My current location is the furthest from an ocean or coast that I have lived in my adulthood, and this thought bothers me occasionally. For a young Houstonian, Galveston Bay and the Gulf of Mexico were close enough to allow frequent swimming, sailing, beachcombing, and birdwatching trips. Sometimes, however, the swimming was off limits or dangerous due to large numbers of Portuguese man-of-war jellyfish (Physalia physalis), which are colonial siphonophores with a bluish gas-filled “float”. The stinging tentacles can be many yards in length, and while most of the time a human victim escapes with one or more painful red welts, a close encounter with Physalia has the potential to result in death.

In oceans worldwide, jellyfish numbers have increased as a result of climate change, eutrophication, and overfishing (read more below), and at the same time, the Great Pacific Garbage Patch has expanded, along with the distribution of marine and beach pollution. Therefore, I’ve embarked on a project that combines issues of altered ocean ecosystems and the problem of plastic pollution, by creating representations of different cnidarian and ctenophore species with plastic bag yarn. The first in the series is Physalia physalis, shown below hanging on my front porch, crocheted with yarn made entirely from plastic newspaper wrappers (I don’t subscribe to the local newspaper, but other people save wrappers for me).

Because they reproduce both asexually and sexually, cnidarians can occur in very large numbers, as “blooms”. Reports of problems associated with jellyfish blooms (stings to swimmers, interference with fishing operations and aquaculture, clogging of seawater intake screens at power plants or desalination plants) have increased over the last two decades (Purcell et al., 2007). Numerous factors may contribute to these blooms; for example, the majority of cnidarian and ctenophore populations studied respond to warmer temperatures with increased numbers. Variations in current patterns and salinity may also contribute to blooms, as does cultural eutrophication along developed coastal regions and in areas that receive agricultural runoff. Eutrophication alters the composition of ocean food webs, causes turbidity, and is associated with low dissolved oxygen content – conditions that favor the survival of cnidarians and ctenophores, and are detrimental to many other marine animals. Both fishing and aquaculture also contribute to jellyfish blooms, by removing predators and competitors, and by providing favorable environmental conditions (Purcell et al., 2007).

Currently, ocean food chains are complex, with diatoms and dinoflagellates as primary producers, various crustaceans as secondary producers, and pelagic fish and whales as top predators. Nevertheless, eutrophication, overfishing, and climate change may lead to conditions that in many ways resemble the oceans of the Cambrian period, when phytoplankton cyanobacteria and flagellates predominated, and the “gelatinous species” (cnidaria and ctenophores) were the top predators. Richardson and colleagues (2009) refer to this as “the never-ending jellyfish joyride”, in which heavy fishing and disturbed marine ecosystems allow jellyfish to sequentially invade habitats where fish might have formerly controlled their numbers, thus creating the potential for a self-enhancing feedback and ultimately, a monoculture of jellyfish. However, there are an number of short- and long-term responses that could put the brakes on the jellyfish joyride (although not stop it completely), including use of cutting nets to destroy jellyfish, enforcing hull-cleaning, reducing overfishing, and use of biocontrol agents.

References:

Purcell JE, Uye S, Lo W-T (2007) Anthropogenic causes of jellyfish blooms and their direct consequences for humans: a review. Marine Ecology Prog. Ser. 350, 153-174 doi: 10.3354/meps07093

Richardson AJ, Bakun A, Hays GC, Gibbons MJ (2009) The jellyfish joyride: causes, consequences and management responses to a more gelatinous future. Trends in Ecology and Evolution 24, 312-322. doi:1.116/j.tree.2009.01.010