fellow in Marine Ecology, James Cook University

This article was originally published on The Conversation. Read the original article.