Looking like a spread of seagrass at first glance, bryopsis, and its more than 60 species, is an algae that falls under the bryopsidaceae family.

Like all species on the planet, Bryopsis play an important ecological role in their balanced ecosystems; however, these species are referred to as pests among aquarium enthusiasts and are commonly associated with green tides and algal blooms.

Let’s take a general look at bryopsis, where they can be found, how they spread, and how they fit into our ever-changing ecosystem.

Bryopsis Taxonomy

  • Kingdom: Plantae
  • Division: Chlorophyta
  • Class: Ulvophyceae
  • Order: Bryopsidales
  • Family: Bryopsidaceae
  • Genus: Bryopsis

Common Species: 

  • Bryopsis hypnoides
  • Bryopsis muscosa  
  • Bryopsis pennata
  • Bryopsis pennatula 
  • Bryopsis plumosa 


From a distance, or at a glance, bryopsis look somewhat like underwater grass, but at closer inspection, you will notice small fern-like fronds instead of grass-like blades. Bryopsis is a soft, bushy sea algae that grows in loose clumps of green and dark green feathery leaves.

Although the algae grows in small “bushes,” it is not likely to branch but, instead, sprout new fronds from its anchoring rhizoids. That said, because of the wide variety of bryopsis species, individuals can be difficult to identify, and their physical appearance can differ. For example, the bryopsis hypnoides, and bryopsis plumosa are both part of the bryopsis family; however, as indicated below, appear to be very different.

Bryopsis-plumosabryopsis hypnoides
Bryopsis plumosa (left) vs. Bryopsis hypnoides (right)

Left image credit: B.Navez; Right image credit: Vishal Bhave – Flickr


Bryopsis is, for the most part, an epilithic species, meaning it can usually be found growing on the surface of a rock or rock-like substance; however, the algae can also be found in the free-floating form.

The species of this fern-like algae genus thrive within a wide range of habitats and are often found inhabiting seaweed beds, fringing reefs, and most conditions of sub-tidal areas.

Bryopsis can be found widely spread among subtropical and tropical regions, where they dominate eutrophic coastal regions. It is also within these highly nutrient-dense areas that some of these algae species become a problem due to overpopulation.


Bryopsis species can reproduce by either asexual reproduction, such as fragmentation or aplanospores, or through sexual reproduction, where both a male and female gamete is required. Not all species of bryopsis reproduce sexually; however, they can all spread through asexual means.

Predators and Threats

Bryopsis grows fast and would, in theory, provide a wonderful feeding ground for our underwater herbivores, but thanks to its natural production of Kahalalide F, many fish and potential predators are deterred.

The biggest predator of bryopsis is sea slugs, sea snails, and nudibranchs. Slugs, such as the sacoglossa, are popular feeders on bryopsis as they can take in the toxins produced by the chloroplast of the plant.

Slugs are the biggest predator of bryopsis, as they can take in the toxins produced by the chloroplast of the plant
Slugs are the biggest predator of bryopsis, as they can take in the toxins produced by the chloroplast of the plant

The chloroplast is pushed to the wavy extensions of the slug’s body, where it photosynthesizes when exposed to light, similar to the way Sea sheep photosynthesize. This makes an otherwise toxic plant an explosive bomb of energy for these tiny crawlers.

Foxfaces and Rabbit fish have also been reported to feed on the algae by aquarium keepers; however, these could be isolated cases due to unnatural environments.

Ecological Impacts and Pests

Algae thrive in well-lit areas with high exposure to sunlight and in water that is dense in nutrients. Bryopsis species are no different and can quickly take over coastal areas when there is a lack of predators or an unbalanced increase in nutrients.

When Bryopsis grows uncontrollably, it can form dense mats that smother native plants and animals
When Bryopsis grows uncontrollably, it can form dense mats that smother native plants and animals

This increase in nutrients (particularly nitrogen) can be caused by various factors such as:

  • The removal of plants and invertebrates that consume high amounts of nitrogen
  • Removing predators, thus upsetting the ecological balance and allowing different species to dominate
  • Through nutrient runoff from farms and cities.

The process of adding nutrients to regions to a point where it impacts the ecology of the area is known as nitrification, and it is this that is the main contributor to green tides.

As bryopsis is chemically protected and therefore has few natural enemies, this, as well as its quick-growing and easily reproduced nature, makes these algae a possible threat to ecological balances along coastal regions.

When nutrients get too high, the algae begins to grow at an exponential rate, quickly covering large areas of rock and coral reef. This rapid spread of algae blocks out the sun, which prevents photosynthesis, and in turn, kills off any diversity of plant life that lives beneath. With no plant diversity, fish diversity decreases, and so the problem continues.

This quick spread of bryopsis is well known to aquarium keepers and is seen as a major pest to these fish-tank carers. Because fish tanks are well-lit and tend to be nutrient-rich, the algae thrive and can quickly destroy a once beautifully replicated landscape. Take this to a large scale, such as in the ocean, and it’s clear how small algae can become a huge problem when we interfere too much.

Controlling Bryopsis

Reducing nutrient-rich runoff, preventing rapid changes in plant and animal diversity, and less effectively – introducing natural predators are all ways to prevent the overgrowth of bryopsis along a coastal region.

In an artificial setting such as with home aquariums, chemical treatments are often the go-to, as the environment is easier to control, but the algae spreads faster.

Certain invertebrates, such as sea urchins or snails, can help to graze on the algae, controlling its growth
Certain invertebrates, such as sea urchins or snails, can help to graze on the algae, controlling its growth

Adding natural bryopsis predators to your tank is also useful to help prevent an algal bloom from spiking before it becomes an issue.


Is Bryopsis invasive?

Bryopsis species are potentially invasive. As they produce chemical deterrents similar to that of killer algae, they are not preyed on by many species and therefore stand the threat of overpopulation.

Does Bryopsis have human value?

Alage and similar plants are often used in the cosmetic industry, and bryopsis is no different, but where its true human-use potential comes through Kahalalide F, which ironically is the chemical that’s keeping so many species away from this tiny sea fern. Kahalalide F is currently under clinical trials for its potential use in fighting several types of cancer and psoriasis.

Are Bryopsis and hair algae the same thing?

Although many people use the two names interchangeably, and at first glance, both appear to be identical, hair algae and bryopsis are two different things. Hair algae most commonly include species that fall under the Oedogonium genus, while bryopsis is a genus upon itself. Hair algae are typically shorter than bryopsis, with a shallower root system that is easier to remove.

How to treat Bryopsis in a reef tank?

Prevention is always better than a cure, which is why it’s a good idea to ensure you are adding clean water to your aquarium, as well as sterilized features such as rocks and logs. Introducing natural bryopsis predators such as the slugs mentioned above is also a good preventative measure. That said, if you are already facing a bryopsis problem in your tank, the fish disease treatment – Fluconazole, has been found to treat the quick-spreading algae.

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