Early life history of deep-water gorgonian corals may limit their abundance

Abstract

Deep-water gorgonian corals are long-lived organisms found worldwide off continental margins and seamounts, usually occurring at depths of ∼200-1,000 m. Most corals undergo sexual reproduction by releasing a planktonic larval stage that disperses; however, recruitment rates and the environmental and biological factors influencing recruitment in deep-sea species are poorly known. Here, we present results from a 4-year field experiment conducted in the Gulf of Maine (northwest Atlantic) at depths >650 m that document recruitment for 2 species of deep-water gorgonian corals, Primnoa resedaeformis and Paragorgia arborea. The abundance of P. resedaeformis recruits was high, and influenced by the structural complexity of the recipient habitat, but very few recruits of P. arborea were found. We suggest that divergent reproductive modes (P. resedaeformis as a broadcast spawner and P. arborea as a brooder) may explain this pattern. Despite the high recruitment of P. resedaeformis, severe mortality early on in the benthic stage of this species may limit the abundance of adult colonies. Most recruits of this species (∼80%) were at the primary polyp stage, and less than 1% of recruits were at stage of 4 polyps or more. We propose that biological disturbance, possibly by the presence of suspension-feeding brittle stars, and limited food supply in the deep sea may cause this mortality. Our findings reinforce the vulnerability of these corals to anthropogenic disturbances, such as trawling with mobile gear, and the importance of incorporating knowledge on processes during the early life history stages in conservation decisions.

Figure 1. Study sites (this study) and locations of abundance transects .

Experimental study sites in the Middle Canyon of the Northeast Channel Coral Conservation Area (Gulf of Maine). The sites are shown relative to 3 upslope transects (‘NEC3’, ‘NEC4a’, ‘NEC4b’) performed in the area in 2006 . Diagram is not to scale.

Figure 2. Array of larval settlement collectors at deployment in 2006 (South wall: 671 m).

Collectors included either a basalt rock (∼7×10×2 cm) supported by a plastic ring (n = 10), or mesh pads (∼7.5×10×1.5 cm, blue ‘Scotch-Brite’ pads, mesh openings: ∼3.5 mm²) (n = 6).

Figure 3. Recruits and adult colonies of deep-water gorgonian coral in Northeast Channel.

Scale bars for adult colonies represent 20 cm. (A) P. resedaeformis. Depth: 288 m. Scale bar in inset represents 1 mm. (B) P. arborea. Depth: 314 m. Scale bar in inset represents 5 mm.

Figure 4. Recruitment of P. resedaeformis on arrays of larval settlement collectors.

Arrays were deployed at 3 locations (north wall: 658 m, south wall: 671 m, floor: 863 m). Settlement collectors included either a basalt rock (n = 10 per location) or mesh pads (n = 6 per location). (A) Mean abundance (+SD) of recruits on all components of the settlement collectors combined. (B) Frequency of presence of recruits on the top flat surfaces of the larval settlement collectors. Mean abundance of recruits (per 100 cm²) on the other components of the settlement collectors (i.e. excluding the top flat surfaces) is indicated at each location and for each surface type.

Figure 5. Relative height-frequency distributions of pooled P. resedaeformis recruits.

Recruits were retrieved on larval settlement collectors with basalt rocks (north wall: n = 732; south wall: n = 391) and mesh pads (north wall: n = 257, south wall: n = 181). Heights indicate the lower ends of 0.5 mm-bins.

Figure 6. Mean relative frequency (+SD) of P. resedaeformis recruits on settlement collectors.

= 10) or mesh pads (n = 6) on arrays deployed at 3 locations (north wall: 658 m, south wall: 671 m, floor: 863 m). The number of polyps reflects early life stages of the recruits: ‘one polyp’ is the primary polyp, and ‘2 or more polyps’ are juvenile colonies.