Colloblasts act as a biomechanical sensor for suitable prey in Pleurobrachia


Ctenophores are a phylum of gelatinous zooplankton known for being voracious ambush predators . Most species in the phylum fall within the class Tentaculata, characterised by the presence of tentacles whose surface is covered with the phylum-defining colloblast cells . Colloblasts are bouquet-shaped with an apical enlargement protruding from the tentacle surface. The present study focuses on Pleurobrachia bachei and Pleurobrachia pileus, two closely related ctenophores of the order Cydippida found in the inshore waters of the Pacific and Atlantic oceans, respectively. Members of this order possess a pair of tentacles with numerous smaller, evenly spaced, tentilla extending perpendicularly off the main tentacle. While hunting, cydippids tend to keep their tentacles and tentilla extended, waiting for prey to drift into the resulting dragnet . When prey make contact with the tentacles, the colloblasts release their adhesive (likely from a collection of internal vesicles) and bond to the prey. This discharge presumably destroys the colloblasts, and they may be continually replaced by differentiation from epithelial stem cells .

Ctenophores’ hunting technique is reminiscent of the ambush strategy of orb weaver spiders  and the release of an adhesive from colloblasts on prey-contact is superficially similar to the harpoon-like stinging cells in cnidarians, called nematocytes or cnidocytes . However, cnidocytes and colloblasts do not share a common evolutionary origin . Many basic questions about colloblasts remain open: What do adhesion events look like? How strong is colloblast adhesive? Understanding the answers to these questions would aid our understanding of colloblast adhesive as a unique biomaterial and inform the potential limitations it puts on ctenophore predation.

In this study, the researcher applied live microscopy techniques to visualise adhesion between probes and tentacles of Pleurobrachia pileus, assessing the fates of individual colloblasts engaging in adhesion. We use this system assess the impact of contact area and surface chemistry on adhesion. We then adapt instrumentation for measuring surface tension to measure the adhesive force exerted by the colloblast adhesive system of Pleurobrachia bachei. Our data demonstrate that ctenophore prey capture is a robust mechanism that acts quickly to ensnare prey under a variety of conditions. Furthermore, the burgeoning understanding of colloblasts is itself integral to our understanding of ctenophore ecology in a rapidly changing marine environment.

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