DOI: 10.5061/dryad.tqjq2bvzp
title: Janzen-Connell effects partially supported in reef-building corals: adult presence interacts with settler density to limit establishment

description: The Janzen-Connell hypothesis predicts increased survival of
 early life stages with decreasing conspecific density and increased
 distance from conspecific adults. Evidence of
 Janzen-Connell effects in maintaining diversity is common in
 plant communities, but its importance in sessile marine
 invertebrates communities remains unclear. Under controlled
 aquarium conditions, we examined the effect of density-dependence and
 adult conspecific water treatments (absent/present) on propagule
 settlement success and settler post-settlement survival, along with
 associated spatial patterns, for six broadcast-spawning, reef-building
 coral species from three families. We also tested if settlement success
 was linked to increasing propagule species diversity for three coral
 species from two families. We found that the probability of settlement was
 density independent and not influenced by adult present water treatments
 Yet, adult present water treatments and settler density did have a
 synergistic negative effect on the probability of short-term settler
 survival for all species examined. Settlers also showed greater spatial
 aggregation as their numbers increased, but were less aggregated in adult
 present water treatments compared to those in adult absent water
 treatments. We further show evidence of significant species interactions
 among propagules, as settlement in single-species trials was four-fold
 higher compared to mixed-species trials. Our findings from controlled
 experimental arenas indicate that the early establishment of corals was
 predominantly limited by density-dependent settler-adult interactions
 among conspecifics and propagule-propagule interactions among
 heterospecifics. Thus, the proximity to established conspecific adults,
 settler density and species diversity of propagules are relevant drivers
 of local coral community diversity and structure. Based on these outcomes,
 we suggest that the Janzen-Connell hypothesis, with demonstrated
 importance for plants, is partially upheld for reef corals.
description: Experiments were conducted at Heron Island Research Station, on
 the southern Great Barrier Reef, during the November 2017 coral spawning
 season. We collected multiple colonies of six common, broadcast-spawning
 coral species (<i>Acropora hyacinthus</i>, <i>A.
 millepora,</i> <i>A. valida,</i> <i>Astrea
 curta,</i> <i>Leptoria phrygia,</i> and <i>Porites
 cylindrica</i>). Experiments were conducted in two outdoor
 raceways. Each experimental replicate was a one litre plastic cylinder
 container with a settlement tile placed into raceways following a
 randomized block design. The containers were immersed in constant
 flow-through seawater to maintain ambient water temperatures. Containers
 were filled with one of two experimental water treatments: flow-through
 reef seawater (“adult absent water treatment”) or water from tubs holding
 two conspecific adult colonies (“adult present water treatment”), and then
 actively swimming coral larval propagules were added to the treatment
 containers. With this set-up, we conducted three experiments: settlement,
 post-settlement survival, and diversity. For
 settlement, we established five replicate containers per combination of
 three crossed variables: six coral species (<i>A.
 hyacinthus</i>, <i>A. millepora</i>, <i>A.
 valida</i>, <i>As. curta</i>, <i>L.
 phrygia</i> and <i>P. cylindrica</i>), three propagule
 densities (10, 50 and 100), and two adult water treatments (absent or
 present). The post-settlement survival experiment extended from the
 settlement experiment, so density (or 'the number of settlers')
 was a continuous variable crossed with coral species (5 levels, no
 <em>P. cylindrica</em>) and adult water treatments (2 levels).
 For the diversity experiment, we tested whether increasing propagule
 species diversity affected settlement success using three species
 (<i>A. millepora</i>, <i>A. valida</i> and
 <i>L. phrygia</i>), and three propagule species diversity
 treatment levels at a set density (one, two and three). We used the five
 replicate containers from the settlement experiment, testing settlement at
 density 50 without an adult, as the single species diversity treatment
 (control). For the two and three propagule species diversity treatments,
 we added 50 (25 of each species) and 51 (17 of each species) propagules to
 each of seven replicate containers. Settlers were
 counted visually on all settlement tile orientations (top, bottom and
 sides) using a dissection microscope and mapped onto a photographic image
 of each tile surface using imaging software. Settlement was scored as the
 number successfully settled versus the number that died on each tile from
 the initial starting density. Post-settlement survival was scored as the
 number that survived versus the number that died on each tile from the
 settlement experiment number settled. The diversity experiment scored
 settlement (number successfully settled versus the number that died) for
 each species on each tile.
description: See ReadMe file. The data for the spatial analyses are not
 provided here as they are included in the manuscript.