Infection of Norway lobster (Nephrops norvegicus) by the parasite Hematodinium sp.: insights from 30 years of field observations

Abstract

The Norway lobster, Nephrops norvegicus, is an important representative of the benthos and also supports valuable fisheries across Europe. Nephrops are susceptible to infection by Hematodinium sp., an endoparasitic dinoflagellate that causes morbidity and mortality. From an epizootiological perspective, the Clyde Sea Area (CSA; west of Scotland) is the best-studied Hematodinium-Nephrops pathosystem, with historical data available between 1988 and 2008. We have revisited this pathosystem by curating and updating prevalence values, differentiating host traits associated with disease exposure and progression, and comparing Hematodinium sp. disease dynamics in the CSA to other locations and to other decapod hosts (Cancer pagurus, Carcinus maenas). Prevalence from a 2018/2019 survey (involving 1739 lobsters) revealed Hematodinium sp. still mounts a synchronized patent infection in the CSA; hence this pathogen can be considered as enzootic in this location. We highlight for the first time that Nephrops size is associated with high severity infection, while females are more exposed to Hematodinium sp. More generally, regardless of the host (Norway lobster, brown and shore crabs) or the geographical area (Ireland, Wales, Scotland), Hematodinium sp. patent infections peak in spring/summer and reach their nadir during autumn. We contend that Hematodinium must be considered one of the most important pathogens of decapod crustaceans in temperate waters.

Keywords: decapod crustaceans; dinoflagellate; disease connectivity; fisheries; long-term datasets; marine parasite.

Figure 1.

(a) Hematodinium sp. patent infection levels (%) in Nephrops norvegicus according to the body colour (BCM) and pleopod (PM; stages 1–4 combined) diagnostic methods. (b) Percentage of infected Nephrops according to stage following the PM method, S1 being few parasites and S4 indicating pleopods are congested with parasites. Note that S0 refers to ‘no visual aggregation of the parasite’. Nephrops were captured from the CSA between November 2018 and October 2019.

Figure 2.

(a) Hematodinium sp. prevalence in Nephrops norvegicus according to the pleopod method (PM; stages 1–4 combined) available from 1990 to 2019 in males and females. (b) Prevalence according to the body colour method (BCM) available from 1989 to 2019 in males and females sampled from the CSA.

Figure 3.

Hematodinium sp. prevalence in Nephrops norvegicus according to the pleopod method (PM; stages 1–4 combined) recorded monthly in the CSA in the years 1992, 1999, September 2007–July 2008, November 2018–2019 in males and females combined.

Figure 4.

Carapace length (mm) of female and male Nephrops norvegicus infected with Hematodinium sp. according to the (a) body colour method (BCM) and (b) pleopod method (PM).

Figure 5.

(a) Sex ratio of Nephrops norvegicus; (b) Hematodinium sp. infection levels in males and females according to the body colour method (BCM)—continuous lines—and pleopod method (PM) (combining PM stages 1–4)—discontinuous lines; and Hematodinium sp. infection levels in (c) males and (d) females Nephrops according to the PM stages captured monthly from the CSA between November 2018 and October 2019.

Figure 6.

Spatio-temporal patterns of patent Hematodinium sp. infection of decapod crustaceans. Prevalence data (%) were averaged across seasons, autumn (Sept, Oct, Nov), winter (Dec, Jan, Feb), spring (Mar, Apr, May) and summer (Jun, July, Aug), for each decapod species and geographical area. Two datasets each from (a) the CSA (1992, 2007/2008) and (b) Ireland (2006) do not cover a full (continuous) 12 months of sampling. Inset: images of Nephrops norvegicus, Cancer pagurus and Carcinus maenas are placed next to their respective sample sites.