Hydrozoan jellyfish: a great many unknowns

6. April 2018

By Dr Anna Kintner, Analytical Services Manager, Europharma Scotland Ltd

When non-scientists and fish farmers talk about jellyfish, they’re usually thinking of the large-bodied species that you see at the beach in summertime – these are the scyphozoan jellyfish that most of us are familiar with. Fish farmers usually notice when this kind of bloom occurs, and major blooms that have serious consequences for fish farms are fortunately rare.

Lately, it’s become clear that we might need to take a much closer look at a different kind of jellyfish.

Hydrozoan jellyfish are much smaller than their scyphozoan cousins (many only about 0.5-10 mm) and aren’t heavily pigmented, so their blooms can be functionally invisible – the only way to spot them at a fish farm is to take water samples and look for them with a microscope. We don’t know how often hydrozoan blooms occur, and we don’t know much at all about what causes them. That means that we often don’t know they’re happening at all until fish start showing signs of disease. It’s likely that disease outbreaks have occurred in the wake of a jellyfish bloom without anyone realizing that a bloom took place.

All jellyfish species have stinging cells on their tentacles called nematocysts, which trigger on contact with prey and can puncture delicate tissues in the gills and gut when they are inhaled or swallowed. Some nematocysts contain micro-doses of venom. A few stings are no big deal, but they can add up in a high-density bloom as the gills are physically injured and further inflamed due to venom toxins.

In northern Norway, at the western side of the Ryggefjord, four mass mortalities of farmed salmon took place between 2002 and 2015. Norwegian biologists have suggested that the culprit may be a hydrozoan jellyfish, Dipleurosoma typicum, which was found blooming locally. This species also occurs in Scotland and is known to occasionally form blooms, but this is the first time it has been identified as a risk to salmon aquaculture.

A few other species have been implicated in similar incidents. In 2008, researchers in Shetland found disease caused by the bacterium Tenacibaculum maritimum in salmon gills after a bloom by a hydrozoan jellyfish called Phialella quadrata, and more recent blooms by Obelia geniculata and Lizzia blondina jellyfish have been followed by outbreaks of amoebic gill disease. It seems safe to say that the gill damage incurred during a jellyfish bloom makes fish vulnerable to secondary infections by microbial pathogens. Research in its early stages now suggests that some pathogenic bacteria may even be part of the natural jellyfish microbiome, opening the possibility that jellyfish could act as vectors for infection.

Hydrozoan jellyfish have been woefully under-researched for a long time, and there are a lot of frequently asked questions about them that we simply don’t have answers to. When and where do blooms occur, and what causes them? Are some sites more vulnerable than others? What will happen to jellyfish populations as the climate changes? And most importantly, especially as the industry grows, what can we do to mitigate the damage that hydrozoan jellyfish blooms can do?

Moving forward, there is a need for increased awareness of jellyfish at a farm site level so that managers can begin to watch out for them. Dedicated hydrozoan monitoring would have a couple of positive outcomes for the industry.

First, if fish farmers check daily whether a jellyfish bloom is taking place, they can respond immediately and start to take measures to help fish recover. We don’t need new technology to start doing this – we can do it right now with some basic personnel training.

Second, if fish farmers are doing these checks, scientists and veterinarians can start to build up datasets to track species and patterns of jellyfish blooms. In the future, a good understanding of jellyfish bloom biology could help in risk-assessing new fish farm sites, understanding the spread disease, and ultimately, forecasting blooms ahead of time.

However, all of this depends on making sure this research happens – without it, we’re likely to keep being unpleasantly surprised when jellyfish turn up to the party.

Dr Anna Kintner is the Analytical Services Manager at Europharma Scotland Ltd and conducted her PhD research into jellyfish population dynamics and their effects on salmon health at the University of St Andrews.