Reduced disease in black abalone following mass mortality: phage therapy and natural selection

Abstract

Black abalone, Haliotis cracherodii, populations along the NE Pacific ocean have declined due to the rickettsial disease withering syndrome (WS). Natural recovery on San Nicolas Island (SNI) of Southern California suggested the development of resistance in island populations. Experimental challenges in one treatment demonstrated that progeny of disease-selected black abalone from SNI survived better than did those from naïve black abalone from Carmel Point in mainland coastal central California. Unexpectedly, the presence of a newly observed bacteriophage infecting the WS rickettsia (WS-RLO) had strong effects on the survival of infected abalone. Specifically, presence of phage-infected RLO (RLOv) reduced the host response to infection, RLO infection loads, and associated mortality. These data suggest that the black abalone: WS-RLO relationship is evolving through dual host mechanisms of resistance to RLO infection in the digestive gland via tolerance to infection in the primary target tissue (the post-esophagus) coupled with reduced pathogenicity of the WS-RLO by phage infection, which effectively reduces the infection load in the primary target tissue by half. Sea surface temperature patterns off southern California, associated with a recent hiatus in global-scale ocean warming, do not appear to be a sufficient explanation for survival patterns in SNI black abalone. These data highlight the potential for natural recovery of abalone populations over time and that further understanding of mechanisms governing host-parasite relationships will better enable us to manage declining populations.

Keywords: Haliotis; abalone; endangered; histology; phage; rickettsial; selection; withering syndrome.

FIGURE 1

Exposure system used in challenge Trials 1 and 2. Abalone from SNI (black circles) and GC (gray circles) were held in replicate tanks (n = 5 in Trial 1 and 2 in Trial 2). The white circles represent tank of uninfected red abalone to test for tank independence in the recirculation system. Seawater flowed from the head tanks (H) holding either infected or uninfected abalone into the exposure tanks. Subsequently seawater flowed into the square sumps and through a series to filters, UV-sterilization and heating systems prior to being returned to the head tank.

FIGURE 2

Mortality curves for infected abalone in four categories. The curves (mean values and 95% confidence intervals) are based on a parametric survival fit analysis with Weibull function. For figure simplification, we did not nest infection within trial. This model was consistent with statistics reported from the proportional hazards model.

FIGURE 3

Kaplan–Meier survivorship curves for (A) RLO-exposed and (B) control (unexposed) black abalone from Trials 1–3 in which abalone from Carmel and San Nicholas Island (SNI) were exposed to three RLOs (WS-RLO, ST-RLO, and RLOv) in Trial 1, two RLOs (WS-RLO and ST-RLO) in Trial 2, or one RLO (WS-RLO) in Trial 3. Survival trends for abalone from the three collection sites are represented by dotted lines (SNI), solid lines (Carmel) and dashed lines (Año Nuevo Island).

FIGURE 4

Flow chart illustrating the relationship among abalone host, WS-RLO parasite, and phage hyperparasite of WS-RLO. Arrows indicate the flow of the effect with an increase in the response denoted with a “+” and a reduction in the response shown by a “-”. WS-RLO exposure results in infection of the posterior esophagus (PE) (A), which leads to metaplasia in the digestive gland (DG) (B). Metaplasia, in turn, provides more target tissue for the WS-RLO to invade and infect this organ, which together result in dysfunction of the DG and catabolism of the pedal muscle as an energy source (C). Visible atrophy of the pedal muscle becomes apparent in the withered and lethargic abalone at the end-stage of disease (D). Phage infection of the WS-RLO (RLOv) reduces the amount of active WS-RLO, thereby reducing metaplasia and DG-RLO, which in turn reduces pedal atrophy and mortality (E). Magnifications: (A) 100×, (B,C) 40×, (D) 1×, (E) 400×.