Deaker DJ et al. (2020), Diet flexibility and growth of the early herbiv...

ECB-ART-49730

PLoS One 2020 Jul 20;157:e0236142. doi: 10.1371/journal.pone.0236142.

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Diet flexibility and growth of the early herbivorous juvenile crown-of-thorns sea star, implications for its boom-bust population dynamics.

Deaker DJ , Mos B , Lin HA , Lawson C , Budden C , Dworjanyn SA , Byrne M .

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The ecology of the early herbivorous juvenile stage of the crown-of-thorns sea star (COTS, Acanthaster spp.) is poorly understood, yet the success of this life stage is key to generating population outbreaks that devastate coral reefs. Crustose coralline algae (CCA) has been considered to be the main diet of herbivorous juveniles. In this study, we show that COTS can avail of a range of algal food. Juveniles were reared on CCA, Amphiroa sp., and biofilm, and survived for 10 months on all three diets. The juveniles fed CCA and Amphiroa sp. reached 15-16.5 mm diameter at ~ 6 months and maintained this size for the rest the experiment (an additional ~4 months). Juveniles fed biofilm grew more slowly and to a smaller maximum size (~3 mm diameter). However, when juveniles were switched from biofilm to CCA they resumed growth to a new asymptotic size (~13.5 mm, 13-20 months). In diet choice experiments, juveniles did not show a preference between Amphiroa sp. and CCA, but generally avoided biofilm. Our results show that juvenile COTS grew equally well on CCA and Amphiroa sp. and can subsist on biofilm for months. Some juveniles, mostly from the biofilm diet treatment, decreased in size for a time and this was followed by recovery. Flexibility in diet, growth, and prolonged maintenance of asymptotic size indicates capacity for growth plasticity in herbivorous juvenile COTS. There is potential for juvenile COTS to persist for longer than anticipated and increase in number as they wait for the opportunity to avail of coral prey. These findings complicate our ability to predict recruitment to the corallivorous stage and population outbreaks following larval settlement and the ability to understand the age structure of COTS populations.

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	Fig 1. Herbivorous juvenile crown-of-thorns sea stars, Acanthaster sp., feeding on coralline algae and biofilms.(A) Juveniles at the start of the feeding experiment. (B-C) Juveniles raised on Amphiroa sp.. The juvenile in (B) is wrapped around the fronds to feed. (D-E) Juveniles raised on biofilm for 292 d, and then provided with crustose coralline algae (CCA) (F, 92 d on CCA). The number in the top right corner of each panel represents the number of days since the experiment commenced. The black arrow (D) indicates a green stomach associated with eating biofilm. The white arrows indicate feeding scars on Amphiroa sp. (B-C), the biofilm plate (E), and CCA (F).
	Fig 2. The growth and survival of herbivorous juvenile crown-of-thorns sea stars on three different diets.(A,B) The diameter (mean ± SE), (C-D) number of arms (mean ± SE), and (E-F) survival (%) of juveniles fed crustose coralline algae (CCA), Amphiroa sp., or biofilm for 292 d (left panels) and juveniles that were raised on biofilm and then fed CCA for an additional 304 d (right panels). The horizontal dashed lines (A-B) represent the approximate minimum size that juveniles can transition to a coral diet [7]. Juveniles were ≤3 months old at the start of the experiment (To).
	Fig 3. The effect of diet and starvation on the diet choice of juvenile crown-of-thorns sea stars.Juveniles raised on a diet of crustose coralline algae (CCA), Amphiroa sp., or biofilm for 292 d were offered three food sources simultaneously before (Fed, 260 d) and after they were starved for 3 d (Starved, 292 d). Their position on the algal foods or bare substrate were recorded each hour for 48 h. Each row represents the position of one individual juvenile over time (n = 10 per treatment), and each box represents the location of that juvenile recorded every hour.
	Fig 4. The mean growth rate of herbivorous juvenile crown-of-thorns sea stars during their exponential growth phase.Four cohorts of juveniles were raised on crustose coralline algae (CCA) in the laboratory. The data were obtained from Yamaguchi [7], Kamya et al. [58], and this study (diets: CCA and Biofilm, then CCA) (S1 Appendix). The growth rate with respect to juvenile diameter was determined by creating an exponential model of the change in mean diameter of each cohort (S1 Table). See supplementary material for equations and R2 values (S2 Table).

References [+] :

Babcock, Assessing Different Causes of Crown-of-Thorns Starfish Outbreaks and Appropriate Responses for Management on the Great Barrier Reef. 2016, Pubmed, Echinobase