Effects of Holothuroid Ichtyotoxic Saponins on the Gills of Free-Living Fishes and Symbiotic Pearlfishes

Abstract

Abstract. Several carapid fishes, known as pearlfishes, are endosymbiotic in holothuroids and asteroids. These echinoderms contain a strong concentration of saponins that are efficient membranolytic repellents to predators. We compared the effects of exposure to saponins from the sea cucumber body wall and from the Cuvierian tubules on the behavior and gill ultrastructure of pearlfishes and free-living fishes. Saponins were extracted from the body wall of two holothuroids, the Mediterranean Holothuria forskali and the tropical Bohadschia atra, and from the water surrounding the Cuvierian tubules of B. atra. Five species of carapids that live in symbiosis with holothuroids and seven species of free-living fishes were exposed to these extracts. The free-living fishes exhibited a stress response and died about 45 times faster than pearlfishes when exposed to the same quantity of saponins. Cuvierian tubules and saponins extracted from the body wall were lethal to the free-living fishes, whereas the carapids were much less sensitive. The carapids did not exhibit a stress response.The high toxicity shown by Cuvierian tubules was not explained by the nature of the saponins that were identified by mass spectrometry, but it is likely due to the higher concentration of saponins in the tubules. Histology and scanning and transmission electron microscopy of the gills of the free-living fishes and pearlfishes showed that saponins act at the level of the secondary lamellae where they induce the detachment of the epithelia, create edema at the level of the epithelia, and induce pores in the epithelial cells that lead to their destruction and the invasion of inner cells (pillar cells and red blood cells). This sequence of events happens 5 min after saponin exposure in free-living fishes and after 1 h in carapids.

Echinometra mathaei and its ectocommensal shrimps: the role of sea urchin spinochrome pigments in the symbiotic association

“…Echinoderm recognition through chemodetection is performed by many ectocommensals, and is one of the main parameters that ensures the transgenerational sustainability of symbiotic associations 6 , 16 , 20 , 23 , 44 , 45 . Two ectocommensal symbioses associated with echinoderms and decapods had previously been investigated in order to identify the nature of the chemical signals allowing host selection.…”

How different sterols contribute to saponin tolerant plasma membranes in sea cucumbers

“…Holothuroid saponins have been classified as being of the triterpenoid class 11 , and over 700 saponins have been described so far in Holothuroidea 12 . This large chemical diversity of saponins is paralleled with a wide variety of biological activities 13 that includes but are not limited to hemolytic 14 , 15 , antibacterial 16 antifungal 17 , 18 cytotoxic 19 , 20 , ichthyotoxic 21 and anti-tumoral 22 activities.…”

“…Most of these biological activities and roles result from the surface-active properties of saponins and the interactions they have with cellular membranes 24 . The ability of the glycosides to form complexes with 5,6-unsaturated sterols of target cell membranes is thought to determine their biological activity including ichthyotoxic action that may protect sea cucumbers against fish predation 21 , 25 – 27 . This complexing reaction leads to the formation of pores, permeabilization of cells and in the case of red blood cells the subsequent loss of haemoglobin in the extracellular medium 21 , 28 .…”

“…The ability of the glycosides to form complexes with 5,6-unsaturated sterols of target cell membranes is thought to determine their biological activity including ichthyotoxic action that may protect sea cucumbers against fish predation 21 , 25 – 27 . This complexing reaction leads to the formation of pores, permeabilization of cells and in the case of red blood cells the subsequent loss of haemoglobin in the extracellular medium 21 , 28 . Although the cytotoxic nature of saponins is well documented, the molecular mechanisms behind this activity is only beginning to be understood.…”

From commensalism to parasitism in Carapidae (Ophidiiformes): heterochronic modes of development?

“…Carapidae were most probably first free living before having commensal relationships with hosts (bivalves and ascidians) deprived of chemical ways of defence. They were then able to enter into echinoderms and resist their chemical defences ( Eeckhaut et al, 2015 ). Some species within this group finally developed parasitic relationships with these echinoderms.…”