String Jellyfish <i>Apolemia</i> spp. (Most Likely <scp><i>A. uvaria</i></scp>) Causes Serious Tissue Damage in Farmed Fish

There has been an increasing number of observations of string jellyfish Apolemia spp. (most likely A. uvaria) in recent years, and during the autumn of 2023, the high occurrence became a serious challenge for fish farmers along the entire Norwegian coast. Several fish farmers reported skin and gill damage on fish exposed to string jellyfish, and in some farms, all fish had to be destroyed or slaughtered due to animal welfare reasons (Sommerset et al. 2023). Apolemia uvaria (order Siphonophorae, family Apolemiidae) is a pelagic colonial jellyfish consisting of several individuals attached to a long chain with a length of up to 30 m (Båmstedt et al. 1998). Observations along the Scandinavian coast have predominantly been of incomplete colonies of 1–2 m (Figure 1A) (Båmstedt et al. 1998). In turbulent seas or encountering physical barriers, the A. uvaria can break into smaller fragments that can enter the fish cages. Apolemia uvaria belongs to phylum Cnidaria and is equipped with specialised cells called cnidocytes containing nematocysts (Figure 2A). Nematocysts consist of a capsule containing a collagenous harpoon-like tubule and venom, which is used for predation, defence and locomotion (Clinton et al. 2021; Beckmann and Özbek 2012). The nematocysts can be located on the entire outer surface (epidermis) but are particularly numerous in the parts of the jellyfish which have tentacles and are specialised for feeding (Clinton et al. 2021; Fautin 2009; Siebert et al. 2013). Following mechanical or chemical stimuli of the sensory hairs (cnidocil) of the cnidocyte, the tubule is expelled from the nematocyst in a harpoon-like manner which can puncture the tissue of the prey and deliver venom (Clinton et al. 2021; Beckmann and Özbek 2012). The venom is known to cause tissue damage in the skin, gills, eyes and gastrointestinal tract of fish (Clinton et al. 2021). Experimental challenges with another jellyfish, Aurelia aurita, showed epithelial loss, necrosis and haemorrhages in the gills already 2 h after exposure. Chronic lesions after 3 weeks were extensive epithelial hyperplasia with fusion of lamellae (Baxter et al. 2011). Jellyfish stings may also stress the fish and lead to panic, abnormal behaviour and reduced appetite (Båmstedt et al. 1998). In November and December 2023, Pharmaq Analytiq received organs for histopathological analysis from 32 cases where jellyfish, suspected to be A. uvaria, had been observed either in or around fish cages. In 10 of the cases, bacterial cultivation from the kidney and, in some cases skin, was performed on blood agar with salt and marine agar. Thirty cases (152 individuals) were from Atlantic salmon (Salmo salar L.) and two cases (10 individuals) were from rainbow trout (Oncorhynchus mykiss Walbaum). Most of the cases were from production areas 2, 3 and 4 (western Norway), but several other production areas were also represented. Formalin-fixed samples of specimens suspected to be A. uvaria collected from affected farms were prepared for histological examination with the aim to increase knowledge of the anatomy of the string jellyfish and to be able to identify structures from the jellyfish in tissue sections of fish submitted for histological examination. The mortality was in some cases very high. Moribund fish were observed standing vertically in the water, with signs of respiratory distress. Some individuals were apparently struggling with the equilibrium. Poor appetite was often reported. The dominating macroscopical findings were wounds, often described as burn marks, on the skin, at the mouth and fins (Figure 1B,C). There were haemorrhage and clouding of the eyes. The gills appeared swollen and fringed, often with small haemorrhages, pale filament tips and areas with necrotic tissue. Several areas in the sections of the string jellyfish suspected to be A. uvaria were interpreted as transverse sections of the organism with an inner endoderm and outer ectoderm containing structures representing nematocysts (Figure 2B,C). These structures had a round shape with an approximate diameter of 15 μm and a refractile capsule. Some nematocysts contained eosinophilic content or tubule-like structures (Figure 2B,C), whereas others appeared empty. These transverse sections of the A. uvaria could represent tentacles or gastrozooids as these are known to be rich in nematocysts. In most of the cases, only mild to moderate occurrences of non-specific changes such as lamellar vascular damage were detected. In samples from rainbow trout, only mild changes were detected. In the most severely affected gills, there were focal to multifocal changes mainly as either (1) acute to subacute tissue changes dominated by necrosis of lamellae and filament (Figure 3A–D) or (2) chronic damage with epithelial cell hyperplasia and lamellar vascular damage, purulent inflammation and extensive fibrin effusion extending to the filament (Figure 3E,F). In cases where necrosis was the dominant finding, filamentous (Tenacibaculum-like) bacteria were often found associated with the lesions. Empty round structures with a diameter of about 15 μm and a distinct, refractile capsule were detected in seven cases (Figures 3 and 5), and most of them were associated with tissue damage. In the skin samples (both salmon and rainbow trout), the main lesions were epithelial loss and necrosis of the dermis (Figure 4A,B) with large amounts of filamentous (Tenacibaculum-like) bacteria in the dermis. In other cases, there were skin ulcers with the detection of filamentous bacteria or a mixture of filamentous and short-rod bacteria. Most of the cases (both salmon and rainbow trout) had other diagnoses than injuries due to jellyfish, which is common in farmed fish with chronic and multifactorial problems. One third of the cases had gill pathology associated with known infectious agents: Paramoeba perurans, Candidatus Branchiomonas cysticola, Desmozoon lepeophterii and salmon gill poxvirus. In addition, there were cases with proven or suspected systemic bacterial infection, including pasteurellosis, mycobacteriosis and infection with Moritella viscosa. Other problems such as cardiac pathology, nephrocalcinosis, parvicapsulosis and mycotic nephritis were also detected. Different species of vibrio (Alivibrio wodanis, Vibrio splendidus, Vibrio sp.) and other opportunistic and possibly environmental bacteria were isolated from wounds and/or kidneys in most cases. Moritella viscosa was isolated from the kidney in two cases, and Tenacibaculum finnmarkense was isolated from the kidney in two cases and from wounds in one case. In histological sections from formalin-fixed string jellyfish suspected to be A. uvaria, several nematocyst-like structures were demonstrated (Figures 2B, 2C and 5A). Similar structures were detected in histological sections from gills from our diagnostic material received during the late autumn of 2023 of fish suspected of encountering A. uvaria. These nematocyst-like structures were round with refractile capsules of approximately 15 μm, with and without contents (Figure 5B). In connection with some of these structures in sections from A. uvaria and gills, a tubule-like structure could be seen, which probably represents the discharged tubule. Based on the findings of nematocyst-like structures in both sections from formalin-fixated A. uvaria and gill tissue from cases with suspected exposure to A. uvaria in the anamnesis, we suggest that the histopathological changes in the gills result from interactions between fish and parts of the string jellyfish. Similar structures could not be detected associated with tissue damage in the skin. The increasing number of jellyfish observations along the Norwegian coast in the last years has resulted in serious fish welfare problems. Although we received many submissions with suspected exposure to A. uvaria, it is often difficult to make a definite diagnosis, especially when no structures from the jellyfish are observed in the histology section. To be fairly certain that tissue damage is caused by exposure to A. uvaria (or other jellyfish), histology samples should be taken shortly after exposure. The damages will develop into chronic changes that cannot be distinguished from other chronic gill pathology. Thus, it can be challenging as it is not always obvious that there has been A. uvaria at the farm, or whether there has been contact between A. uvaria and fish. In several cases, the histological picture was complex with histopathological changes that could be related to known gill pathogens or systemic bacterial infection. This makes the histopathological assessment challenging, especially in cases where no medical history is given. Furthermore, histopathological changes in the gills that are often related to jellyfish are nonspecific and could have other causes such as exposure to other irritating material in the water, mechanical damage after non-medicinal delousing and systemic bacterial infections. Of all histology submissions to Pharmaq Analytiq in November and December 2023 with the detection of histopathological changes that could be related to exposure to irritating material in the water, including jellyfish, suspicion of A. uvaria was mentioned in the anamnesis in 45% of the cases. The number of cases with A. uvaria may be underestimated, and a thorough and relevant anamnesis is essential to be able to put together clinical observations and histopathological changes and make a diagnosis. Damage to the skin and gills caused by jellyfish can lead to secondary bacterial infection, and especially infection with Tenacibaculum spp. has been reported (Ferguson et al. 2010; Delannoy et al. 2011; Småge et al. 2017). By histological examination, we demonstrated Tenacibaculum-like bacteria in the gills and/or skin in a total of 16 cases. T. finnmarkense was isolated from skin ulcers and kidneys in three cases, whereas in several cases, various pathogenic, opportunistic and possible environmental bacteria were isolated. There was a high incidence of string jellyfish suspected to be A. uvaria along the Norwegian coast during the autumn of 2023, and this together with concomitant and opportunistic infections led to extensive damage and high mortality in farmed fish. Here we have summarised our diagnostic findings, which show a clear association between parts of A. uvaria and tissue damage in gills. The tissue damage caused by A. uvaria can inflict on the fish's gills and skin and have serious consequences for the fish's health and welfare. There is a need for more knowledge about A. uvaria and other jellyfish in the fish farming industry. Mette Hofossæter: conceptualization, investigation, writing – original draft, visualization, writing – review and editing, formal analysis. Marta Alarcón: conceptualization, investigation, writing – original draft, visualization, writing – review and editing, formal analysis. Helene Wisløff: conceptualization, investigation, writing – original draft, visualization, writing – review and editing, formal analysis. The authors would like to express gratitude to Herman Høgenes Kvinnsland for pictures of farmed fish exposed to A. uvaria, to farmers and fish health personnel for submitting samples and to colleagues at Pharmaq Analytiq for processing samples in the lab. The authors declare no conflicts of interest. The data that support the findings of this study are available from the corresponding author upon reasonable request.