Mon, Aug 02, 2021:On Demand
Background/Question/Methods: For animals with complex life cycles, the motile larval phase is an important contributor to adult distribution patterns. Recruitment relies on an initial step of larvae settling to optimal habitat. But is the settlement process moderated by a directed choice behavior rather than a random process? Examples of marine invertebrate larvae settling in response to specific settlement cues suggests larval settlement can be far from random. Some marine invertebrate larvae are unable to settle unless the right habitat (cue) is encountered; this behavior invoking creative titles such as the “Death Before Dishonour Hypothesis”. Other species delay settlement until optimal habitats are encountered, but retaining the ability to settle in sub-optimal habitats if they become desperate (Desperate Larval Hypothesis). Information on whether sponge larvae actively explore and choose sites to settle is limited. This study will address the question: Do sponge larvae actively settle to habitat-related cues? Larval choice settlement assays were undertaken using four sponge species. Larvae were exposed to treatments that represented signals of reef habitat and corresponding settlement quantified. Habitat related treatments tested included physical (surfaces with engineered surface topography), chemical-based cues (extracts of conspecifics, microorganism biofilms or coralline algae) and treatments of no cue.
Results/Conclusions: During a demersal phase, larvae from all four species exhibited “exploration” behaviors of intermittent swimming and contact with the “experimental benthic surface”. For chemical-based cues, sponges consistently settle more rapidly, and in higher numbers, in the presence of all experimental treatments in comparison to controls of no cue. A significant effect of physical-based cues was also demonstrated with larvae choosing to settle in surface pits that closely resemble larval sizes (~200 µm) rather than larger pits (400-1000 µm) or no surface topography at all. That these sponges show directed choices to settle to a range of habitat-related cues supports an interpretation that some species can actively choose settlement sites and clearly showing that larval settlement for some sponges is far from random. Furthermore, evidence of directed settlement choices in this aneural group highlights some capacity for decision-making; decisions presumably centered on the potential of cellular communication to sense, process, and respond to external stimuli (external environments) thereby aiding post-settlement success and recruitment to adult populations.
Results/Conclusions: During a demersal phase, larvae from all four species exhibited “exploration” behaviors of intermittent swimming and contact with the “experimental benthic surface”. For chemical-based cues, sponges consistently settle more rapidly, and in higher numbers, in the presence of all experimental treatments in comparison to controls of no cue. A significant effect of physical-based cues was also demonstrated with larvae choosing to settle in surface pits that closely resemble larval sizes (~200 µm) rather than larger pits (400-1000 µm) or no surface topography at all. That these sponges show directed choices to settle to a range of habitat-related cues supports an interpretation that some species can actively choose settlement sites and clearly showing that larval settlement for some sponges is far from random. Furthermore, evidence of directed settlement choices in this aneural group highlights some capacity for decision-making; decisions presumably centered on the potential of cellular communication to sense, process, and respond to external stimuli (external environments) thereby aiding post-settlement success and recruitment to adult populations.