Phenotypic Plasticity or a Reproductive Dead End? Primnoa pacifica (Cnidaria: Alcyonacea) in the Southeastern Alaska Region

Abstract

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.

Deep coral habitats of Glacier Bay National Park and Preserve, Alaska

“…This may be an indication that Glacier Bay’s population is reproductively successful, whereas researchers believe that P . pacifica populations in some of Alaska’s other southeastern fjords are not currently successfully reproducing [ 49 ]. Portions of transects where P .…”

“…New recruits and size classes are important to record because they indicate that there are different age cohorts in these populations. This may be an indication that Glacier Bay's population is reproductively successful, whereas researchers believe that P. pacifica populations in some of Alaska's other southeastern fjords are not currently successfully reproducing [49]. Portions of transects where P. pacifica colonies were present in high abundance and large size that the substrate and other organisms beneath were not visible were classified as areas of dense "thicket habitat" [41] in the data set.…”

Understanding growth and age of red tree corals (Primnoa pacifica) in the North Pacific Ocean

“…However, the variability in frequency and the number of bands per year is not consistent with only spring tides. We instead hypothesize that high seasonal and interannual variability of primary productivity [42] and/or energy allocation to reproduction [43,44] combined with spring tides can influence the variable banding and growth pattern. Additional collections of colonies spanning the size range of the species coupled with time series in situ measurements of primary productivity, specifically flux (POC or particulate organic carbon) delivery to the seafloor, and studies on reproductive seasonality of P. pacifica in the eastern Gulf of Alaska could elucidate the relationship between these factors and the sub-annual skeletal development.…”

Shallow-emerged coral may warn of deep-sea coral response to thermal stress

“…This keystone species 3 is conspicuous given its large size, tree-like morphology, and brilliant coloration, forming thickets at depths between 150 and 350 m in the eastern Gulf of Alaska 1 (GOA). Large branching colonies up to 5 m in height and width modify small-scale ocean currents and provide extensive habitats that support high associated biodiversity 1,4 . For more than a century these corals have inadvertently been brought to the surface by fishermen, tangled in nets and lines, and have become symbolic of the rich fauna and diverse communities in Alaska's deep marine waters.The value of RTC thickets in Alaska to commercially important species prompted the North Pacific Fisheries Management Council (NPFMC) to designate those habitats as Essential Fish Habitat in 2000 5 .…”

“…These populations of RTCs in the cold and dark waters typical of glacial fjords were likely established through pioneer recruitment from deep populations outside the fjords. With such deeper populations often inaccessible to researchers due to location, weather, and expense, discovery of these populations in shallow water provides a rare opportunity to access specimens for seasonal life-history studies, including reproduction 4,10,11 .RTCs are gonochoric broadcast spawners, producing oocytes that require up to 16 months to fully develop and male gametes that develop over the course of approximately one year 11 . In both female and male colonies, multiple stages of gametes are commonly found at the same time 11 .…”