Hyperpigmentation in North Sea dab Limanda limanda. I. Spatial and temporal patterns and host effects

Abstract

Hyperpigmentation is a term describing a specific pigment anomaly affecting common dab Limanda limanda in the North Sea and, less frequently, in adjacent areas, e.g. the English Channel, Irish and Celtic Seas, western Baltic Sea and Icelandic waters. Other North Sea flatfish species are also affected, but at a markedly lower prevalence. The condition is characterised by the occurrence of varying degrees of green to black patchy pigment spots in the skin of the upper (ocular) body side and pearly-white pigment spots in the skin of the lower (abocular) body side. In the course of fish disease monitoring programmes carried out by Germany and the UK (England and Scotland), a pronounced spatial pattern of hyperpigmentation has been detected in the North Sea. An increase in prevalence has been recorded in almost all North Sea areas studied in the past 2 decades. The prevalence recorded in hot spot areas of the condition increased from 5 to > 40% between 1988 and 2009. Analysis of the German data indicates that the prevalence and intensity (degree of discolouration) of hyperpigmentation increase with size and age, indicating a temporal progression of the condition with size and age. Intense hyperpigmentation is associated with increased growth (length) and decreased condition factor. Potential causes of the condition (UV-B radiation nutrition, water temperature increase, demographic changes) and, in particular, of the spatial/temporal patterns recorded as well as the relationship to host-specific factors (sex, age, length, growth, condition factor) are discussed.

Hyperpigmentation in North Sea dab Limanda limanda. II. Macroscopic and microscopic characteristics and pathogen screening

“…However, when Stentiford et al (2010) investigated the age at first occurrence of several disease conditions affecting dab in the North Sea and the Irish Sea where the smallest fish sampled were in the range of 10 to 14 cm (the majority of these being 1 yr old), the first occurrence of hyperpigmentation in North Sea dab was found to be at 3 yr . Similarly, data from a different area of the North Sea recorded the first occurrence of hyperpigmentation in 2 yr old dab, the smallest with 12 cm but higher prevalence observed in >15 cm fish (Grütjen et al 2013). These results together with our own data, where higher prevalence of hyperpigmentation was always observed in individuals greater than ~15 cm (data not shown), suggest that the factors involved in the genesis of this condition must therefore be more complex.…”

“…experimentally exposed to moderate UV-B radiation, 30% more melanin was recorded in the skin compared to the non-exposed control group (Häkkinen et al 2002). Data from our study, analysing grade 2 and 3 fish, and from other groups in different areas of the North Sea also suggest that the higher pigmented categories are more prevalent among larger fish (Grütjen et al 2013). Interestingly however, the data suggest that higher pigmented fish have a lower condition factor (Grüt-jen et al 2013).…”

“…Hyperpigmentation in common dab shows an apparent increasing trend in areas of the North Sea (Grütjen et al 2013) and clearly involves hyperplasia of dermal melanophores and iridophores. We found no evidence of specific association with microorganisms or parasites by light or electron micro scopy or by standard bacteriological and virological assessment.…”

“…Fish displaying hyperpigmentation were sorted into 3 groups representing the different severity grades ( Fig. 1; described in Grütjen et al 2013), based on the number and spread of pigment spots on the ocular side according to the Biological Effects Quality Assurance Monitoring Programme (BEQUALM; www. bequalm.org).…”

“…The histopathology and ultrastructure of the changes are described, and other potential non-infectious origins are discussed (ontogenic disturbance, UV radiation, nutrition and environmental factors). Grütjen et al (2013) provide information on spatial and temporal characteristics of the phenomenon in the North Sea and adjacent areas.…”