Geology topics

Julia Johnstone

Publications and source records attributed to Julia Johnstone.

3 recordsLinked to original sources

USGS2026

Mesophotic octocoral larval settlement preferences and behaviors

Introduction Lab-based coral reproduction is advancing restoration efforts to support coral populations injured by the 2010 Deepwater Horizon oil spill. Objectives This study aimed to answer foundational questions concerning coral spawning and larval settlement that need to be resolved for these mesophotic and deep-sea coral species before restoration efforts can be scaled up. Methods From 2021 to 2023, fragments of the mesophotic octocorals Swiftia exserta and Thesea nivea were collected from the continental shelf offshore from Texas to Florida and transported to a federal facility in Florida. Multiyear spawning allowed for in-depth analysis of spawning periodicity, egg development, larval behavior, settlement rate, and substrate preference. In addition, we determined the likelihood of successful development and survival during early stages of development. Results Fertilization rates remained high throughout spawning periods of both species, with S. exserta spawning during a 1-month period and T. nivea continuing to spawn year-round. Larvae from both corals significantly preferred tiles seeded with crustose coralline algae, and there were species-specific preferences in substrate material. Additionally, larval survival or mortality was typically evident within a few days when in the presence of settlement substrates. In the absence of substrates, we observed larvae surviving for up to 2 months without settling. Conclusions This work answers critical questions to inform the long-term logistical planning for eventual out-planting of these species. Further, it highlights avenues that could accelerate future restoration-based reproduction efforts into unexplored mesophotic and deep species.

Alabama, Louisiana, Mississippi, Texas↗

USGS2025

Ecophysiology of two mesophotic octocorals intended for restoration: Effects of light and temperature

Light and temperature are driving forces that shape the evolution and physiology of mesophotic organisms. On the Mississippi-Alabama continental shelf, octocorals dominate the mesophotic seascape and provide habitat for many fish and invertebrate species. Gaps in knowledge regarding the fundamental physiological responses of these species to light and temperature are of particular interest to restoration activities following the Deepwater Horizon oil spill. To address these gaps, the photobiology and thermal tolerance of Swiftia exserta and Muricea pendula were assessed in the field and laboratory. Pulse amplitude modulated fluorometry, histology, light microscopy, and epifluorescence imaging revealed low densities of photosynthetic endobionts in samples of S. exserta and none in samples of M. pendula collected near the determined bottom of the euphotic zone (51.45 m). Response to the recorded monthly mean habitat temperature range (18.5–25.4°C) was assessed using respirometry and polyp activity data from live corals exposed to temperatures between 18°C and 26°C. There was no significant difference in oxygen consumption for either species between 18°C and 26°C, and calculated Q 10 values were not significantly different from 1, thus suggesting that both species have a low sensitivity to the local thermal environment. However, a negative correlation between temperature and polyp activity suggests that M. pendula is more sensitive to higher temperatures than S. exserta . This study improves the understanding of the effects of light and temperature on mesophotic octocoral physiology and lays the foundation for future work to explore the thermal thresholds of each species and the endobiont–host relationship in S. exserta .

Alabama, Mississippi, South Carolina↗

USGS2019

Phenotypic plasticity or a reproductive dead end? Primnoa pacifica (Cnidaria: Alcyonacea) in the southeastern Alaska region

Red tree corals ( Primnoa pacifica ) are abundant in the eastern Gulf of Alaska, from the glacial fjords of Southeast Alaska where they emerge to as shallow as 6 m, to the continental shelf edge and seamounts where they are more commonly found at depths greater than 150 – 500 m. This keystone species forms large thickets, creating habitat for many associated species, including economically valuable fishes and crabs, and so are important benthic suspension feeders in this region. Though the reproductive periodicity of this species was reported in 2014 from a shallow fjord (Tracy Arm), this study examined reproductive ecologies from 8 sites – two within Glacier Bay National Park and Preserve, three on the continental shelf edge, one within Endicott Arm (Holkham Bay) and two time points from the Tracy Arm (Holkham Bay) study. Male reproductive traits were similar at all sites but there were distinct differences in oogenesis. Though per polyp fecundity mostly showed no significant difference between sites, there was a non-significant trend of increasing number of oocytes with depth. In addition, the average oocyte size from Tracy Arm (the shallowest site) was 105 μm, whereas from Shutter Ridge (one of the deepest sites) the average size was 309 μm. Moreover, the maximum oocyte size at Endicott Arm was 221 μm and at Tracy Arm was 802 μm (both shallow sites), whereas at Dixon Entrance (a deep site) it was 2120 μm, a difference not usually observed within a single species. We propose two theories to explain the observed differences, (a) this species shows great phenotypic plasticity in reproductive ecology, adjusting to different environmental variables based on energetic need and potentially demonstrating micro-evolution; or (b) the fjord sites are at a reproductive dead end, with the stress of shallow-water conditions effectively preventing gametogenesis reaching full potential and likely limiting successful reproductive events from occurring, at least on a regular basis.

Alaska↗