Date Published: April 29, 2019
Publisher: Public Library of Science
Author(s): Rahul Mehrotra, Coline Monchanin, Chad M. Scott, Niphon Phongsuwan, Manuel Caballer Gutierrez, Suchana Chavanich, Bert W. Hoeksema, Shashank Keshavmurthy.
Recent studies revealed that reef corals can eat large-sized pelagic and benthic animals in addition to small planktonic prey. As follow-up, we document natural ingestion of sea slugs by corals and investigate the role of sacoglossan sea slugs as possible prey items of scleractinian corals. Feeding trials were carried out using six sacoglossan species as prey, two each from the genera Costasiella, Elysia and Plakobranchus, and four free-living solitary corals (Danafungia scruposa, Fungia fungites, Pleuractis paumotensis and Heteropsammia cochlea) as predators. Trials were carried out under both in-situ and ex-situ conditions with the aim to observe ingestion and assess signs of prey consumption based on tissue loss of prey individuals over time. Significant differences were observed in both ingestion time and consumption state of prey between prey species, with three of them being ingested more rapidly and preferentially consumed over the others. Additionally, prey size was found to be a significant factor with larger prey (>12 mm) being ingested more slowly and rarely than smaller ones (<6 mm and 6–12 mm). Comparisons of consumption capability among predators showed no significant difference with all coral species showing similar preferences for prey species. While no specific mechanism of prey capture is proposed, we also document instances of kleptoparisitism and resuspension of prey items by wrasses. This study highlights the important distinction between opportunistic prey capture and true predation events.
Sea slugs (Mollusca: Gastropoda: Heterobranchia) are more commonly known as predator than as prey, which is largely attributed to the chemical defences acquired from their prey species [1–3]. Though less extensive, a growing number of instances of sea slugs as prey species have been recorded, as part of in-situ observations and experiments, in particular with cnidarian predators [4,5]. However, once a prey is captured, it is not sure that it will also be consumed because it may become released and eventually escape .
Among the six sacoglossan prey species, three were found to be consumed more frequently than rejected (Table 2): Elysia pusilla, C. usagi and C.cf. kuroshimae were only rejected in 2.5%, 20% and 17.5% of trials, respectively. Additional trials of over 120 mins were carried out on each to confirm that these species were not being rejected at a later point. Under in-situ conditions, the mean PIOL for all non-rejected trials was 62 mins and under ex-situ conditions, the mean PIOL for all non-rejected trials was 131 mins. Mean consumption score for E. pusilla, C. usagi and C.cf. kuroshimae (Fig 6) independent of coral species, indicates that after an hour, individuals typically showed at least some tissue degradation, with E. pusilla showing significantly greater consumption scores than all other prey species. In contrast, E. cf. japonica rarely showed signs of consumption, being rejected 52.5% of the time with a mean consumption score of 0.15 ± 0.05. Both Plakobranchus species were typically rejected, with P. cf. ocellatus and P. cf. papua being rejected 95% and 90% and having consumption scores of 0.03 ± 0.02 and 0 respectively. These scores remain low, despite sparse instances where prey items were not rejected, as upon extraction prey items showed little to no signs of tissue degradation, and multiple instances where ingestion remained incomplete. With all trials for all prey species combined, most rejected prey were found to have a consumption score of 0 (n = 77), with the remaining having a consumption score of 0.5 (n = 6) and all rejected prey items were alive post rejection and motile shortly after rejection. No rejected prey items showed heavy signs of tissue degradation.