Observation: A "monoculture" of "flowering" sea cucumbers has replaced a formerly diverse marine biological community at the south tip of Trial Island near Victoria, BC, Canada. I have been diving in these waters regularly for 35 years as a commercial octopus and scallop fisherman, and I am intimately familiar with many of the subtidal habitats in the Victoria Bight.
The subtidal habitat at this location is rocky and experiences strong tidal currents. This habitat formerly hosted a diverse variety of colourful marine life including sponges, anemones, encrusting algae and corals, swimming and rock scallops, giant barnacles, starfish, etc. The south tip of Trial Island is located proximal to two deep water sewage outfalls that discharge untreated municipal wastewater from the Victoria area into the junction of Haro and Juan de Fuca Straits.
During the past 10 to 15 years patches of these small brown "flowering" sea cucumbers have crept in and covered much of the rock substrate between 30 and 120 ft of water. In 35 years of commercial diving experience I have seen this kind of colonization only at one other location. That area, Brotchie Ledge, is also located within the tidal ranges of the raw sewage discharges and that colonization progressed in a similar time frame as the Trial Island colonization.
I'm not exactly sure what species of sea cucumber they are or what changes in ocean chemistry (e.g. nutrients, etc) have enabled them to gain an advantage at these locations. They have a "flowering" head with a peanut sized body covered with small white tendrils for attaching to the bedrock. Their morphology is identical to the larger orange sea cucumber common in the Pacific Northwest (Cucumaria miniata). Allan Crow Fishing, Victoria, BC.
Ocean Integrity Research Consult: LEO BC Coordinator, Dr. Tom Okey writes, "Suspension-feeding sea cucumbers are in the order Dendrochirotida of the class Holothuroidea in the phylum Echinodermata. Dr. Philip Lambert (see his consult below) suggested that these "carpets" of sea cucumbers could be made up largely of the Black Brooding sea cucumber Pseudocnus curatus (Cowles, 1907) (formerly Cucumaria curata). The photographs of P. curatus found online, however, show this species as having all black tentacles, while the tentacles of the species in this observed barrens are a lighter color. The numerically dominant species observed here also resembles the orange-footed cucumber Cucumaria frondosa, which is normally distributed in the North Atlantic Ocean and the Arctic, not in the Pacific. Positive identification of this observed species would require closer investigation such as close-up photographs."
"The video of this "cucumber barrens" provided by Mr. Crow appears to show occasional patches of the orange sea cucumber (C. miniata) within the dense carpets of this other dominant species, whatever species it is."
Alternative hypotheses can be identified to explain the replacement of the formerly more diverse subtidal biological community in this location with the observed "carpets" of these suspension-feeding sea cucumbers:
One hypothesis is that the general reduction of seastar predators in the region, due to seastar wasting disease, may have released this cucumber species from predation pressure. Solaster stimpsoni (the Sun Star), Luidia foliolata (the Sand Star) are two of the seastar predators of C. miniata that have declined, for example.
Another hypothesis is that this shift may have been directly or indirectly caused by the exposure of this site to sewage effluent which may have increased mortality or prevented recruitment of other species, or given these suspension-feeding sea cucumbers a competitive advantage, or both. If this were true, these cucumber barrens would be a good indicator of sewage outfall impacts in rocky, high-current habitats.
A third hypothesis is that the predominant species of sea cucumber in this "barrens" is invasive and becomes competitively dominant in this novel system without its normal competitors and predators. C. frondosa is in fact normally distributed in the North Atlantic Ocean and the Arctic, not in the Pacific
Any combination of these alternate hypothesis could explain this "sea cucumber barrens."
This cucumber barrens is remarkably reminiscent of documented anemone barrens in the Galapagos, which were likely caused by a combination of a high heat mortality event, which opened space on the reef, in combination with depletion of predators that would otherwise control the Aiptasia sp. anemones there (Okey et al. 2003, Okey 2004). In both of these cases, these carpets of suspension feeders, once established, are persistent and stable because they consume the propagules of any species that might recruit to this habitat." - Dr. Tom Okey, University of Victoria School of Environmental Studies, and Ocean Integrity Research.
Royal BC Museum Consult: Curator Emeritus Philip Lambert writes,"The sea cucumbers that Allan Crow observed are most likely Pseudocnus curatus (Cowles, 1907) (formerly Cucumaria curata). They occur in vast aggregations like the one he describes. They are a brooding species hence the large aggregations. I have seen them like that in various places in Juan de Fuca wherever there is a high current and suitable flat rocky habitat. Of course sewage has been flushed into the area for many years so I suspect [the] suggestion of reduced predation because of the sea star die-off is the best explanation. Their main predators are seastars, particularly 3 species of Sunstars and the Sunflower star, which have declined recently."
Videos:
Click here for a video of the "sea cucumber barrens" at Trial Island
For broader context, the following is a video addressing the historical impacts of the Victoria sewage outfalls on subtidal marine environments and biodiversity, titled, WHAT YOU CAN'T SEE FROM ABOVE. (rt 4:45)
Media:
Times Colonist – Diver in Victoria waters sees firsthand the need for sewage treatment. By Allan Crow. March 26, 2016
References:
Lambert, P., 1997. Sea cucumbers of British Columbia, Southeast Alaska and Puget Sound. UBC Press.
Okey, T.A., S.A. Shepherd and P.C. Martinez. 2003. A new record of anemone barrens in the Galápagos. Noticias de Galápagos 62: 17-20.
Okey, T. A. 2004. Discovery of Anemone barrens in the Galapagos and potential explanations. Chapter 3 in Shifted community states in four marine ecosystems: some potential mechanisms. PhD Thesis, University of British Columbia, Vancouver.
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