Summer 2020

Document Type

Master's Thesis (Open Access)

Degree Name

Master of Science (M.S.)


Moss Landing Marine Laboratories


The Eastern Lau Spreading Center (ELSC), located between 19°20’S and 22°45’S, is a back-arc basin containing active hydrothermal vents, and is characterized by gradients of several geological and chemical variables that follow along a north – south axis. The northern section of the ELSC spreads faster than the southern section resulting in farther distance from the Tofua Arc and vent geology and chemistry more akin to mid-oceanic ridges. In the Southern section, where distance from the arc is less, substrates are more heavily influenced by water resulting in andesitic substrates as opposed to basaltic substrates in the north. There are also north to south biological patterns, where in the northern vent peripheral zone communities are dominated by anemones, and by sponges in the south. This project used a replacement type experiment to test whether the anemone and sponge community distributions are due to substrate preferences (either basalt or andesite) or location effects. In September 2006, 17 basalt and 18 andesite rock blocks were set out in the peripheral vent zone at three locations in the Lau Basin; 42 months later, in May 2009, the rocks were recovered. After collection and identification (to lowest taxonomic level) of all invertebrates, rock block surface area and roughness was determined. This report found that while the basalt rock blocks used were smaller and had smoother surface roughness than andesite rocks, both rock types were evenly distributed throughout each location. Through this conclusion, rock type was determined not to affect the invertebrate community and that there was some other location specific effect responsible for the invertebrate community, possibly sulfide distribution. Southern locations had more brecciated substrates, allowing for more sulfide to distribute farther laterally, resulting in more primary productivity, and thus an increase in taxa belonging to groups Copepoda, Polychaeta, and Gastropoda. Specifically, southern sites showed higher densities of the copepod Amphiascus sp. and from the family Dirivultidae, the molluscs Lepetodrilus sp. and from the family Sutilizonidae, and polychaetes from the families Serpulidae, Ampharetidae, and Hesionidae.