Reaching for the Stars: Ecology and Skeletal Morphology of Pacific Northwest Sea Stars

Abstract

Intertidal habitats exist at the interface of land and water, requiring the organisms that live there to inhabit two very different fluids, while also generating webs of interaction that span terrestrial and marine environments. This thesis addresses the functional groups and functional traits of intertidal organisms related primarily to trophic interactions, how these interactions change with habitat heterogeneity (e.g., seagrass on soft sediment, crevices on rocky shores), and skeletal morphology, in terms of three-dimensional ossicle organization, as a novel set of traits in sea stars (Asteroidea). Tidal flats exposed at low tide frequently consist of a mosaic of habitat types. In Washington State, USA, tidal flats typically contain eelgrass (Zostera marina) patches intermixed with areas devoid of vegetation. Seagrasses represent a functionally important coastal habitat, serving as protection and nursery grounds for aquatic organisms, but less is known regarding avian use at low tide. Waterbirds in different functional feeding groups may interact differently with vegetation on tidal flats, and birds may make different choices when foraging or resting. In this study, community scientists collected observational data bimonthly during daytime low tides in summer (April to August). Scan surveys and focal follows of waterbirds were carried out in paired patches of two lower intertidal habitats (eelgrass and unvegetated) across five sites. Ninety-nine percent of birds seen were gulls, crows, herons, and geese, and half of all birds observed were actively foraging. Total bird abundance within a patch did not differ between habitat types; among the four functional groups (corvids, gulls, herons, waterfowl), herons were more abundant in eelgrass than unvegetated patches. Waterbirds overall were more likely to be observed foraging in eelgrass than in unvegetated patches, and this difference also was seen in the activity budgets of herons. These tidal flats were occasionally visited by humans during low tide surveys. Human presence nearby did not appear to alter bird abundance overall or for any functional group. Overall, while low-tide eelgrass patches were valuable for foraging for some functional groups, a mosaic of habitats was used by waterbird communities. Global declines in eelgrass could lead to population decreases of herbivorous birds that directly consume eelgrass as well as wading birds, such as herons, that take advantage of prey living within seagrass beds. Ochre sea stars (Pisaster ochraceus) are an apex, keystone predator on wave-exposed coastlines of the US Pacific Ocean, responsible for establishing the vertical zonation of mussel beds and therefore controlling benthic diversity. The studies that have formed the basis of understanding of this sea star’s diet have primarily occurred at low tide and in areas with dominating mussel beds. However, given that intertidal organisms must live in both air and water, often with differing tasks or behaviors depending on where they find themselves, it is critical to study their behavior holistically. Additionally, P. ochraceus can be found in relatively high abundance in rocky intertidal habitats devoid of mussels, which may alter the functional role of this predator. This observational study addressed the diet and feeding behavior of P. ochraceus in the field through the tidal cycle at three sites around San Juan Island, Washington, USA, a region that is not currently dominated by mussels. Study design accounted for their time in and out of water, the microhabitats they use, and their size. Sea stars were more likely to be found feeding at high than low water, when outside than in crevices, and as body size increased. When P. ochraceus were feeding, they consumed larger prey at low than high water, when inside than outside crevices, and as body size increased. Mussels were rare on these shores and represented <1% of feeding records. With no mussels to consume, P. ochraceus may not play the role of a keystone predator in this area. This effort to further understand the role of sea stars in intertidal communities is important because P. ochraceus is widespread in many wave exposure regimes but was reduced in density by the recent sea star wasting disease (SSWD) events that caused major population losses of this predator along the US Pacific Coast. Armor in sea stars (Asteroidea) comes in the form of a highly articulated endoskeleton made up of individual pieces called ossicles. Many descriptive studies have been conducted on the basic patterning of sea star skeletons, with differences in shapes of ossicles forming the basis of early echinoderm phylogenies. However, the functional value of sea star ossicles is not necessarily in the characteristics of individual pieces, but rather in how those pieces are arranged and their relative contributions to the skeleton as a whole. In this study, micro-computed tomography was used to measure and compare morphological differences in the patterns of endoskeleton allocation of nine sea star species local to Washington, USA. Based on 14 quantitative morphological traits, analyses addressed: 1) correlations and tradeoffs among these traits and 2) correspondence of morphological similarity to two possible predictors, specifically ecology and phylogeny. Given the ecological and phylogenetic diversity of stars included in this study, expected patterns included 1) differences in the amount of armoring (relative volume of skeleton) and 2) those differences arise from varying allocation and shape of ossicles across different regions of the body. While all sea stars share the same five basic types of ossicles, the amount of skeletal armoring across the body varied by at least an order of magnitude across species and differed in its distribution across ossicle types. Heavily armored stars invest in larger, boxy body wall ossicles, whereas a reduction in armor volume was often paired with more intricately shaped body wall ossicles and an increase in the number and complexity of spines. Skeletal patterning and allocation are more correlated with phylogenetic than ecological differences. More research is needed to understand which morphological traits are phylogenetically conserved, which could provide an opportunity to better use phylogeny to understand trait evolution