Symbiosis in Fishes

The Biology of Interspecific Partnerships

Ilan Karplus

• Publisher: Wiley-Blackwell
• Year: 2014
• ISBN:
  9781118759776
• Edition: 1
• Topic: Tecnologia e ingegneria
• Subtopic: Industria ittica e acquacoltura

Symbiosis in Fishes provides comprehensive coverage of the biology of partnerships between fishes and invertebrates, ascending the phylogenetic scale, from luminescent bacteria, sponges and coelenterates to molluscs, crustaceans and echinoderms. Both facultative and obligatory partnerships are reviewed with emphasis on the behavioral, ecological and evolutionary aspects of fish symbiosis. Each of the eight chapters of this book focuses on a different group of partners. The structure, physiology and anti-predatory strategies of each group are described to provide the necessary background for the understanding of their partnerships with fishes. The formation of the associations, the degree of partner specificity and its regulation, as well as the benefits and costs for the fishes and their associates, communication between partners and their possible co-evolution are discussed in each chapter.

This is the first attempt to critically review in a single volume all associations of fishes with invertebrates based on the latest studies in these areas, together with studies published many years ago and little cited since then.

Symbiosis in Fishes provides a huge wealth of information that will be of great use and interest to many life scientists including fish biologists, ecologists, ethologists, aquatic scientists, physiologists and evolutionary biologists. It is hoped that the contents of the book will stimulate many to further research, to fill in the gaps in our knowledge in this fascinating and important subject. Libraries in all universities and research establishments where biological sciences are studied and taught should have copies of this exciting book.

THREE
The Associations between Fishes and Anthozoans

Sea Anemones

Sea anemones belong to the phylum Cnidaria and, as such, they possess a relative simple structure that consists of only a single opening to the gastric cavity and two layers of living cells, the ectoderm and endoderm, with the mesoglea, a gelatinous layer, in between. They have a basic radial symmetry and possess cnidae, specially structured stinging organelles that occur only in members of this phylum. Sea anemones belong to the Anthozoa, the largest class of the Cnidaria, which consists of over 6000 living species that include, in addition to the sea anemones, also the hexa and octo corals. All members of this class, which is strictly marine, possess only the polyp body form but not the medusa morph. Anthozoans cnidae typically lack an operculum and cnidocil and their mitochondrial DNA is circular. They possess one or more ciliated grooves in their tubular pharynx which leads from the mouth to the coelenteron, their gastric cavity that is partitioned by mesenteries or septa.

Sea anemones are generally large solitary polyps ranging in length from 1.5 to 10 cm and from 1 to 5 cm in diameter. Anemones usually possess six, or more than eight, simple or branched tentacles which surround their mouth; however, the tentacles may be small and densely scattered over the entire oral disc. Anemones inhabit the shallow and deep coastal zones attached to rocks or shells with their pedal disc or burrow in the sand or mud. Although anemones are usually visualized as sedentary, they can actually move about by slow muscular gliding on their pedal disc. Some species change their position by swimming, thrashing their tentacles or bending their column or by being passively carried by the currents after releasing their anchorage, which is secured by the pedal disc, and inflating their body with gas. Sea anemones usually feed on various small invertebrates; however, large species feed on crabs, shrimp, wave dislodged clams and even fish. The prey is captured with the tentacles, paralyzed and secured by the cnidae, then carried to the mouth. The lips, which can be inflated, assist in prey capture. Some species are suspension feeders, trapping plankton in mucus that is secreted over their body surfaces.

Many sea anemones harbor zooxanthellae in their endoderm, particularly in the tentacles and oral disc. These unicellular algae obtain from their sea anemone host inorganic compounds containing nitrogen and phosphorus, carbon dioxide and organic molecules and a substratum deployed in the sun. The sea anemone Anemonia sulcata receives a large fraction of the zooxanthellae’s photosynthetic products; these are utilized for respiration, growth and reproduction (Stambler and Dubinsky, 1987; Achituv and Dubinsky, 1990).

Asexual reproduction by fragmentation or fission is common among sea anemones. Pedal laceration, which is a special form of fragmentation, occurs either by the detached fragments moving away from the pedal disc or by leaving such fragments behind when the sea anemone moves away. Both longitudinal and transverse fission occur, however, the former is more common. Anemones may be either gonochoric or hermaphroditic; their eggs may be fertilized in the sea or inside the coelenteron where they may also develop. The free swimming planula metamorphoses into a small attached polyp.

The most common defensive response of sea anemones is the retraction of the oral disc and tentacles into the column and its closure by the sphincter muscle. Some species retract into rock fissures, the sediment or a protective tube, avoiding any further contact with a predator. Other species move away by crawling, swimming or body inflation. According to Mebs (2009) sea anemones use two defensive mechanisms which involve toxins: (i) injection of paralyzing polypeptides/proteins through stinging cnidae; (ii) covering their body with a mucus coat which contains cytolytic proteins as a deterent. The latter are very effective hemolysins and ichthyotoxins. Nonsymbiotic fish exposed to low concentrations (less than 0.5 µg/ml) of these substances die. Mortality seems to be due to severe damages to the gills, which leads to complete breakdown of the organism’s physiological functions such as osmoregulation and gas exchange. In the mucus of sea anemones, toxins may act just as a repellent due to fish moving away from the anemone; however, they may exert lethal effects when being injected through nematocysts (Mebs, 1994).

The Stinging Cells and their Release Mechanism

The cnidae are small complex intracellular organelles that usually do not exceed 50 µm in diameter and serve mainly to capture prey and deter predators. Each of these sphere-shaped organelles is located close to the outer surface of a cnidae-bearing cell – the cnidocyte. There are three basic types of cnidae: the nematocysts, the spirocysts, and the ptychocysts. The members of the classes Scyphozoa and Hydrozoa possess only nematocysts, while some of the members of the class Anthozoa possess all three types of cnidae. The nematocyst typically consists of a long, hollow everted barbed tube that is coiled and immersed in fluid inside a rigid proteinaceous capsule that is strengthened by lignin. When ejected, the barbed thread penetrates the skin of the prey/predator and injects a potent neurotoxin. The tubes of the spirocysts, which are also coiled inside the capsule, lack barbs or spines but possess minute sticky threads that radiate from the wall of the tubule and adhere to the surfaces of prey/predator. Finally, the ptychocysts are adhesive but lack minute radiating sticky threads and are stored in the capsule in a compact zig-zag, like a collapsed fire hose. When ejected, they form the tough felt-like protective tubes of the tube anemones (Ceriantharia).

The cnidocysts of sea anemones are most common in the ectoderm of the tentacles and the oral disc but also occur in the entodermal filaments, which extend from the mesenteries, possibly to quell swallowed prey. The mechanism of release and expulsion of the cnidocyst was mainly studied in hydrozoan nematocysts. The release mechanism involves both chemical stimulation (e.g., reduced gluthathione, which is released from wounded prey) concomitantly with mechanical stimulation of the cnidocil, a rigid cluster of modified cilia. There is still a debate with regard to the mechanism of nematocyst expulsion. One of the hypotheses is that upon stimulation many of the ions, especially Ca²⁺, in the capsule fluid dissociate from larger macromolecules, leading to an increase in the intracapsular osmotic pressure. This change in osmotic pressure leads to a rapid penetration of water into the capsule, leading to a drastic increase of pressure of up to 140 atmospheres....