Predation of newly settled Dungeness crab by juvenile sunflower sea stars

Sunflower sea stars (Pycnopodia helianthoides) are voracious, generalist predators that consume a wide variety of benthic invertebrates across intertidal and subtidal zones (Duggins, 1983; Shivji, 1983; Sloan & Robinson, 1983). Historically abundant from the northern Aleutian Island chain in Alaska to Baja, California, populations of P. helianthoides have declined precipitously (>94% across the range) following the outbreak of sea star wasting disease in 2013 (Hamilton et al., 2021). P. helianthoides is currently listed as Critically Endangered (IUCN, 2020) with estimated population losses numbering in the billions (Hamilton et al., 2021; IUCN, 2020). As one of the largest and fastest sea stars, P. helianthoides are capable of actively pursuing and capturing highly mobile prey, a trait that distinguishes them from many other sea stars. This capacity for active predation mirrors patterns seen in kelp forest consumers, which often derive substantial portions of their diet from both pelagic and benthic sources (Elliott Smith et al., 2021). While numerous studies have documented P. helianthoides trophic interactions and ecological roles (Burt et al., 2018; Galloway et al., 2023; Shivji, 1983), previous work has focused on adult stars, and the diets of juvenile P. helianthoides, and the consumption of pelagic-sourced production in general by this key predator remains unexplored. Here, we define “juvenile” P. helianthoides as stars with a radius of <10 cm, following Hodin et al. (2021). In June 2025, during a collection dive for P. helianthoides in Port Orford, Oregon, USA, we observed dense aggregations of recently settled Dungeness crab (Metacarcinus magister) juveniles, as previously described by Galloway et al. (2017). These recruits, based on their size, were likely settled from their life as pelagic zooplankton megalopae within days to weeks of this observation. At the time of the observation, the juvenile crabs were primarily concentrated in small sand pockets, between the rocky reef habitats where P. helianthoides were collected. Fourteen juvenile P. helianthoides (4.2–7.9 cm radius) were collected from Port Orford, Oregon, USA, on 1 June and 3 June 2025 from two locations: Port Orford Jetty (42.737689 N, −124.498104 W) and Tichenor Cove (42.738672 N, −124.504046 W). Dense aggregations of newly settled M. magister recruits were present at both sites, but no direct predation by P. helianthoides was observed while underwater. Once P. helianthoides were collected, they were placed in a cooler and transported to the Oregon Institute of Marine Biology, Charleston, Oregon, USA, which served as a temporary holding prior to their transfer to the Oregon Coast Aquarium in Newport, Oregon, USA, where they were going to be held for future experiments and ultimately released, as per our ODFW permits (no. 28855 [M. magister] and no. 28880 [P. helianthoides]). Once the P. helianthoides were removed from the cooler and placed into the holding tanks, multiple M. magister recruits were observed at the bottom of the cooler, including one live individual. As predation on juvenile crab by P. helianthoides has not been previously reported, this observation represents a novel trophic interaction. P. helianthoides were transported to Oregon Coast Aquarium, Newport, USA, on 4 June 2025. After sizing, weighing, and measuring each individual, they were separated into individual tanks and given four M. magister recruits (approximately 0.40–1.10 cm carapace width). Once M. magister individuals were captured by the P. helianthoides, a new crab was added to maintain a constant density of four M. magister per tank. The feeding trial lasted approximately 47 h, during which the 14 P. helianthoides individuals consumed a total of 145 M. magister recruits (Figures 1b and 2). This corresponds to an average of 10.4 recruits per individual over the trial, or roughly 5.2 per day. Consumption rates varied among individuals, ranging from 4 recruits (~2 per day) to 16 recruits (~8 per day) (Appendix S1: Table S1). This variation was associated with the radius of P. helianthoides, as prey consumption increased significantly with body size (Poisson GLM: β = 0.187, p = 0.025). Although the recruits were markedly faster than P. helianthoides, the sea stars were able to capture them by extending their arms to corner the prey and seizing them with tube feet or pedicellariae, small, pincer-like structures on the aboral side (Figure 1a). Our observation captures a previously unreported but potentially ecologically consequential moment where a benthic predator directly consumes newly settled recruits soon after they have arrived from the plankton, providing a clear example of cross-ecosystem subsidies which link pelagic-derived prey to nearshore benthic food webs (e.g., Zuercher & Galloway, 2019). Like benthivorous reef fishes, which can derive up to 98% of their diet from pelagic sources (Docmac et al., 2017), P. helianthoides may similarly access recently settled crab recruits as a highly concentrated energy input during the juvenile life stage for both organisms. These observations and feeding trials provide the first documented evidence that juvenile P. helianthoides actively consumes recently settled M. magister. P. helianthoides is known to sometimes consume various adult “unknown” crab species (Duggins, 1983; Herrlinger, 1983; Mauzey et al., 1968; Shivji, 1983). There are also a few observations of adult P. helianthoides eating n = 1 Cancer crabs, including Cancer oregonensis (Paul & Feder, 1975), Cancer productus (Appendix S1: Figure S1, Galloway unpublished, Herrlinger, 1983), and M. magister (Feder & Hoberg, 1980). However, crabs are generally rare in stomach content analyses, ranging from >1% to 10% of the identified prey in these studies. While P. helianthoides is a known generalist predator and scavenger with broad diets, meaningful predation on early life stages of decapod crustaceans has not been previously reported, except for an observation of one megalopa of an unknown crab species found in one P. helianthoides in Central California (Herrlinger, 1983). The finding here that juvenile P. helianthoides can eat ~5 juvenile crabs per day suggests that this predator could exert direct top-down control on crabs soon after their benthic settlement and recruitment, potentially influencing population dynamics of M. magister at local scales. Miles E. Rough: Conceptualization; data curation; formal analysis; investigation; methodology; writing original draft; writing review and editing. Aaron W. E. Galloway: Resources; supervision; formal analysis; writing original draft; writing review and editing. We would like to acknowledge that collections of specimens were conducted on the traditional lands of the Tututni people, part of the Confederated Tribes of Siletz Indians, who have stewarded this region since time immemorial. We thank all the staff at the Oregon Coast Aquarium specifically Tiffany Rudek, Steve Vogel, and Jim Burke. We would also like to thank Sarah Gravem, Ian Kittell, Talia LaForte, Katie Rumsey, Aubrey Taradash, Kersten Schnurle, Caroline Rice, and Faith Townsend for instrumental diving support and field assistance. This work was partially supported by the Oregon Kelp Alliance (NOAA Restoration Center Cooperative Agreement NA25NMFX469C0012) and The Nature Conservancy (P119034) awarded to Aaron W.E. Galloway. The authors declare no conflicts of interest. Data (Rough & Galloway, 2026) are available in Figshare at https://doi.org/10.6084/m9.figshare.31403556.v2. Appendix S1. 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