Chasing transparent needles in a moving haystack!

by Michael Blackett,
Sir Alister Hardy Foundation for Ocean Science

Jellyfish represent a bewilderingly diverse array of zooplankton that play an often under appreciated role in planktonic ecosystems.

When conditions are favourable, jellyfish can grow and reproduce extremely quickly. Voracious predation by dense seasonal blooms can influence the structure of the planktonic community and restrict energy transfer to higher trophic levels.

Physophora hydrostatica © Mike Blackett, SAHFOS

Unfortunately, we know surprisingly little about the biology and ecology of these ubiquitous yet enigmatic creatures. This makes predicting the causes and consequences of their blooms challenging. On this cruise we have deployed the ‘jelly-net’, which is designed to carefully sample these fragile organisms. This has allowed us to examine the jellyfish species assemblage and collect specimens for molecular genetic analysis back on terra firma.

Unlike the highly sophisticated LOPC, mentioned earlier in the bog, the jelly-net samples do require tedious hours behind the microscope. Taxonomic identification requires an extensive knowledge of jellyfish anatomy and is challenging at best. The task is even more difficult on a ship in the North Atlantic. Imagine trying to find needles in a haystack, only the needles are transparent and the haystack is moving with the pitch and roll of the boat!

Rathkea ocptopunctata ©Mike Blackett, SAHFOS

Positive identification of the species encountered means we can validate the images captured by the VPR (see entry Flying through microspace) and investigate the fine-scale vertical distribution of discrete components of the jellyfish community that exhibit highly variable behavioural and functional ecology.

Molecular genetic characterisation allows us to explore the genetic similarity of geographically isolated populations, unravel phylogenies and evolutionary lineages and produce ‘genetic barcodes’ for the identification of specimens based on their genetic makeup.

Since the beginning of Leg II of this expedition, it is remarkable to see how the jellyfish communities have developed. An explosion of new recruits and reproductive stages has succeeded the few characteristic winter species as the jellyfish begin to exploit the intense productivity of the spring bloom.