A sea pen field in shallow water in the Amakusa Islands, southern Japan

Abstract

Sea pens are ecologically important habitats for associated marine organisms, serving as ecosystem engineers in sandy or muddy seafloor environments. In such areas, sea pens can form habitats with high population densities known as sea pen fields . However, the presence and importance of sea pen fields have not been well studied in shallow waters in East Asia. Here, we report a sea pen field of Virgularia sp. aff. gustaviana in the shallow waters of Ushibuka Marine Park, in the Amakusa Islands of southern Japan. The average colony numbers of the field across all depths (7-20 m) was 10.3 colonies/m 2 (live colonies) to 13.6 colonies/m 2 (all: live+dead colonies+holes), and the area of the sea pen field was at least ∼50,000 m 2 . At a depth of 15 m, the substratum consisted of sand and fallen leaves of terrestrial origin, and the highest sea pen density was observed (averages=17.2 live colonies/m 2 , =25.8 total (live+dead+holes) colonies/m 2 ). At a depth of 20 m, the substratum consisted of broken shells and rocks and had the lowest density (live colonies: average=0.8 colonies/m 2 , all: average=1.0 colonies/m 2 ). There were significant differences in colony number of Virgularia sp. aff. gustaviana between the sand , sand+leaves , and broken shells/rocks substrates. We hypothesize that the strength of the water currents caused by local geographic features and tidal movements produce suitable sedimentation and habitat for this species of sea pen. Therefore, we suggest that preserving the natural coastline is crucial to protect this and other sea pen fields in shallow waters and their benthic marine communities.

Exploring the trends of adaptation and evolution of sclerites with regards to habitat depth in sea pens

“…Similarly, Virgularia aff. gustaviana , which also has no sclerites, is an ecosystem engineer forming sea pen fields with remarkably high densities ( Kushida et al, 2020 ). By not producing sclerites and saving energy, faster growth rates may increase colony numbers in sandy or muddy bottoms, although there may be higher predation risks due to the lack of sclerites.…”

“…For example, sea pen fields have been designated as Vulnerable Marine Ecosystems (VMEs) by the Northwest Atlantic Fisheries Organization NAFO and as “Threatened and/or Declining Habitat” by the Oslo and Paris Conventions (OSPAR) ( e.g. , Kushida et al, 2020 ). As the biomineralization of octocorals is an important factor in create their biodiversity, understanding the mechanisms of sea pens’ ecological traits, including their roles as ecosystem engineers via more basic and ecological studies are urgently needed.…”