Japanese brown seaweed (Sargassum muticum) – an alga with a migration background
Fig. 2: Underwater shot of several thalli of Japanese brown seaweed (Sargassum muticum) floating on the surface of the water in a protected tidal channel of the rocky Helgoland Wadden Sea coast. Photo: Sami Domisch, Alfred Wegener Institute
Description
The brown seaweed Sargassum muticum originates from Pacific waters. Since the mid-1980s, this species is a member of the benthic communities in wave-protected shallow-water habitats on the rocky shores of the island of Helgoland as well as in mussel and oyster banks in the Wadden Sea. With the help of a disc-shaped holdfast this rapidly growing alga attaches to hard surfaces on the seafloor. Here, S. muticum grows to a length of up to four metres within a summer while in the tropics it can reach a length of twelve metres. S. muticum has small, berrylike gas-filled bladders (pneumatocysts, Fig. 1) to absorb light as effectively as possible for gaining energy by photosynthesis. These bladders gave the alga its name and support its upright position in the water. Moreover, pneumatocysts allow the upper parts of the alga to float at low tide on the water surface (Fig. 2). When the first autumn storms set in, S. muticum is torn off, except for the holdfast and a few stems measuring 20 to 40 centimetres in length, which sprout again in the following spring (pseudo-perennial lifecycle). The torn-off algal “branches” drift with the oceanic currents and serve to disperse the algae, thus significantly boosting the rapid spread of the species.
The influence of the newcomer Sargassum muticum on native species diversity
Fig. 2: Wadden Sea herring use Japanese brown seaweed as cradles by attaching their whitish spawn to the algae “branches”. The gas-filled pneumatocysts are identifiable as brown berries. Photo: Christian Buschbaum, Alfred Wegener Institute
Foto: Christian Buschbaum
The arrival of foreign species may lead to dramatic changes in the interactions among species in communities that had developed over many centuries. This happens, for example, because newcomers do not have any natural enemies and can thus spread unchecked and suppress native species. As a consequence of such “biological invasions”, the new species may, among other things, cause considerable changes in species diversity.
This interrelationship was demonstrated for S. muticum off the West Coast of North America. Staff members of the Wadden Sea station on Sylt and of the Biologische Anstalt Helgoland in the Alfred Wegener Institute for Polar and Marine Research conducted experiments at both islands to determine the extent to which the presence of S. muticum has had an impact on the species diversity of the resident benthic communities. The studies indicate that just as many (rocky Helgoland coast) or more (Sylt Wadden Sea) animals and algae live in the “branches” of S. muticum as in native species of algae. Particularly the snake pipefish Entelurus aquoreus and sea stickleback Spinachia spinachia related to seahorses profit greatly from the existence of Japanese brown seaweed since it provides additional habitat. The Japanese brown seaweed is also used as a spawning substrate by native fish species, such as the herring, which sticks its eggs onto the algae (Fig. 1). Furthermore, field experiments on Helgoland showed that communities under the canopy of S. muticum did not differ from communities that live under the canopy of a “cousin” of the Japanese brown seaweed, the native sea oak (Halidrys siliquosa), neither in terms of composition nor diversity of species. These studies demonstrate that foreign species are not necessarily harmful for preservation of the native animal and algae world from the outset, but can definitely unfold a positive impact.