Introduction

Earth’s oceans are full of life—they are home to many different species. Certain locations in the ocean, such as coral reefs, have a high diversity of species, meaning many different types of organisms live there. In the Mediterranean Sea, seagrass meadows are well-known diversity hot spots—but there are others! High diversity is essential for an ecosystem’s health and stability. Healthy coastal ecosystems provide food and shelter for fish species, including species that humans eat, so it is important that the diversity of ocean ecosystems be maintained. We investigated a less-studied type of ocean habitat to understand its diversity—beds of red macroalgae. We were surprised to find that these habitats are thriving with marine organisms such as sea stars, anemones, and tube worms! In the remainder of this article, we will explain why macroalgae are important habitats, and we will focus on the diversity of one fascinating group of inhabitants—the tube worms.

Sessile Marine Animals Need Space!

In the ocean, we can find many animals that have evolved to live attached to the ocean floor or some other surface—they are called sessile animals. Unlike animals found on land, sessile animals cannot move around during their adult life. Corals are the most well-known example of sessile marine animals, but other examples are barnacles and sponges. The high number of sessile animals in the ocean requires a lot of free space for them to settle down. This can create a space shortage, and as a result, any free area will quickly be colonized by sessile animals. This free space can be anything, from rocks and man-made structures like jetties or ships to other living organisms like plants and macroalgae.

A Special Kind of Marine Worm

Worms that live in the ocean, called marine worms, come in all shapes and sizes. One group of worms, called annelids, is characterized by their segmented bodies. This means that their bodies consist of a long chain of similar parts—like the wagons of a long train. Sessile annelids called tube worms attach themselves to a surface when they are larvae, and they cannot move around once they become adults. These worms build solid tubes around themselves for protection against predators. Since they cannot move around, they filter tiny food particles from the passing currents with delicate, feather-like tentacles called radioles (Figure 1). Two things are very important for tube worms: an excellent spot to attach to and access to passing water currents so they can filter out enough food. The perfect home for a tube worm is a water-current-exposed surface, like the leaves of seagrasses or the bodies of marine macroalgae that grow upright in the water [1].

The Red Algae Apartment Complex

Red algae are photosynthetic organisms that can use sunlight to produce energy to grow—like plants do on land. Macroalgae species, including red algae, form dense mats and provide habitats for many species (Figure 2). Their leaves and bodies provide living space and can also change the environment for their inhabitants. For example, they keep out predators or strong water currents [2], just like a real house! Because of their ability to shape the environment, these species are called ecosystem engineers [3]. Well-known examples of ecosystem engineers on land include trees, which create homes for birds, and beavers, which create ponds with their dams.

Red algae mats function like giant apartment buildings with all kinds and sizes of flats. At the bottom of the mats, closer to the ocean floor, water movement is reduced, and it is much darker than it is in the top layers [2]. The leafy areas on top of an algae mat (Figure 3A) provide more prominent places for settlement, and they are exposed to water currents. Red macroalgae grow only a few centimeters per year and can live more than 2 years [4], so they form relatively stable long-term habitats. As a result, every animal species can find an apartment tailored to its needs, and a diverse community can develop.

When we began our study, red algae mats had not been studied very much. We were surprised to find many groups of sessile organisms living in red algae mats, each in high abundance (numbers) and with high diversity. Since tube worms are such fascinating animals, we wanted to learn more about them and about their relationship with the red algae.

The Inhabitants of the Algae Mats

To look at the importance and health of the habitat formed by red algae, we investigated several factors: the total number of tube worms present, the number of different species of tube worms that lived there (diversity), and how many tube worms of each species were there (abundance).

We found high numbers of tube worms inside the 5 cm thick mats of red macroalgae. The numbers we found were similar to those found in another well-studied tube worm habitat: seagrass mats. However, we found more types of tube worm species on the macroalgae, meaning tube worm diversity was higher in the macroalgae compared to seagrass. We also looked at the abundance of each different species that we found. We observed that only a few tube worm species were very abundant on the seagrass. However, on the macroalgae, many different species of tube worms were present in large numbers (Figure 3). These results show that macroalgae mats are a valuable habitat for tube worms.

In a healthy and stable habitat, the distribution of different species is more balanced than in a damaged habitat [4]. The more even distribution of tube worm diversity on the red macroalgae shows that the community is probably more resilient to any disturbances, like pollution or climate change. That makes it less likely that a rare group of tube worms will disappear completely.

Conclusion

Many species—including tube worms—are losing their homes because of human activity near the coast, and some habitats have already disappeared. For example, the seagrass in the Mediterranean Sea is in danger because of climate change and water pollution [5]. Larger sessile animals, like Mediterranean corals, have died off after warming events in the last few years [6]. Macroalgae have been observed to take over damaged areas in tropical reefs, but this shift is often accompanied by a loss of diversity loss [7], meaning the ecosystem becomes less healthy and resilient. But our studies showed that macroalgae could provide homes for many different species—creating a habitat with similar or even higher diversity than previously known habitats like seagrass. Red algae mats have not been studied very much, and we were surprised to find such large numbers of tube worms and so many other animal species. While environmental changes generally lead to loss of habitat and decreased diversity, macroalgae mats might be a hidden gem of diversity, giving us hope that natural richness can still survive in unexpected locations. There is still a lot to discover!

Glossary

Diversity: ↑ A measure of the variety of species in a habitat. Considering the number of different species and their abundances.

Macroalgae: ↑ Large marine organisms that can perform photosynthesis, like plants do on land.

Sessile: ↑ Attached to a surface and unable to move around. Sessile animals typically have a filtering mechanism to get food, since they cannot graze or hunt for prey.

Annelids: ↑ Worms that have a body built by many similar sections, called segments, and therefore look like they have rings around their bodies. The earthworm is a well-known annelid.

Segmented: ↑ In biology, this means a body is divided into a series of similar parts. Like many passenger wagons on a train.

Radioles: ↑ Feather-like tentacles that help marine worms to catch food and to take up oxygen from the water.

Ecosystem Engineers: ↑ A species that creates or modifies a habitat. These species may have huge impacts on the diversity of an area.

Abundance: ↑ The number of individual organisms per area.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

The authors would like to thank Jenny Tuček and Mischa Schwarzmeier (IfMB), Reiner and Regina Krumbach (Campese Diving Center) for logistical support throughout our study. We also thank Susann Roßbach for providing helpful feedback on the manuscript and Anette Reh for support in sampling activities. This study was supported by baseline funding of the Marine Ecology Department, University of Bremen, and the Institute for Marine Biology (Campese, Italy). MB received funding via the ERASMUS+ program.