Spotted Wolffish Broodstock Management and Egg Production: Retrospective, Current Status, and Research Priorities

Novel Application of Non-Invasive Methodological Approaches in Biomedical Sciences Towards Better Understanding of Marine Teleost Ocular Health and Disease

Seafood is an important resource for global nutrition and food security, with both land and marine aquaculture playing pivotal roles. High visual acuity is key for health and survival of farmed, cultured, and wild fish. Cleaner fish technology to control parasite infestation has become important in marine aquaculture and highlights the importance of visual acuity in the efficacy of cleaner fish species. New clinical diagnostic approaches towards understanding and optimising fish visual health could benefit both aquacultured and wild fish populations. Opportunities for developing and using advanced non-invasive clinical assessment and diagnosis of ocular health in wild, cultured, and experimental fish are key to more rapidly realising how threats to eye health in these animals might be better understood and mitigated. Ophthalmoscopy can rapidly and non-invasively image anatomical aspects of retinal and anterior ocular tissues and has been used in mammalian biomedicine since the turn of the 20th century. More now than ever, labour-intensive post-mortem approaches for ocular analysis such as histology are increasingly being replaced or supplemented by application of various forms of optical coherence tomography (OCT) imaging of ocular tissues in mammalian biomedicine. Advances and availability of other methodological approaches such as three-dimensional printing and computer science make instrument customisation affordable and adaptable. This review article will outline how ophthalmoscopy, OCT, and other methodologies are being applied towards understanding ocular health in teleost fish species and will describe some of the future opportunities that technological advances might afford in advancing ocular imaging in fish health and disease in general.

Aquaculture potential of Atlantic wolffish (Anarhichas lupus): stress and physiological responses to acute handling

The Atlantic wolffish (Anarhichas lupus) is a cold-water species with the potential to diversify aquaculture in Northern countries. Few studies have investigated the stress physiology of Atlantic wolffish, and the current knowledge on stress in wolffish species is largely derived from the closely related spotted wolffish (Anarhichas minor). In the current study, Atlantic wolffish were exposed to handling stress mimicking common husbandry conditions in aquaculture such as repeated air exposure and net-chasing. Samples were taken prior to stress exposure (pre-stress; control) as well as 5- and 24-h post-stress. A series of primary and secondary acute stress response parameters were assessed: plasma cortisol, glucose and lactate levels, hematological indices (hemoglobin, Hb; hematocrit, Hct; mean corpuscular hemoglobin concentration, MCHC), and osmoregulatory capacity through plasma osmolality and gill Na+/K+ATPase (NKA) activity. Other secondary stress responses with implications for fish health and welfare are intestinal integrity and transport functions. These parameters were assessed using the Ussing chamber technique. The cortisol peak values were low in comparison to other fish species studied after acute handling stress and occurred as late as 24 h post-stress, suggesting that Atlantic wolffish is a slow and low cortisol responder. Plasma glucose remained stable, whereas lactate concentrations significantly decreased between 5 and 24 h after stress. There was no effect on pH, Hb, or Hct, although a significant increase in MCHC was found after 5 h and 24 h, originating from a small increase in Hb. This result suggests a minor increase in Hb synthesis after stress exposure. The intestinal integrity and transport functions as well as gill NKA-activity remained unchanged after stress exposure. In conclusion, Atlantic wolffish appears to exhibit a relatively moderate stress response, characterized by a slow and low primary stress response and minimal secondary effects following husbandry-related acute stress. These findings contribute to the understanding of the species' potential for development as a candidate for marine, cold-water aquaculture.

High protein requirements of juvenile Atlantic wolffish, Anarhichas lupus: Effects of dietary protein levels on growth, health, and welfare

The objective of the present study was to investigate the optimal dietary protein requirement and the effect of varying protein levels on the growth and health of juvenile, wild-caught Atlantic wolffish, Anarhichas lupus, a promising candidate for cold-water aquaculture diversification. Six iso-energetic (ca. 18.3 MJ kg-1), fish meal-based experimental diets were formulated with crude protein levels ranging from 35% to 60%, with graded increments of 5% in a 12-week feeding trial in a recirculating aquaculture system (RAS). Weight gain, specific growth rate (SGR), and condition factor (K) were evaluated in response to dietary protein levels. Liver, muscle, and blood parameters were assessed for possible changes in protein and lipid metabolism and welfare. Overall growth was highly variable throughout the experiment on all diets, as expected for a wild population. The feed with highest in protein (60%) inclusion resulted in the highest growth rates, with an average weight gain of 37.4% ± 33.8% and an SGR of 0.31% ± 0.2% day-1. This was closely followed by feeds with 55% and 50% protein inclusion with an average weight gain of 22.9% ± 34.8% and 28.5% ± 38.3%, respectively, and an SGR of 0.18% ± 0.3% day-1 and 0.22% ± 0.3% day-1, respectively. Fish fed the high protein diets generally had increased hepatic lipid deposition (17%-18%) and reduced free fatty acid levels (3.1-6.8 μmol L-1) in the plasma relative to fish that were fed the lower protein diets (35%-45%). No effects of diet were found on plasma protein levels or muscle protein content. Furthermore, stress parameters such as plasma cortisol and glucose levels were unaffected by diet, as were plasma ghrelin levels. Overall, these results suggest that a high protein inclusion in the diet for Atlantic wolffish is required to sustain growth with a minimum protein level of 50%.

Unravelling spermatogenesis in spotted wolffish: Insights from the ultrastructure of juvenile male testes to the cryopreservation of broodstock sperm

The aim of this study was to deepen our understanding of the reproductive biology of male spotted wolffish (Anarhichas minor) using two different experimental approaches involving juvenile and mature broodstock fish. The first approach consisted of a detailed histological examination of the testes to identify the onset of gonadal maturation and characterise the spermatogenic stages in two- and three-year-old juvenile specimens. Light microscopy analysis revealed clear differences between the age groups. Two-year-old fish displayed well-defined interstitial tissue, Sertoli cells and cysts housing spermatogonia stem cells in which meiosis had not yet begun. In contrast, three-year-old fish exhibited cysts containing spermatocytes, spermatids and abundant spermatozoa, indicating the initiation of the spermatogenic cycle, albeit with asynchronous puberty. Histochemical staining revealed a significant presence of smooth myoid cells in the interstitial tissue of sexually mature fish, while electron microscopy further revealed synaptonemal complexes indicating the onset of meiosis and centriolar structures that gave rise to flagella. The second approach focused on optimising semen freezing and cryopreservation procedures in mature broodstock individuals over the age of 10 years. Seven freezing extenders (KT, TS-2, OP, MT, MH, HBSS, or SR), with seawater (SW) as a control, were assessed along with two cryoprotectants dimethylsulfoxide (DMSO) or methanol to evaluate their impact on pre- and post-thaw semen quality. Results showed that the MT and HBSS extenders were superior in total sperm kinetics at 1:3 dilution, and that DMSO showed optimal results in sperm motility and velocity variants. Moreover, the MT and HBSS groups demonstrated consistent sperm viability after cryopreservation, with values similar to fresh samples. Based on the viability results of the SYBR-green-14/PI assay comparing fresh and cryopreserved sperm using MT and HBSS, the MT extender emerged as the most effective freezing medium for cryopreservation of spotted wolffish broodstock sperm. In conclusion, this study provides a comprehensive understanding of the reproductive dynamics of male spotted wolffish, offering valuable insights for both scientific research and aquaculture management.

Increased water temperature contributes to a chondrogenesis response in the eyes of spotted wolffish

Adult vertebrate cartilage is usually quiescent. Some vertebrates possess ocular scleral skeletons composed of cartilage or bone. The morphological characteristics of the spotted wolffish (Anarhichas minor) scleral skeleton have not been described. Here we assessed the scleral skeletons of cultured spotted wolffish, a globally threatened marine species. The healthy spotted wolffish we assessed had scleral skeletons with a low percentage of cells staining for the chondrogenesis marker sex-determining region Y-box (Sox) 9, but harboured a population of intraocular cells that co-express immunoglobulin M (IgM) and Sox9. Scleral skeletons of spotted wolffish with grossly observable eye abnormalities displayed a high degree of perochondrial activation as evidenced by cellular morphology and expression of proliferating cell nuclear antigen (PCNA) and phosphotyrosine. Cells staining for cluster of differentiation (CD) 45 and IgM accumulated around sites of active chondrogenesis, which contained cells that strongly expressed Sox9. The level of scleral chondrogenesis and the numbers of scleral cartilage PCNA positive cells increased with the temperature of the water in which spotted wolffish were cultured. Our results provide new knowledge of differing Sox9 spatial tissue expression patterns during chondrogenesis in normal control and ocular insult paradigms. Our work also provides evidence that spotted wolffish possess an inherent scleral chondrogenesis response that may be sensitive to temperature. This work also advances the fundamental knowledge of teleost ocular skeletal systems.