Eisenia bicyclis bed coverage off Oshika Peninsula, Japan, in relation to sporophyte survival and Strongylocentrotus nudus abundance

Abstract

It has recently been reported that the Eisenia bicyclis beds on the coast of Tomarihama in the Oshika Peninsula in Miyagi Prefecture (northern part of Japan) have been progressively receding from the offshore side of the beds every year. In this study, to determine the factors causing the change in the distribution of E. bicyclis beds as well as the extent of the change, an ecological survey of E. bicyclis and grazing animals was conducted. A 150 m-long fixed line was set up in the rocky coastal area on the Oshika Peninsula in Miyagi Prefecture, and the number of E. bicyclis individuals inhabiting a 1-m width was then measured. In addition, the numbers of individuals of sea urchins (Strongylocentrotus nudus) and abalone (Haliotis discus hannai) were measured. Although the E. bicyclis bed extended to the 64-m point on the fixed line at the time of the survey in August 1999, by July 2001, its lower limit was at the 53-m point; thus, a retreat of the outer edge of the bed of 11 m was observed in 2 years. The density of adult E. bicyclis sporophytes in the bed was reduced from 1.08 to 1.80 individuals (ind.) m −2 , respectively. Juvenile sporophytes were also observed, but most of those that had settled near the lower limit of the bed failed to survive to become adult sporophytes. There were few sea urchins, the main grazing animal on the site, inside the E. bicyclis bed (3.5 ind. m −2 in October 2001) while there were many near the bed lower limit and on the deep-site side (12.9 ind. m −2 in October 2001). These results indicate that the high grazing pressure of sea urchins reduced the survival of juvenile sporophytes near the bed lower limit, thus impeding the recruitment of juvenile sporophytes to replace adult sporophytes that died due to senescence, storm action and/or grazing.

Effects of macroalgal expansion triggered by the 2011 earthquake and tsunami on recruitment density of juvenile abalone Haliotis discus hannai at Oshika Peninsula, northeastern Japan

“…Before the earthquake and tsunami, grazer‐resistant crustose coralline algae dominated in the CCA, and the recruitment of canopy‐forming algae was limited by the grazing pressure of the sea urchin M. nudus (Fig. c) as described by previous studies conducted at the same site (Sano et al ., ; Muraoka, ). While after the earthquake and tsunami, previously unrecorded recruitments of juvenile E. bicyclis were observed in the CCA (Fig.…”

“…The CCA areas can be maintained by the grazing activities of the herbivores, especially sea urchins. The strong grazing pressure of the sea urchin M. nudus , which densely inhabited CCA areas before the tsunami, removed macroalgae including new macroalgal recruits on CCA surfaces leaving the grazing resistant CCA (Sano et al ., ; Muraoka, ). Therefore, the large‐scale removal of the sea urchin because of the tsunami disturbance was also predicted to change the spatial distribution of the algal communities (Takami et al ., ; Kawamura et al ., ; Muraoka et al ., ).…”

“…We also monitored possible factors affecting abalone recruitments such as changes in the distribution pattern of abalone with its growth associated with algal communities, and impacts of the tsunami on the structure of algal communities in addition to the densities of adult abalone and sea urchin. At the monitoring site Tomarihama, which is the nearest coast from the epicenter of the earthquake, many ecological studies dealing with dynamics of benthic grazer‐algal interactions (Kawamura et al ., ; Yamada et al ., ; Taniguchi, ; Sano et al ., ; Muraoka, ), trophic structures of benthic animals (Won et al ., , , ) and recruitment of the abalone H. discus hannai (Sasaki and Shepherd, ) have been done since 1987. Based on these profound ecological findings and the direct effects of the tsunami disturbance described above, we aim to examine the drivers of recruitment variability in H. discus hannai .…”

Effect of Water Flow on Grazing by the Sea Urchin (<i>Strongylocentrotus Nudus</i>) in the Presence of Refuge Habitat

“…They provide suitable habitats for many commercial fishes and benthic animals [1,3,4]. However, subtidal macrophyte habitats around the world have declined due to human pollutants [5,6], overgrazing of sea urchins [4,[7][8][9] and herbivorous fishes [10], as well as natural disturbances [11][12][13]. Sea urchins are important agents of disturbance and are frequently regarded as proximate determinants of community structure and abundance of macroalgal habitats [14].…”

“…In Vestfjorden, northern Norway, overgrazing by the sea urchin Strongylocentrotus droebachiensis has resulted in the decrease of large kelp forests and has remained a barren ground dominated by crustose coralline algae [7]. Losses of Eisenia bicyclis habitat in Miyagi Prefecture, Japan, have been ascribed to the heavy grazing by the sea urchin Strongylocentrotus nudus [9].…”