L’s diet in marine reef aquariums.

In the world of marine reef aquariums, herbivorous fish such as surgeonfish (Zebrasoma, Acanthurus,...), blennies (Salarias,...), and rabbitfish (Siganus,...) play a fundamental role in maintaining the ecological balance of these systems.

Although they are a must in our tanks, their dietary needs are often downplayed and ignored, leading to major health problems and an extremely reduced life expectancy over the long term.

Understanding their dietary needs is essential to ensuring health, longevity, and resistance to disease.

This article explores the physiological, metabolic, and anatomical differences between herbivorous and omnivorous/carnivorous fish, with the aim of providing a scientifically accurate overview that can help hobbyists give them a better life.

Typical characteristics of herbivorous species:

L’s digestive system represents a unique evolutionary adaptation, developed to make use of a diet consisting primarily of algae. Understanding the structure and function of their digestive system is essential to meeting their nutritional needs in an aquarium and ensuring their long-term health.

Herbivores have a digestive tract designed to handle a diet rich in fiber, complex carbohydrates, and minerals.

Unlike carnivores or omnivores, which mainly consume protein-rich prey that is easier to digest, herbivores must extract nutrients from algae containing polysaccharides that are much harder to digest, such as cellulose, agar, or carrageenan.

Although they too rely on chemical digestion involving the production of specific enzymes such as proteases, lipases, and carbohydrases, the overall anatomy of herbivorous fish digestive systems has evolved in a very particular way, developing specific functional adaptations for their diet that rely on an extremely functional gut microbiota.

Among the characteristic anatomical adaptations, the following are particularly important: mouth and dentition, stomach, intestinal length and morphology, pyloric caeca, gut microbiota.

Mouth and dentition

The oral cavity is the first tool for acquiring food:

The mouth is generally small, terminal or subterminal in position, and equipped with specialized dentition adapted to the diet. For example, in surgeonfish, the dentition may take the form of a rake or a comb, and its main specialization is scraping epilithic algae and surface films. In other fish, the dentition resembles scissors or blades and is better suited to cutting filamentous algae (e.g., blennies) or tough macroalgae (e.g., rabbitfish).

Stomach

Algae-eating fish have a poorly developed or even rudimentary stomach with extremely limited functionality. This deficiency is compensated for by the evolution of a very long and highly specialized intestine.

The stomach wall is thin and contains digestive enzymes for the initial hydrolysis of proteins. In algae-eaters such as Siganus, the stomach acts as a temporary reservoir, but the primary role in digestion belongs to the intestine.

In many species, such as surgeonfish, primary digestion takes place in the intestine, where food remains for long periods to be fermented and broken down by the bacteria that make up their microbiota.

Intestine

L’intestine is the heart of the digestive system in algae-eating fish and hosts a symbiotic bacterial flora that ferments the complex polysaccharides typical of algae (e.g., cellulose, carrageenan, etc.).

Maintaining a healthy gut microbiota is extremely important for their health.

Although the intestine is folded back on itself several times, it is extremely long (up to 10 times the body length in several surgeonfish), has thick walls and highly developed villi to optimize nutrient absorption.

Likewise, numerous loops and convolutions are present, exponentially increasing the total surface area available for nutrient absorption.

Pyloric caeca

Pyloric caeca are outpocketings of the intestine located at the junction with the stomach (or where it would be). They are essential because they harbor the fermentative bacteria that break down algae into absorbable compounds.

Their presence serves to increase the digestive surface area, improving the efficiency of algae assimilation.

Gut microbiota

The gut microbiota of algae-eating fish is a crucial evolutionary adaptation that enables them to extract nutrients from a diet rich in fiber and complex, poorly digestible polysaccharides found in algae. This microbiota consists of a community of bacteria, fungi, and other symbiotic microorganisms that work together to ferment structural carbohydrates and produce compounds that the host organism can use.

The microbiota in algae-eating fish performs four main functions:

• Fermentation of cellulose and other typical polysaccharides: Symbiotic bacteria produce enzymes that break down cellulose and other complex polysaccharides into simpler sugars and absorbable compounds. Algae-eating fish do not produce cellulase themselves; it is synthesized entirely by symbiotic bacteria. For example, the bacterium Fibrobacter succinogenes, known for its high cellulolytic activity, has been isolated from Zebrasoma scopas.
• Production of short-chain fatty acids (SCFAs): Bacterial fermentation produces short-chain fatty acids, such as acetate, propionate, and butyrate, which are absorbed through the intestinal walls and used as an energy source (providing up to 30% of the fish's energy requirements).
• Vitamin synthesis: Some microbiota bacteria synthesize essential vitamins, such as B12, which support the body's overall metabolism.
• Detoxification: Some bacteria are involved in detoxifying secondary compounds produced by algae, such as tannins and polyphenols, which could interfere with digestion and nutrient absorption.

Diet is one of the factors that most strongly influence the gut microbiota of these fish: a diet without algae and fiber-rich foods alters the microbiota, consistently causes digestive problems, and reduces life expectancy. Similarly, using feed that is high in animal protein and low in algal polysaccharides can quickly reduce the diversity of the gut microbiota, causing serious health problems for our aquarium inhabitants.

Diet in the wild and in the aquarium

To ensure the health and longevity of algae-eating fish in aquariums, it is essential to fully understand their dietary needs, which are closely linked to their physiology.

As mentioned earlier, these species have a specific diet based on evolutionary adaptations that allow them to make the most of algae, a nutrient-rich but extremely difficult-to-digest food source.

In the wild, these fish feed continuously, spending much of the day scraping algae off rocks and dead corals. Their diet often includes particles of organic debris and small organisms associated with algae.

These occasional servings of animal protein are useful but make up a secondary part of their diet. A well-balanced diet should provide the key nutrients they obtain in the wild, primarily from algae and, to a lesser extent, from zooplankton and small benthic organisms.

Nutritionally, algae represent a unique combination of fiber and plant protein, with a relatively low lipid content.

This balance is perfectly suited to herbivores’ metabolism, as they use the carbohydrates and volatile fatty acids produced by intestinal fermentation as their primary source of energy. Although present in moderate amounts, proteins should preferably be plant-based to avoid metabolic overload.

A key element of herbivores’ nutrition is fiber, which not only supports digestion but also promotes the growth and stability of the gut microbiota by acting as a prebiotic. Symbiotic bacteria in herbivores’ intestines ferment fiber and complex polysaccharides, producing short-chain fatty acids such as acetate, propionate and butyrate. These compounds are not only a source of energy but also contribute to overall gut health.

From a mineral perspective, algae are an essential source of iodine, magnesium, iron and other essential micronutrients for many physiological functions. L’absence of these minerals in the diet can lead to metabolic problems, head and lateral line erosion (HLLE), immune dysfunction and other conditions. The vitamins and numerous bioactive compounds found in algae also play a key role in their health, promoting tissue regeneration and overall metabolism.

In a controlled environment such as an aquarium, it is essential to replicate the natural diet of herbivorous fish as closely as possible. Plant-based feeds specifically designed for them, enriched with algae, Spirulina, and Chlorella and combined with regular supplementation with fresh or freeze-dried algae, provide a solid foundation.

Likewise, macroalgae play an irreplaceable role in aquarium feeding. They provide not only the energy needed for daily activity, but also a range of useful molecules that support their overall health, metabolism, and well-being.

Among the most widely used macroalgae are Ulva, Porphyra, Palmaria, and Halymenia, each with unique nutritional characteristics and specific benefits. Among the less nutritionally interesting but readily available options are Caulerpa, Enteromorpha, and Chaetomorpha.

Ulva

Ulva lactuca, commonly known as sea lettuce, is probably one of the best-known and most widely used macroalgae. Thanks to its availability and balanced nutritional profile, it is an excellent choice for feeding herbivorous fish.

L'Ulva is particularly rich in fiber and protein, with a protein content that can range from 18% to 30% of its dry weight, depending on the time of harvest. The proteins in this seaweed are rich in essential amino acids, such as lysine and methionine, which are essential for fish metabolism.

Another interesting aspect of Ulva is the presence of complex polysaccharides, such as ulvan, which not only provides energy but also plays an important role in stimulating the immune system.

L's high content of vitamins, including vitamin C and some B vitamins, and essential minerals such as iodine, iron, and magnesium, makes this seaweed a complete and extremely versatile food. In addition, its soft texture and the ease with which it can be fed make it ideal for herbivorous fish.

Porphyra

This red alga is also valuable in the diet of algae-eating fish, thanks to its very high protein content, which can reach 40% of its dry weight. It is one of the most protein-rich macroalgae, with a complete and well-balanced amino acid profile, making it ideal for supplementing the diet of fish during periods of growth or increased activity.

Porphyra are also very rich in fiber, which supports intestinal health, and omega-3 fatty acids, such as EPA, which promote cell membrane health and reduce inflammation.

The presence of bioactive pigments, such as carotenoids and porphyrins, also gives it strong antioxidant properties.

Thanks to its composition, rich in minerals including iron, potassium and calcium, and vitamins A, C and K, this alga offers a complete range of essential nutrients and can be used to balance the micronutrient and bioactive molecule content of other macroalgae.

Palmaria

 

Palmaria palmata, also known as “Dulse”, is a red alga distinguished by its balanced nutritional profile, its high protein content, and its high palatability. In nature, this alga grows in cold waters, developing a soft structure that makes it easily digestible for algae-eating fish.

Nutritionally, Dulse is one of the most balanced macroalgae.

It contains a significant amount of protein, between 20% and 25%, with a complete amino acid profile.

It is rich in fiber and complex carbohydrates, which provide slow-release energy, but what makes it truly special is its high mineral content, including iodine and iron. These nutrients support vital functions in fish, such as metabolism and thyroid health. It is also one of the few algae to contain vitamin B12, which is especially useful for energy metabolism and red blood cell formation.

Halymenia

LHalymenia, often called "sea silk", is characterized by a soft, gelatinous texture. It is a red alga that grows in tropical and subtropical waters, and its delicate structure makes it an extremely appetizing food for herbivores. It can easily be cultivated in an aquarium to provide a high-quality food source.

Nutritionally, Halymenia stands out for its high carbohydrate content, which can exceed 50%, and for the presence of polysaccharides such as agar and carrageenan, known for their prebiotic properties. It also contains a fair amount of protein, about 10-15%, and is rich in potassium and iron, which support muscle function and energy production.

This alga is particularly suitable for fish such as Siganus and Zebrasoma, which enjoy its soft texture.

Enteromorpha

Species of Enteromorpha are filamentous green algae characterized by a tubular structure.

They also grow easily in aquariums and are especially prized for their high fiber content and carbohydrates. These algae provide about 20% fiber, essential for maintaining a healthy gut in herbivores, and a generous amount of complex carbohydrates, which can reach 50%.

An interesting feature of Enteromorpha is its ability to accumulate minerals such as calcium and magnesium, which help regulate osmotic balance and support the fish’s overall health.

Its tubular structure encourages herbivores to behave as they would in nature, nibbling on it directly from the substrate or clips. This alga is particularly suitable for species such as Ctenochaetus, which feed on filamentous algae and associated detritus.

Caulerpa

Caulerpa is a green macroalga that stands out for its rapid growth and versatility. Often used as a decorative alga or for nutrient control in aquariums (in specific tanks), it is also an excellent food source for herbivores.

Caulerpa provides a good amount of fiber (15-20%) and complex carbohydrates (40-50%), making it ideal for maintaining efficient digestion. It also contains phenolic compounds with antioxidant properties that help protect tissues from oxidative damage. The most common species, such as Caulerpa prolifera and Caulerpa lentillifera, are especially appreciated for their crunchy texture, which stimulates fish chewing behavior.

However, Caulerpa requires careful monitoring in an aquarium, as it can release toxins and its growth can easily become invasive if not properly controlled.

Chaetomorpha

Chaetomorpha is a filamentous green macroalga often used in refugiums for its ability to absorb excess nutrients such as nitrates and phosphates.

Although it is not highly digestible, its filamentous structure is particularly appealing to species such as Ctenochaetus strigosus and Paracanthurus hepatus. Although it is better known for its ecological and filtration roles, it is also a fairly interesting food source for herbivores.

Chaetomorpha is rich in fiber, with a content exceeding 20%, and contains about 40% carbohydrates, providing long-term energy without overloading the fish's metabolism. It is also a good source of minerals such as iron and iodine, which are crucial for metabolism and hormonal regulation.

Conclusion

Understanding the differences between herbivores and carnivores at the anatomical, physiological, and metabolic levels is crucial to ensuring the health of fish in an aquarium.

Herbivorous fish, with their unique adaptations, require careful, specific dietary management.

Only by replicating natural conditions and providing a balanced diet can we ensure these fish live long and healthy lives. Please do not underestimate this extremely important aspect.

The vast majority of problems in keeping surgeonfish are caused by an entirely unsuitable diet.

Hoping this brief overview will be useful to you, happy reefing, everyone.