Evidence of an inflammatory-like response in non-normally pigmented tissues of two scleractinian corals

Abstract

Increasing evidence of links between climate change, anthropogenic stress and coral disease underscores the importance of understanding the mechanisms by which reef-building corals resist infection and recover from injury. Cellular inflammation and melanin-producing signalling pathway are two mechanisms employed by invertebrates to remove foreign organisms such as pathogens, but they have not been recorded previously in scleractinian corals. This study demonstrates the presence of the phenoloxidase (PO) activating melanin pathway in two species of coral, Acropora millepora and a massive species of Porites, which both develop local pigmentation in response to interactions with a variety of organisms. L-DOPA (3-(3,4-dihydroxyphenyl)-L-alanine) substrate-based enzyme activation assays demonstrated PO activity in healthy tissues of both species and upregulation in pigmented tissues of A. millepora. Histological staining conclusively identified the presence of melanin in Porites tissues. These results demonstrate that the PO pathway is active in both coral species. Moreover, the upregulation of PO activity in areas of non-normal pigmentation in A. millepora and increased melanin production in pigmented Porites tissues suggest the presence of a generalized defence response to localized stress. Interspecific differences in the usage of pathways involved in innate immunity may underlie the comparative success of massive Porites sp. as long-lived stress tolerators.

Figure 1

Macroscopic signs of pigmentation response in (a) A. millepora showing blue pigmentation encircling a wound and normal coloration of the surrounding healthy branches, and (b) massive Porites sp. showing pink pigmentation encircling lesions and normal coloration of the surrounding healthy tissue. (Photos credited to Giles Winstanley.)

Figure 2

Comparison of mean (±s.e.) change in PO activity, as measured by the oxidation of l-DOPA over time, between healthy and pigmented samples of A. millepora (n=28; t=2.212, p=0.031).

Figure 3

Comparison of mean (±s.e.) number of zooxanthellae per 100 μm² of gastrodermis between healthy and pigmented samples of A. millepora (n=15; U=7.0; p<0.01).

Figure 4

Histological sections showing (a) healthy free body wall epithelial layers of A. millepora, with abundant zooxanthellae in the gastrodermis (H&E); (b) pigmented free body wall epithelial layers of A. millepora, with granular cell layers and depleted numbers of zooxanthellae in the gastrodermis (H&E); (c) pigmented free body wall epithelial layers of A. millepora showing no melanin deposits (Fontana–Masson stain); (d) Porites free body wall epithelial layers showing pigment cells in the gastrodermis of a healthy sample; (e) Porites free body wall epithelial layers showing pigment cells in both the epidermis and the gastrodermis of a pigmented sample; (f) Porites free body wall epithelial layers showing black stained melanin deposits (Fontana–Masson stain) in the same pigmented sample as in (e). E, epithelium; G, gastrodermis; M, mesogloea; Z, zooxanthellae; Me, melanin.

Figure 5

Comparison of mean (±s.e.) change in PO activity, as measured by the oxidation of l-DOPA over time, between healthy and pigmented samples of Porites sp. (n=20; t=0.479, p=0.634).

Figure 6

Porites healthy versus pigmented tissue comparisons for mean (±s.e.) densities of: zooxanthellae (n=15, U=669.0, p<0.01; white bars), epidermal granular cells (n=15, U=52.5, p=0.012; grey bars) and gastrodermal granular cells (n=15, U=73, p=0.101; black bars).