Symbiosis and Parasitism in Marine Organisms Flashcards

Core Concepts of Symbiosis and Interspecific Interactions

• Definitions and Scale of Associations:     * Symbiosis: Defined as a close, long-term association between two or more different species of organisms. The involved organisms are termed symbionts.     * Spectrum of Relationships: Symbiotic relationships cover a broad spectrum, ranging from:         * Random         * Casual         * Facultative (optional)         * Obligatory (necessary for survival)

• Types of Interspecific Interactions:     * Competition ($-/-$): Both species are negatively affected.     * Predation ($+/-$): One benefit, one is harmed.     * Mutualism ($+/+$): A form of symbiosis in which two species associate for their mutual benefit. Partners are termed symbionts.     * Commensalism ($+/0$): Association where there is a clear example of benefit to one member while not harming the other.     * Parasitism ($+/-$): One benefits at the expense of the other.

• Specialized Subdivisions:     * Inquilinism: A subdivision of commensalism where one animal lives in the home of another, or in its digestive tract, without being parasitic.     * Terminology: In commensal and inquiline relationships, the partner gaining the advantage is the commensal, and the other is the host.

Distribution and Comparison of Higher Animal Taxa

• Ecosystem Comparison: Symbiotic associations are significantly more common in marine environments than in terrestrial ones.

• Taxonomic Data (Table 1.4):     * Marine Ecosystem: 32 Phyla, 73 Classes (Not counting phyla not subdivided into classes; includes parasites).     * Terrestrial Ecosystem: 12 Phyla, 25 Classes.     * Freshwater Ecosystem: 16 Phyla, 33 Classes.     * Note on Classification: Mesozoa are considered two phyla, Pogonophora as a polychaete, and Pentastomida as a class of arthropods.

Algae-Invertebrate Symbiotic Associations

• General Characteristics:     * Usually involve autotrophic unicellular algae.     * Geographic Distribution: Most common in tropical waters; less common in temperate waters; virtually absent in polar waters.     * Environmental Restrictions: Restricted to shallow waters, intertidal regions, and the upper pelagic realm due to the photosynthetic requirements of the autotrophs.

• Association Types:     1. Intracellular/Intercellular: The entire autotrophic cell resides within the host organism. These are usually restricted to specific tissues and found within vacuoles inside tissue cells or in spaces between/within tissue layers.     2. Organelle Incorporation: Only the functioning chloroplasts from the algal cells are incorporated into the invertebrate tissues.

• Classification of Algal Symbionts (by color):     1. Zooxanthellae: Brown, yellow, or golden cells.     2. Zoochlorellae: Green cells (e.g., found in Snakelocks Anemone and Flatworms).     3. Cyanellae: Blue-green cells.

• Transmission Methods:     * Vertical Transmission: Passed to the next generation via the eggs (common in Cnidaria).     * Re-infection: Must be replenished from the environment in each new generation (e.g., some Gastropods and Porifera).

• Case Study: Cassiopeia Jellyfish:     * Unlike most jellyfish, Cassiopeia sp. lies upside-down on the seabed.     * This behavior exposes their oral tentacles, which contain algae, to sunlight.

Animal-Animal Symbiotic Associations

• Living Arrangements:     * Endozoites: Organisms living inside other organisms.     * Epizoites: Organisms living on the surface of other organisms. Many epizoites are not strictly commensal; their association is often due to settlement on sessile or slow-moving hosts.     * Ciliate Protists: Found on external surfaces or internally (gills and digestive tract).

• Case Study: U-Shaped Burrow of Urechis caupo:     * The Echiurid Worm (Urechis caupo) hosts several commensals in its burrow:         1. Fish (Clevelandia ios)         2. Scale Worm (Hesporone adventor)         3. Pinnotherid Crab (Scleroplax granulata)         4. Long-Fingered Shrimp (Bataeus longidactylus)

• Convergent Evolution and Specific Examples:     * Cling Fish (Diademichthys lineatus) and Aeoliscus strigatus: Exhibit similar adaptations for living among sea urchin spines.     * Pearlfish (Carapus sp.): Lives inside the bodies of Sea Cucumbers (Holothurians).     * Portuguese Man-of-War (Physalia sp.) and Fish (Nomeus gronovii): The fish lives among the stinging tentacles.     * Goby Fish (Cryptocentrus lutheri) and Blind Shrimp (Alpheus djiboutensis): A mutualistic partnership where the goby acts as the eyes for the burrow-digging shrimp.

• The Boxer Crab (Lybia tesselata):     * Interaction: The crab holds sea anemones in its front claws (chelopeds).     * Adaptation: Claws have small teeth evolved specifically for holding anemones; feeding is performed by modified front walking legs.     * Benefit to Crab: Defense via the anemone's stinging tentacles.     * Benefit to Anemone: Mobility for finding food and access to scraps from the crab's meals.

Fundamentals of Marine Parasitology

• Definitions of Parasitism:     * Klaus Rohde (2005): "Close association of two organisms, in which one – the parasite – depends on the other – the host – deriving some benefit from it."     * Robert Poulin (2006): "An organism living in or on another organism, the host – feeding on it, showing some degree of structural adaptation to it, and causing it some harm."     * Oxford English Dictionary (2008): Defined as obtaining nutrients at the expense of the host organism, which it may directly or indirectly harm.

• Quantitative Metrics in Parasitology:     * Prevalence: The proportion of infected hosts at a given time.         * $\text{Prevalence} = \frac{\text{Infected Hosts}}{\text{Total Hosts Sampled}}$         * Example: 4 infected, 6 uninfected ($\text{Total} = 10$); Prevalence = $0.4$ or $40 \%$.     * Intensity: The number of parasite individuals of a particular species in each infected host.     * Mean Intensity: The total number of individuals of a particular parasite species in a sample divided by the number of infected individual hosts.         * Example: Parasite counts of 5, 4, 4, 3 in 4 infected hosts; Mean Intensity = $\frac{5+4+4+3}{4} = 4$.     * Abundance: Total number of individuals of a particular parasite species in a sample divided by the total number of hosts (infected and uninfected).         * Example: Counts in 8 hosts (0, 0, 5, 4, 0, 0, 4, 3); Abundance = $\frac{16}{8} = 2$.

Parasite Life Cycles

• Direct Life Cycle: The parasite passes from the definitive host (adult stage) through a larval stage back to a definitive host without intermediate hosts.

• Indirect Life Cycle: Involves one or more intermediate hosts.     * Definitive Host: Where the adult stage of the parasite resides.     * 1st Intermediate Host: Usually where larval stage L1 develops (often a mollusc in trematodes).     * 2nd Intermediate Host: Supports larval stage L2.     * Paratenic Host: A "transport" host where the parasite survives but does not undergo further development.

Case Studies in Marine Parasitism

• The Tongue-Eating Louse:     * Parasite: Cymothoa exigua (Crustacean).     * Host: Lutjanus guttatus (Spotted Rose Snapper).

• Trematodes (Flukes):     * Typical Cycle: Eggs $\rightarrow$ Miracidium (free-living) $\rightarrow$ Sporocyst/Redia in 1st intermediate host (mollusc) $\rightarrow$ Cercariae (free-living) $\rightarrow$ Metacercariae in 2nd intermediate host $\rightarrow$ Adult in vertebrate definitive host.     * Ecological Impact: Metacercariae in the foot of a cockle prevent it from burrowing deep into sediment. This results in limpets and sea anemones co-inhabiting cockle shells, altering infaunal diversity and community structure.

• Acanthocephala (Thorny-headed Worms):     * Anatomy: Features a proboscis with hooks, neck, lemniscus, and cement glands.     * Life Cycle: Involves amphipods, fish, and seals.     * Host Manipulation: Changes the behavior of the host. For example, Gammarus infected with P. laevis show altered phototactism (reactions to light). This is linked to altered serotonin activity in the Gammarus brain (specifically the Tritocerebral Giant Neuron - TGN).

• Rhizocephala (Sacculina sp.):     * Host: Crabs such as Carcinus maenas.     * Anatomy: Consists of an Externa (reproductive part outside the host) and an Interna (root-like system inside the host).     * Effects on Host:         1. Extensive damage to endocrine organs, CNS, and gonads.         2. Hypertrophy and degeneration of the androgenic gland.         3. Sterilization of the host.         4. Feminization of male hosts (e.g., changes to the abdomen).         5. Alteration of growth (molting) and behavior (e.g., the crab treats the parasite externa as its own eggs—"egg mimicry").     * Developmental Stages: Nauplius $\rightarrow$ Cypris $\rightarrow$ Kentrogon $\rightarrow$ Vermigon (injected into host via stylet).

• Microsporidia:     * Host: Amphipods (e.g., Gammarus, Echinogammarus).     * Life Cycle and Feminization: Species with vertical life cycles can feminize genetic males into phenotypic females.     * Transmission Logic: Vertical transmission is typically maternal (via the egg). Males are a transmission "dead-end." By converting males to females, the parasite increases the frequency of the transmitting sex.     * Research Data: Gammarus duebeni infected with Nosema granulosis or Dictyocoela duebenum show significantly higher percentages of female offspring compared to uninfected broods.