Larval longevity and competency patterns of Caribbean reef-building corals

The potential for long-distance larval dispersal depends on the longevity of planktonic, free-swimming larvae and their capacity to successfully recruit to reef habitat. We present multi-year laboratory observations of the persistence of planular larvae and settlement competency over time for cohorts derived from the same parental populations of the most important Caribbean reef building coral species, Orbicella faveolata and Acropora spp. Despite variability among years/cohorts, larvae of both species display capacity for extended longevity (up to 83 d) and competency (demonstrated at up to 48 d). Both species also displayed signiﬁcantly reduced survivorship and lower realized settlement under elevated temperatures. Although the observed levels of settlement in 24 h competency assays was extremely variable, the timing of onset of competence was highly consistent among years/cohorts but distinct between species. Orbicella faveolata displayed onset of competence during day 3-5 or 4-7 (with or without exposure to positive settlement cue) after spawning; whereas, onset for Acropora spp. was day 7-8 or day 10-11 (with or without cue, respectively). This longer pre-competency period for Acropora spp. nonetheless corresponded to a greater persistence of A. palmata larvae to this age of competence (71-83% of initial cohort compared to 54-55% for O. faveolata ). Such life history variation implies meaningful diﬀerences in likely dispersal potential between these imperiled reef-building species. The potential for long-distance larval dispersal depends on the longevity of planktonic, free-25 swimming larvae and their capacity to successfully recruit to reef habitat. We present multi-year laboratory observations of the persistence of planular larvae and settlement competency over 27 time for cohorts derived from the same parental populations of the most important Caribbean 28 reef building coral species, Orbicella faveolata and Acropora spp. Despite variability among 29 years/cohorts, larvae of both species display capacity for extended longevity (up to 83 d) and 30 competency (demonstrated at up to 48 d). Both species also displayed significantly reduced 31 survivorship and lower realized settlement under elevated temperatures. Although the observed 32 levels of settlement in 24 h competency assays was extremely variable, the timing of onset of 33 competence was highly consistent among years/cohorts but distinct between species. Orbicella whereas, spp. pre-competency results highlight the intra- and inter-cohort variability within spawning output, potential for dispersal, and larval behavior which may pose

111 Florida Keys reefs in August is approximately 30 o C in modern times, approximately 1 o C higher 112 than the turn of the 20 th century (Kuffner et al. 2015). 113 Water changes (approximately 75% volume) in each dish were conducted every other day 114 with filtered reef water. Full water changes were conducted during counts of the larvae 115 remaining in each dish, conducted every other day for the first two weeks and then less 116 frequently (approximately weekly) thereafter. These counts scored organisms as 117 pelagic/swimming larvae, settled polyps, or MUPs (metamorphosed, unattached polyps). Larvae 118 that metamorphosed in the water column (MUPs) were never observed to attach so their viability 119 as recruits is unclear, but we chose to exclude them from both estimates of larval supply and 120 mortality. Settled polyps remained in the bowls in 2015, but during 2016 and 2017, all settlers 121 were removed during each count. Data are reported as larval supply (remaining swimming 122 planulae larvae) over time (i.e. age) and mortality over time (initial # minus remaining 123 swimming planulae minus cumulative # settlers minus cumulative # MUPs). 124 There was a discontinuity during the 2015 and 2016 experiments at the completion of our 125 field expedition (approximately 1 month following spawning). The individual dishes were 126 lidded and transported in a dry cooler from the field lab in Key Largo back to the Southeast 127 Fisheries Science Center in Miami FL. In 2015, the recirculating temperature-controlled baths 128 were re-assembled, and the temperature treatments were maintained until all larvae expired or 129 settled. In 2016, the warm treatment was terminated when this move was made, and only a 130 single water bath for the cool treatment was carried on. In 2017, all experiments were curtailed 131 at 23-25 dAS due to forced evacuation before Hurricane Irma.

132
At the conclusion of the experiment, larval longevity under the temperature treatments 134 packages in R, with replicate dish as a random factor. Kaplan-Meier survival curves were 135 estimated for each treatment within a cohort and compared using a log rank test.

136
For the competency assays, larger batch cultures of larvae were maintained in round 137 culture chambers that had a 100-μm mesh bottom and a continuous, recirculating slow drip 138 around its walls. Culture chambers were housed in outdoor recirculating seawater systems 139 containing reef-collected seawater with temperature control, salinity maintained at 35-37 ppt by 140 addition of distilled water to compensate for evaporation, a UV sterilizer, and 5μm filtration.
141 Partial exchange with new reef water was conducted periodically to maintain water quality. No 142 effort was made to deprive these larvae of settlement cues since raw reef water was exchanged 143 periodically and other tanks in the recirculating seawater system contained natural reef rubble. 144 Beginning two to three days after spawn, replicate aliquots of larvae were counted out (10 for 145 Acropora spp.; 20 for O. faveolata) into individual wells in two 6-well polystyrene culture plates 146 along with a fragment of substrate with crustose coralline algae to provide positive settlement 147 cues (n=12; Fig 1A). In most years, this substrate was a fragment chipped from a freshly 148 collected piece of reef rubble, though in 2014 chips of artificial ceramic substrate that had been 149 conditioned in local reef habitat for over two months were used. Rubble or ceramic chips were 150 approximately 1 x 1 cm in size. Starting in 2015, glass inserts were placed inside the polystyrene 151 6-well plates such that settlement took place in the absence of plastic.  Fig 1B) was scored under a fluorescent dissecting 155 microscope. After each scored assay, larvae were discarded and a fresh selection of larvae and 157 daily, with reduced frequency to every other day after about 2 weeks and were continued as long 158 as healthy-looking larvae (i.e., elongated and swimming) were available from the initial batch 159 culture with the following exceptions. In the first year of the study (2012), the competency 160 assays were not initiated until day 14 and were conducted less frequently. An additional 161 competency assay for Acropora spp was performed at a later date (after the initial batch culture 162 was expended) using remaining larvae from the longevity assays in 2016 (the discontinued 163 'warm' treatment larvae when the experiment was moved back from the field lab) and in 2017 164 (all larvae from the longevity assays prior to required hurricane evacuation). Lastly, in 2013, 165 two separate cohorts (termed a and b) collected on sequential nights from different sites were 166 tested separately.   Table 1) with larvae derived from the same general parent population (i.e., collected 172 from 2-3 sites within ~ 12 km distance), yet overall settlement rates and competency patterns 173 over time were extremely variable. In contrast, onset of competency was quite consistent among 174 experimental trials and cohorts within a species, while being distinct between species.

175
The pattern of larval supply (i.e., remaining swimming planula) over time for each trial is  Though the settlement response is highly variable, some characteristics are remarkably 266 consistent between years, though contrasting between species (summarized in Table 3). For 267 example, the initial observation of metamorphosis in the longevity studies (i.e., in absence of 268 settlement cue) was either 10 or 11 dAS for both Acropora species over three years and either 269 day 4 or 7 for O. faveolata. Given these different ages of competency, the percent of larvae 271 than for O. faveolata (55-58 % on day 4-7) in the longevity study under the more benign (cool) 272 temperature treatments (Table 2, Fig 2). Knowledge of these consistent, species-specific 273 characteristics can be leveraged in developing and optimizing production pipelines for larval 274  These results provide a more solid basis for predicting dispersal of the key reef-building 298 Caribbean corals than has been previously available and, hence, the connectivity potential of 299 metapopulations of these foundation species. However, the realization of this potential is 300 predicated on successful larval production and successful recruitment into the receiving habitat 301 (i.e., settlement and survivorship to maturity). There is evidence that the latter process is failing    Manuscript to be reviewed

Summary of species-specific differences in observed larval characteristics
Values given in the first three rows are drawn from one or two corhorts of Orbicella faveolata and three cohorts of Acropora palmata (Fig 2). The last two rows are drawn from six cohorts of O.faveolata and three cohorts of A.palmata (Fig. 3, 4). Only one cohort of A.cervicornis was addressed in this study. dAS = days After Spawn