Red algae use color tricks to warn fish they taste bad

Marine life, including algae, can present a kaleidoscope of color, sometimes produced by microstructures rather than pigments. This unique method, known as structural color, is a topic scientists have explored in creatures from fish to seaweeds.

Recent findings show that certain red algae use structural color in ways that go beyond simple aesthetics. One researcher who has observed this phenomenon for years is Hiroshi Kawai, an expert in marine biodiversity and ecology at Kobe University.

Algae’s unexpected color shift

“Being a diver, I have long been aware that some red algae have a much whiter appearance than their usual red when observed in water,” said Kawai. While exploring coastal waters, experts came across young shoots with a blueish tint near their tips. 

Further sampling required specialized gear due to the algae’s fragile nature and deep location. Still, enough specimens were gathered for close inspection with advanced imaging methods.

Researchers classify Asparagopsis taxiformis in the Bonnemaisoniales order, placing it among seaweeds that show diverse structures. Its delicate branches can break under minor stress, prompting careful handling techniques.

A qualified phycologist, someone who studies algae, can notice subtle traits that casual observers miss. Long hours underwater revealed more than just color variations, including the potential for chemical defenses within the tissues.

Field research in deeper zones sometimes requires special permits for scuba operations and environmental guidelines. Strict adherence to these rules ensured minimal disturbance to local ecosystems.

How algae reflect light

Scientists observed that certain gland cells in the algae contain tightly packed spheres of reflective materials, each measuring about the same size. Uniform spheres reflect light of a single color, which explains why the tips appear blue.

As the algae mature, the sphere sizes vary and scatter multiple wavelengths of color. That color shift results in a whitish tint that can mask the original red tone.

Rhodophyta, the broader group of red algae, includes species that rely on pigments like phycoerythrin. Structural color adds another layer of visual effects beyond the red shades typically seen.

Different wavelengths bounce off micro-scale structures, producing colors without relying on traditional pigments. This phenomenon has also been recorded in certain land plants and insects.

Some marine biologists stress that structural color is often more stable than pigment-based hues. Changes in water clarity or sunlight intensity can still influence how it appears to observers.

Colors protect algae from predators

At the site of these color shifts, researchers found anti-herbivory chemicals that may deter hungry fish. Some experts propose that the bright hue signals a warning, making the algae less appetizing to potential grazers.

Where the algae’s fruiting structures develop, the lighter tones may help it blend in. This trick could be beneficial in waters where vision-based predators roam.

The presence of bromides in certain algae species has been documented as a possible deterrent. Some analyses suggest these substances can interfere with fish digestion, encouraging them to seek meals elsewhere.

A shift in shading at different growth stages points to complex interactions between the algae and its environment. Defensive strategies might change as the algae transitions from young shoots to fruiting bodies.

Anti-herbivory strategies in algae can vary, with some relying on tough external layers instead of chemicals. The dual approach of color and substances is less common, drawing renewed interest in laboratory studies.

Impact of warming seas

“As warm-water fish migrate further north, they might pose a significant threat to the algae there that are not protected by such coloration,” explained Kawai.

Global warming can shift tropical fish populations into cooler regions, increasing grazing pressure on algae not prepared for such threats. 

Such color shifts could lead to reduced numbers of red algae without these visual defenses. This raises concerns about ecosystem changes in regions previously free from tropical fish.

Rising temperatures can alter marine habitats faster than some organisms can adapt. Species with built-in protective traits might outcompete those lacking similar adaptations.

Marine ecologists suggest ongoing surveys to track how expanded grazing ranges affect underwater forests of algae. In some spots, these forests also serve as shelters for small marine animals, amplifying the impact of fish movement.

Warmer conditions also affect the reproductive cycles of many algae species. Faster or altered reproductive phases can intersect with shifting fish populations, compounding the stress on these underwater communities.

Structural color of algae

Researchers from Hokkaido University contributed to the effort, hinting that structural color might be more prevalent in marine plants. Some brown algae were also found to display similar optical features in earlier work.

Such color patterns may serve multiple roles, from camouflage to signaling. Future studies may uncover additional strategies across other algae groups.

Chemical compounds linked to defense could attract interest from various industries. Discoveries about unusual pigments or reflective properties sometimes pave the way for new materials or pharmaceuticals.

Future research directions

Stronger global collaboration may spur further research on deep-water algae and their hidden qualities. Closer ties between institutions also help support the extensive fieldwork required to observe these species in their natural habitats.

Collaboration between universities offers a broader perspective on how widespread algae color phenomena may be. Sharing data across continents can reveal patterns that single-site studies might overlook.

The study is published in the European Journal of Phycology.

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