Morphology and ichthyotoxicity as defensive strategies in soft corals from the Great Barrier Reef

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Three collections of common soft corals were made for an ichthyotoxicity study. Morphological features relating to structural defence against fish predation were also assessed. The first set of samples, comprised of 68 specimens derived from 16 genera and 57 species was collected from Britomart Reef, Davies Reef, Slashers Reef and Orpheus Island between August and October, 1980. The second set of 36 specimens derived from 9 genera and 34 species, was collected from Britomart Reef, Rib Reef and Myrmidon Reef between August and October 1982. A final set of samples was collected at Lizard Island and was comprised of 67 specimens derived from 16 genera and 55 species.Initially, 16 attributes (each with between 2 and 4 levels) presumed to be of adaptive significance in morphological defence against fish were recorded for 68 alcyonacean corals from a wide range of species. These characters fell into the more general categories of: gross colony form (encrusting or erect); colony texture (including sharpness or spikiness, hardness, flexibility, and penetrability); presence of mucus; color; polyp retractility; sclerite morphology and distribution (including density of sclerite packing, length, sharpness, placement, and orientation within the colony - assessed microscopically). The soft coral species used in this study were then categorised from most toxic to least toxic (1-9) based on previous ichthyotoxicity studies of behaviour and mortality levels in test fish. A second set of observations concentrated on 28 specimens of the genus Sinularia and 28 specimens from the family Nephtheidae, which vary widely in both their morphological and ichthyotoxic characteristics. A more restricted and clearly defined set of morphological characters were considered: consistency of exposed colony parts; superficial armament of the polypary; mineralization of coenenchymal mass; anthocodial armament; and polyp retractility. In this study, soft coral species were classified as toxic or non-toxic based on relative mortality levels of test fish after 12 hours.
This research was initiated to investigate the relationship between morphological features and ichthyotoxicity in defence against fish predation in soft corals.

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Statement: Statement: Toxicity data used in this study were derived from previous research described in the following publications:Coll JC, La Barre SC, Sammarco PW, Williams WT and Bakus GJ (1982) Chemical defences in soft corals (Coelenterata: Octocorallia) of the Great Barrier Reef: a study of comparative toxicities. Marine Ecology Progress Series 8: 271-278.Coll JC and Sammarco PW (1983) Terpenoid toxins of soft corals (Cnidaria, Octocorallia): their nature, toxicity and ecological significance. Toxicon Suppl 3: 69-72.La Barre SC, Coll JC and Sammarco PW (1986) Defensive strategies of soft corals (Coelenterata: Octocorallia) of the Great Barrier Reef. II. The relationship between toxicity and feeding deterrence. Biological Bulletin 171: 565-576.Common types of fish predators occurring naturally in the mid- and outer-shelf regions were taken into consideration when identifying potential functional morphological defenses. Characters which might serve as defenses against browsers, which attack the colony as a whole (eg parrotfish, triggerfish, pomacanthids and labrids) were considered as well as characters serving as a defence against fish, which attack individual polyps, such as butterflyfish (chaetodonts).Categories used to describe morphological characteristics of soft corals for the initial analysis:Gross colony form1 - encrusting2 - mushroom-shaped3 - erect, tree-like4 - stoloniferousEncrusting subtype1 - completely flat2 - massive (eg Sinularia) 3 - extensively lobedErect subtype1 - thin lobes2 - thick lobes 3 - spherical Texture 1 - sharp and spiky 2 - smooth Hardness 1 - hard 2 - medium 3 - soft Flexibility 1 - flexible 2 - moderately flexible 3 - rigidPenetrability1 - cortical only 2 - homogeneously tough 3 - no apparent toughness Basal sclerites (density)1 - dense 2 - sparse Basal sclerites (length)1 - over 3 mm 2 - 1 to 3 mm 3 - less than 1 mmBasal sclerites (sharpness)1 - sharp 2 - blunt Upper sclerites (density)1 - dense 2 - sparse 3 - forming an exoskeleton Upper sclerites (length)1 - over 3 mm 2 - 1 to 3mm3 - less than 1 mm Upper sclerites (sharpness)1 - sharp 2 - blunt Retractility1 - polyps retractile 2 - polyps not retractile Mucus1 - mucus present 2 - mucus absent Colour1 - conspicuous 2 - not conspicuousSet of morphological characters used for analysis of the genus Sinularia and the family Nephtheidae:Consistency of exposed colony partsa) Firm but brittle tissue (such as that which may be found in Lobophytum pauciflorum) or soft but easily torn tissue (such as that found in Sarcophyton glaucum)b) Tough leathery tissue, found in many encrusting alcyoniids. Examples would include Sinularia dura or the more elastic tissue found in the lacuna-rich stems of many Nephtheidae such as Dendronephthya.Superficial armament of the polyparya) Weak, where sclerites on the exterior are too small, blunt, or sparse, or are entirely absent, so as to provide no physical deterrence to fish predation. (eg Sinularia flexibilis and Lemnalia spp.)b) Strong, where interpolypal sclerites, even if sparse, are long, sharp, and protrude from the colony (eg Dendronephthya spp. and several digitate and laminar species of Sinularia, including Sinularia dura)Mineralization of coenenchymal massa) Lightly mineralized interior (eg such fleshy alcyoniids as Sinularia flexibilis, Sinularia capillosa, and Sarcophyton glaucum as well as the nephtheid Lemnalia spp.)b) densely mineralized interior, where the sclerites, even though not necessarily large or sharp, are tightly packed, particularly towards the colony base (eg several species of Sinularia with low relief - Sinularia compacta, Sinularia dura or Sinularia variabilis.Anthocodial armamenta) Weak, where the sclerites are too small, sparse, or inadequately arranged to be effective in defence of the polyps against fish predation (eg all alcyoniids, including Sinularia flexibilis, Xenia sp, and such nepththeids as Nephthea sp. a - BH-23, Lemnalia spp. and Paralemnalia spp.)b) Strong, where polypal sclerites are well developed and adequately arranged so as to directly shield the individual polyp (eg Capnella spp.) or harm a predator (eg Nephthya sp. b - BH -45).Polyp retractilitya) Non-retractile, where the polyp and its tentacles cannot be completely withdrawn inside the coenenchymal mass (eg Nephthea sp.)b) Retractile, where polyps and tentacles can be completely withdrawn into the coenenchyme (eg all alcyoniids, including Sinularia flexibilis, Sinularia gyrosa, and Sarcophyton spp. as well as the neptheids Lemnalia spp. and Paralemnalia sp.The morphological feature "superficial armament of the polypary" used in this study is described in: Bayer FM, Grasshoff M, Verseveldt J (1983) Illustrated trilingual glossary of morphological and anatomical terms applied to Octocorallia. Brill EJ, Backhuys W, Leiden, p 75.Toxicity categories used in the second study:1 - Toxic (some or all test fish died within 12 hours)2 - Non-toxic - no mortality within 12 hours)

Notes

Credit
Sammarco, Paul W, Dr (Principal Investigator)