Neopilina sp.: Tracking Monoplacophorans on the ocean floor.

The Monoplacophorans are a poorly understood group of Molluscs superficially resembling Limpets, but quite distinct from the Gastropods. Like Gastropods they have a muscular foot, a shell and a scraping radula (tongue), but unlike them they lack eyes (or much of a head at all) have gills on each side of their body, which form a series of pairs, and their shell is attached to their body by eight pairs of dorso-ventral muscles. Monoplacophorans are known in the fossil record from the Cambrian to the Devonian, and for a long time were thought to be extinct, till a live specimen was recovered from the deep sea in 1952. Since more than 35 species of Monoplacophorans have been described, all from the deep oceans, although only two species have been encountered more than once, and almost nothing is known about their habitat or ecology.

In a paper published in the journal Marine Biodiversity on 7 March 2018, Julia Sigwart of the Marine Laboratory at Queen’s University Belfast, and the Museum of Paleontology at the University of California, Berkeley, Mary Wicksten of the Department of Biology at Texas A&M University, Matthew Jackson of the Department of Earth Science at the University of California Santa Barbara, and Santiago Herrera of the Department of Biological Sciences at Lehigh University, report the first direct observations of what appears to be a new species of Monoplaophoran from seamounts in American Samoa.

Sigwart et al. report that non-periodic, sinusoidal trails clear of sediment were observed on ferromanganese-encrusted lava flows were observed at a depth of 3760 m on the Leoto Seamount and 3837 m on the Utu Seamount on the Manahiki Plateau while they were being explored by the remotely operated vehicle (ROV) Deep Discoverer and the camera platform Seirios, operated from the NOAA Ship Okeanos Explorer, during an expedition to the area between February and April 2007. Further examination of these trails revealed a Monoplacophoran trail-maker on the Utu Seamount.

Trackways attributed to Neopilina sp. in still images from HD video. (a) Overview of rock surface marked with feeding trails at Leoso Seamount (where no animals were observed). (b) Trackways at Utu Seamount; red dots are laser sights 10 cm apart. (c) Neopilina sp., at Utu seamount, close-up showing apparent radular scratches within trackway. Original video data from the NOAA Office of Ocean Exploration and Research. Sigwart et al. (2018). 

The trail-maker is thought to be a member of the genus Neopilina, and measures about 20 mm in length and about 12 mm in width, exceptionally large for a modern Monoplacophoran, even a member of the large-bodied genera Adenopilina, Vema, and Neopilina (prior to this only six modern specimens larger than 10 mm in length had been observed, all from depths of greater than 3000 m. However, the observed specimens differ from other previously described members of the genus in several ways, most notably the presence of longer, more slender gills. This is unsurprising, as the nearest previous sighting of a member of the genus was from Hawaii, 4000 km away, and leads Sigwart et al to conclude that this is a new species, though they stop short of describing it as such, due to the absence of any recovered material. In living on ferromanganese-encrusted lava flows, this Monoplacophoran inhabits an ecological niche where the group has not previously been observed, though other species have been recovered from ferromanganese nodules dredged from the seafloor (most known modern Monoplacophorans have been lifted from the seafloor on some sort of object). As such if opens the possibility that Monoplacophorans are much more abundant than previously realised, as seamounts and lava flows cover much of the deep ocean floor.

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Cyclone Gita reaches New Zealand.

Cyclone Gita made landfall on South Island, New Zealand, on Tuesday 20 February 2018, bringing flooding and high winds to the area around Christchurch in the Canterbury Region. There are no reports of any casualties associated with the storm in New Zealand at this time, though it has caused flight cancellations and several people have had to be rescued from floods. 

Debris on a swollen river near Bainham in the Tasman Region of South Island. Reuters.

Cyclone Gita first formed to the southeast of the Solomon Islands in the first week of February 2018. It passed to the south of Samoa and American Samoa between 8 and 11 February, causing flooding in both groups of islands, and wind damage in parts of American Samoa. The storm passed directly over Tonga on 12 February, causing extensive damage, to the islands' infrastructure, damaging over a thousand buildings, and totally destroying over a hundred, including the nation's parliament building. One person is known to have died in Fuaʻamotu on Tongatapu (the largest island of Tonga and seat of the national government), with at least 30 more being injured, three of them seriously. After Tonga Cyclone Gita passed to the south of Fiji on 13 February, again causing flooding and wind damage, but this time no reported casualties.

Storm damage on Tonga following the passage of Cyclone Gita. John Pulu/AFP.

Tropical storms are caused by solar energy heating the air above the oceans, which causes the air to rise leading to an inrush of air. If this happens over a large enough area the inrushing air will start to circulate, as the rotation of the Earth causes the winds closer to the equator to move eastwards compared to those further away (the Coriolis Effect). This leads to tropical storms rotating clockwise in the southern hemisphere and anticlockwise in the northern hemisphere.These storms tend to grow in strength as they move across the ocean and lose it as they pass over land (this is not completely true: many tropical storms peter out without reaching land due to wider atmospheric patterns), since the land tends to absorb solar energy while the sea reflects it.

 

The passage of Cyclone Gita till 6.00 pm GMT on Monday 19 February 2018  (thick line) with its predicted future path (thin line, circles represent the margin of error on the predictions). Colours indicate the strength of the storm. Tropical Storm Risk.

 

The low pressure above tropical storms causes water to rise there by ~1 cm for every millibar drop in pressure, leading to a storm surge that can overwhelm low-lying coastal areas, while at the same time the heat leads to high levels of evaporation from the sea - and subsequently high levels of rainfall. This can cause additional flooding on land, as well as landslides, which are are a common problem after severe weather events, as excess pore water pressure can overcome cohesion in soil and sediments, allowing them to flow like liquids. Approximately 90% of all landslides are caused by heavy rainfall.

 

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Magnitude 6.8 Earthquake between Tonga and Samoa.

The United States Geological Survey detected a Magnitude 6.8 Earthquake at a depth of 10 km, about 86 km to the northeast of the island of Niuatoputapu (sometimes known as Traitor's Island or Keppel Island) at about 10.00 pm local time on Friday 3 November 2017 (about 4.00 am on Saturday 4, GMT). While this was a large quake, it was unlikely to present any danger due to its distance from shore, though it was felt on the islands of Upolu in Samoa and Tutuila in American Samoa.

The approximate location of the 3 November 2017 Tonga Earthquake. USGS.

The islands of Tonga and Samoa lie on opposite sides of the boundary between the Pacific and Australian Tectonic Plates. The Pacific Plate is being subducted beneath the Australian Plate along the Tonga Trench, which forms the boundary between these two plates, with the volcanic islands that make up the Tonga archipelago being formed as the subducting plate is melted by the heat of the planet's interior, so that lighter minerals rise up through the overlying plate as liquid magma, which fuels the volcanoes that build the islands. This subduction is not a smooth process, with the two plates constantly sticking together until the pressure generated by their movement builds up sufficiently to break them apart, causing a sudden forward jerk we experience as an Earthquake.

Diagram showing subduction along the Tonga Trench, and how this feeds the volcanoes of the Tonga Volcanic Arc. York University.

Witness accounts of Earthquakes can help geologists to understand these events, and the structures that cause them. The international non-profit organisation Earthquake Report is interested in hearing from people who may have felt this event; if you felt this quake then you can report it to Earthquake Report here.

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