A Review of Asteroid Biology in the Context of Sea Star Wasting: Possible Causes and Consequences

Abstract

AbstractSea star wasting-marked in a variety of sea star species as varying degrees of skin lesions followed by disintegration-recently caused one of the largest marine die-offs ever recorded on the west coast of North America, killing billions of sea stars. Despite the important ramifications this mortality had for coastal benthic ecosystems, such as increased abundance of prey, little is known about the causes of the disease or the mechanisms of its progression. Although there have been studies indicating a range of causal mechanisms, including viruses and environmental effects, the broad spatial and depth range of affected populations leaves many questions remaining about either infectious or non-infectious mechanisms. Wasting appears to start with degradation of mutable connective tissue in the body wall, leading to disintegration of the epidermis. Here, we briefly review basic sea star biology in the context of sea star wasting and present our current knowledge and hypotheses related to the symptoms, the microbiome, the viruses, and the associated environmental stressors. We also highlight throughout the article knowledge gaps and the data needed to better understand sea star wasting mechanistically, its causes, and potential management.

Figure A1.

Heat map representation of 18S and 28S rRNAs recovered from metagenomes targeting material <0.2 μm during longitudinal study of asteroid wasting progression in control tissues, artificial scars, and wasting lesion margins. Red hues indicate greater numbers of library reads associated with taxonomy; black indicates no reads associated with taxonomy. There were no operational taxonomic units (OTUs) that were consistently present in wasting tissues across individuals but that were also absent from asymptomatic tissues on the same animal at the same time or in artificial scar tissues. C, asymptomatic tissue; A, artificial scar tissue; N, wasting lesion margin; T0, experiment initiation; Tf, time at death or for controls the end of observation period; Ti, time of appearance of lesion. Data derived from Hewson et al. (2020a).

Figure 1.

Examples of sea star species affected by sea star wasting (SSW). The left column represents animals with no or with early symptoms. The right column represents animals with late symptoms. (A) Pycnopodia helianthoides; the right image shows terminal stages of SSW. Photo credit: Mark Nayer. (B) Asterias forbesi; the arrow on the left image shows a small lesion. (C) Pisaster ochraceus; the arrow on the left image shows a small lesion. Photo credit: Melissa Miner. (D) Orthasterias; the arrow on the left image shows a small lesion. Photo credit: Feiro Marine Life Center (left).

Figure 2.

Progression of gross lesions in the aboral side of sea star wasting in Asterias forbesi. (A) Arm tip of an affected sea star showing a progression from healthy-looking tissue (chevron, top right) to broad areas of discoloration and loss of cuticle and epithelial tissue (narrow arrow) and severe ulcers leading to loss of tissue at the tip of the arm and exposure of underlying ossicle and the tip of the radial canal (broad arrow). (B) Close-up of early-stage lesion with small pinpoint white lesions (broad arrow) and early degradation of the spines. (C) Close-up of a more advanced lesion showing further development of pinpoint white lesions (broad arrow) and spine degradation. (D) Close-up of the tip of the arm in (A) showing advanced lesions with total loss of epithelium (narrow arrow) and exposure of the underlying tissues and ossicles and the tip of the radial canal (broad arrow).

Figure 3.

Arm twisting is a behavior that can be present in both healthy and wasting sea stars and, thus, cannot always be attributed to sea star wasting (SSW). (A, B) Pycnopodia helianthoides. (A) shows the wound, and (B) shows how the non-wasting sea star covers this wound by twisting its arms. (C) Pisaster ochraceus. The twisting behavior is here observed in a sea star that showed no other signs of SSW. Photo credits: (A, B) Julia Kobelt; (C) Mike Dawson.

Figure 4.

Early lesions of the dorsal epithelium of an Asterias forbesi individual with sea star wasting (SSW). (A) Vacuolation and necrosis of epithelial cells with influx of hemocytes (arrows) and disruption of the cuticle. (B) Edema of underlying connective tissues. (C) Necrosis of muscle and associated underlying connective tissues. Scale bars = 32 μm.

Figure 5.

Common lesions noted in the epidermis and dermis. On the left (Pisaster ochraceus), within the box, there is multifocal epidermal degeneration and lytic necrosis characterized by a loss of cell distinction, nuclear condensation, and fragmentation (karyorrhexis). On the right (Pycnopodia helianthoides), there is coagulation-type epidermal necrosis characterized by loss of cellular detail, particularly nuclear and cell margin distinction with increased uptake of eosin stain (pink staining) by the cytoplasm. Both types of lesions were associated with microscopic epidermal loss or ulceration in some specimens. The right image also shows the two most common lesions within the dermis: separation of the connective tissue fibers by edema and infiltrations of coelomocytes consistent with inflammation. A low instance of dermal degeneration and necrosis also occurred. Scale bars = 50 μm.