Field Assessment of the Predation Risk - Food Availability Trade-Off in Crab Megalopae Settlement

Abstract

Settlement is a key process for meroplanktonic organisms as it determines distribution of adult populations. Starvation and predation are two of the main mortality causes during this period; therefore, settlement tends to be optimized in microhabitats with high food availability and low predator density. Furthermore, brachyuran megalopae actively select favorable habitats for settlement, via chemical, visual and/or tactile cues. The main objective in this study was to assess the settlement of Metacarcinus edwardsii and Cancer plebejus under different combinations of food availability levels and predator presence. We determined, in the field, which factor is of greater relative importance when choosing a suitable microhabitat for settling. Passive larval collectors were deployed, crossing different scenarios of food availability and predator presence. We also explore if megalopae actively choose predator-free substrates in response to visual and/or chemical cues. We tested the response to combined visual and chemical cues and to each individually. Data was tested using a two-way factorial design ANOVA. In both species, food did not cause significant effect on settlement success, but predator presence did, therefore there was not trade-off in this case and megalopae respond strongly to predation risk by active aversion. Larvae of M. edwardsii responded to chemical and visual cues simultaneously, but there was no response to either cue by itself. Statistically, C. plebejus did not exhibit a differential response to cues, but reacted with a strong similar tendency as M. edwardsii. We concluded that crab megalopae actively select predator-free microhabitat, independently of food availability, using chemical and visual cues combined. The findings in this study highlight the great relevance of predation on the settlement process and recruitment of marine invertebrates with complex life cycles.

Mangrove expansion into temperate marshes alters habitat quality for recruiting Callinectes spp.

“…During the study dates, lagoon waters near the study site had a mean salinity (± SD) of 33 ± 4 and mean temperature of 25 ± 3°C (sensor 0054, Harbor Branch Oceanographic Institute Land/Ocean Biogeochemical Observatory). Vegetation used in each study was collected fresh, scrubbed, and rinsed with fresh water to remove biofilms and epibionts (van Montfrans et al 2003).…”

“…Each study was populated with high but realistic densities of settlers (110−120 megalopae or 65−88 J1 per m 2 in preference trials and 75−94 megalopae or J1 per m 2 in survival trials; maximum documented natural recruitment is ~150 megalopae per m 2 : Moksnes 2002, van Montfrans et al 2003. Developmental stage did not influence results in any trial, thus megalopae and J1 data are analyzed and presented together.…”

“…In preference tests, we simultaneously offered each vegetation type to settling crabs within a subdivided circular arena (van Montfrans et al 2003). Each 200 l arena (bottom surface = 0.28 m 2 ; 64 cm bottom width [77 cm top width] × 42 cm height) contained a center stand pipe for infrastructure, aeration, and drainage (Fig.…”

“…Equal numbers of settling crabs (6−8 J1 or 10−11 megalopae) were added to each section to simulate a random distribution, the expected condition for no preference (van Montfrans et al 2003). After 5 min of acclimation, dividers were removed to allow crabs to freely move about the arena for 12−14 h between 18:30−09:30 h, when megalopae are most active due to natural nocturnal ingress behavior (Epifanio 1995, Tankersley et al 2002, van Montfrans et al 2003, Moksnes & Heck 2006. At the end of each trial, dividers were simul taneously returned to each arena and water was drained down to ~10 cm depth through the central standpipe (lined with < 500 µm mesh to prevent settler loss).…”

“…The evolution of transient, mobile settler stages (e.g. megalopae) is attributed in part to the importance of finding and recruiting to optimal habitat by recognizing and choosing among cues for structure, food, and reduced predation (Welch et al 1997, Rittschof et al 1998, Tapia-Lewin & Pardo 2014. Settling recruits responded to both patch-level habitat cues and isolated vegetation structures.…”