An apicomplexan parasite drives the collapse of the bay scallop population in New York

Abstract

The bay scallop, Argopecten irradians, represents a commercially, culturally and ecologically important species found along the United States' Atlantic and Gulf coasts. Since 2019, scallop populations in New York have been suffering large-scale summer mortalities resulting in 90-99% reduction in biomass of adult scallops. Preliminary investigations of these mortality events showed 100% prevalence of an apicomplexan parasite infecting kidney tissues. This study was designed to provide histological, ultrastructural and molecular characteristics of a non-described parasite, member of the newly established Marosporida clade (Apicomplexa) and provisionally named BSM (Bay Scallop Marosporida). Molecular diagnostics tools (quantitative PCR, in situ hybridization) were developed and used to monitor disease development. Results showed that BSM disrupts multiple scallop tissues including kidney, adductor muscle, gill, and gonad. Microscopy observations allowed the identification of both intracellular and extracellular stages of the parasite. Field surveys demonstrated a strong seasonal signature in disease prevalence and intensity, as severe cases and mortality increase as summer progresses. These results strongly suggest that BSM infection plays a major role in the collapse of bay scallop populations in New York. In this framework, BSM may synergistically interact with stressful environmental conditions to impair the host and lead to mortality.

Figure 1

Schematic representation of the experimental workflow for parasite (Bay Scallop Marosporida) characterization and seasonal monitoring.

Figure 2

Peconic Bay in Long Island, New York. The location icon indicates the Orient Harbor sampling site.

Figure 3

Micrographs of histological sections presenting healthy (A) and infected (B and C) scallop kidneys (H&E stain). (C) represents a high magnification capture of the area pointed in B and underlines the extensive disruption of kidney tubules caused by heavy BSM (Bay Scallop Marosporida, black arrowheads) infection. ec: epithelial cell, lnt: lumen of nephridial tubule, nt: nephridial tubule.

Figure 4

Micrographs of histological sections presenting BSM infection in different scallop tissues using H&E stain (A, C, E, G and I) and in situ hybridization (B, D, F, H, and J). (A) and (B): kidney, (C) and (D): gills, (E) and (F): adductor muscle, (G) and (H): ovary, (I) and (J): testis. The black arrowheads indicate parasite cells.

Figure 5

Micrographs of unstained (A to F) and stained (with DAPI, G and H) fresh kidney preparations. Red and black arrowheads represent single and dividing BSM, respectively. Letters indicate kidney cell membrane (kcm), kidney concretions (kc).

Figure 6

Micrographs of the most common BSM forms (A) found in histological section of bay scallop kidney (H&E stain). (B) Microgamont. Letters indicate kidney cell nucleus (kcn), kidney cell membrane (kcm), macrogamont (ma), macrogamont nucleus (n), macrogamont nucleolus (nu), microgamont (mi), trophozoites (t).

Figure 7

Ultrastructure of the most common BSM forms found in scallop kidney. (A) and (B): general views of kidney cells infected with multiple parasite cells. Different presumptive life stages are visible: early trophozoites (white arrowheads), growing trophozoites (red arrowheads) and gamonts (black arrowheads); (C) and (D): early trophozoites present an ellipsoidal form, contain a large mitochondrion (mt) posterior to the nucleus (n), and remnant components of the apical complex (ac), all hallmarks of the infectious sporozoite life stage that precedes the growing trophozoite stage; (E): a growing trophozoite presenting a round form and containing multiple vacuoles; (F): details of a macrogamont (sexual form) containing a large nucleus (n), several mitochondria (mt) and multiple granules. ag: amylopectin granule; dg: dense granule, hcn: host cell nucleus, kc: kidney concretion, pv: parasitophorous vacuole, nu: nucleolus.

Figure 8

Ultrastructure of the BSM apical complex (A to C) and apicoplast (D and E). (A): general view of an early trophozoite with a visible apical complex (ac). (B) and (C) represent enlargement of the boxed areas in (A) and (B), respectively. Note the presence of micronemes (mi), rhoptries and conoids (co). (D): general view of an early trophozoite presenting an apicoplast (ap). (E): Larger magnification of the apicoplast surrounded by four membranes pointed out by the white arrowheads. ag: amylopectin granule; dg: dense granule, mt: mitochondria.

Figure 9

Phylogeny of the bay scallop Marosporida. (A). Maximum-likelihood phylogenetic tree based on 18S rRNA gene sequences (1,742 nucleotide sites) reconstructed with evolutionary model GTR + F + I + G4, showing the position of BSM with closely related members of the Apicomplexa. Grey circles indicate that the corresponding nodes were recovered in the reconstructed tree based on the maximum-parsimony algorithm; black circles indicate that the corresponding nodes were recovered based on both the neighbor-joining and maximum-parsimony algorithms. Bootstrap values (> 50%) are listed as percentages at the branching points. Bar: 0.1 substitutions per nucleotide position. GenBank accession numbers for 18S rRNA genes are indicated in parentheses. (B). The maximum-likelihood phylogenetic tree based on the amino acid alignments of 22 proteins from apicoplast genomes showing the position of BSM in relation to closely related members of the Apicomplexa. Phylogenetic analysis was performed with the LG + F + I + G4 model using 4,485 amino acid positions. Bootstrap values (> 50%) are listed as percentages at the branching points. Black circles indicate that the nodes were recovered based on both the neighbour-joining and maximum-parsimony algorithms. Bar: 0.2 amino acid substitutions per site. GenBank accession numbers for apicoplast sequences are indicated in parentheses.

Figure 10

BSM infection intensity by fresh preparation score (A) and qPCR (B) in scallops collected from Orient Harbor between May 3 and August 19, 2021. Polyserial correlation between both detection techniques is presented in (C) where the Maximum Likelihood Estimator (MLE) is given. Asterisks indicate significant differences between May and all other sampling dates (P-values are given for the ANOVA on ranks in A and parametric ANOVA on Log10 transformed values in B).

Figure 11

Monthly temperature anomalies calculated from 2007 to 2022 (USGS Orient Harbor Station 01,304,200). Warmer anomalies are presented in red while colder anomalies are in blue. The recurrent scallop outbreaks were first noted in summer 2019, and are systematically associated with warmer spring/summer temperatures.