Molecular mechanisms mediating stiffening in the mechanically adaptable connective tissues of sea cucumbers

•Echinoderms possess the ability to voluntarily stiffen or soften their collagenous tissues through the release of specialized effector proteins.•Using a combination of in silico analyses, Hf-(D)Tensilin, a stiffening protein from the dermis of the sea cucumber Holothuria forskali, was identified.•Specific antibodies revealed that this protein is localized in the secretory granules of type 2 juxtaligamental-like cells, a mutable collagenous tissue specific cell type.•Recombinant tensilin stiffens pieces of dermis and aggregates collagen fibrils in vitro.•Assays with truncated recombinant proteins and with various compounds disentangled the molecular interactions which mediate tensilin aggregating effect on collagen fibrils.•Understanding the molecular mechanism underlying sea cucumber dermis mutability is a prerequisite for the development of biomimetic smart dynamic materials.