Symbiosis and Parasitism

Introduction to Symbiosis and Interspecific Interactions

• Terminology and Framework: Symbiotic relationships represent a broad spectrum of associations ranging from random and casual to facultative (optional) or obligatory (essential for survival).

• Definition of Symbiosis: A close, long-term association between two or more different species of organisms. The participants in these relationships are termed symbionts.

• Major Types of Interspecific Interactions:     * Competition (-/-): Both species are negatively impacted by the interaction.     * Predation (+/-): One species (predator) benefits while the other (prey) is harmed.     * Mutualism (+/+): A form of symbiosis where both involved species derive a benefit from the association.     * Commensalism (+/0): An association where one member clearly benefits, while the other (the host) is neither helped nor harmed.     * Parasitism (+/-): One species (the parasite) benefits at the expense of the other (the host).

• Specialized Subdivisions:     * Inquilinism: A subdivision of commensalism where one animal lives within the home or digestive tract of another without being parasitic. The benefiting partner is the commensal, and the provider is the host.

Symbiosis in the Marine Environment

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

• Biodiversity Statistics (Table 1.4):     * Phyla:         * Marine: $32$         * Terrestrial: $12$         * Freshwater: $16$     * Classes:         * Marine: $73$         * Terrestrial: $25$         * Freshwater: $33$     * Note: These counts consider Mesozoa as two phyla, Pogonophora as a polychaete, and Pentastomida as a class of arthropods.

• Broad Groupings of Symbiotic Associations:     1. Associations between algae and various invertebrate animals.     2. Associations between different animals (both invertebrates and vertebrates).     3. Associations between bacteria and animals.

Associations Between Algae and Marine Invertebrates

• General Characteristics:     * Typically involve autotrophic unicellular algae.     * Highest prevalence in tropical waters; lower prevalence in temperate waters; virtually absent in polar waters.     * Restricted to shallow waters, intertidal regions, and the upper pelagic realm due to the photosynthetic requirements of the algae.

• Mechanical Types of Association:     1. Whole Cell Integration: The entire autotrophic cell resides within the invertebrate, usually restricted to specific tissues. They are found within vacuoles inside tissue cells or in spaces between/within tissue layers.     2. Organelle Integration: Only the functioning chloroplasts from the algal cells are incorporated into the tissues of the invertebrate host.

• Taxonomic Classification (by color):     1. Zooxanthellae: Brown, yellow, or golden cells.     2. Zoochlorellae: Green cells (e.g., found in Snakelocks Anemones and certain flatworms).     3. Cyanellae: Blue-green cells.

• Key Examples:     * Snakelocks Anemone and Flatworms: Utilize Zoochlorellae.     * Cassiopeia Jellyfish: Known as the "upside-down jellyfish," it lies on the seabed to expose its oral tentacles (which contain algae) to sunlight.

• Transgenerational Transmission:     * Vertical Transmission: Passed via the eggs to the next generation (e.g., Cnidaria).     * Re-infection: New generations must be replenished from the environment (e.g., some Gastropods and Porifera).

Commensal and Mutualistic Animal Associations

• Living Locations:     * Endozoites: Organisms living inside other organisms.     * Epizoites: Organisms living on the surface of other organisms. Many epizoites are not strictly commensal; their presence may be due to settling on sessile or slow-moving hosts.

• Commensal Protists: Ciliate protists are often found on external surfaces or internally within the gills and digestive tracts of hosts.

• The U-Shaped Burrow Ecosystem: In the burrow of the Echiurid Worm (Urechis caupo), several commensals may reside simultaneously, including:     * Fish (Clevelandia ios).     * Scale Worm (Hesporone adventor).     * Pinnotherid Crab (Scleroplax granulata).     * Long-Fingered Shrimp (Bataeus longidactylus).

• Convergent Evolution and Mimicry: Examples of different species evolving similar symbiotic traits or appearances to live with hosts:     * Cling Fish (Diademichthys lineatus) and the shrimp Stegopontonia commensalis.     * Pearlfish (Carapus sp.): Lives inside the bodies of Sea Cucumbers (Holothurians).     * Portuguese man-of-war (Physalia sp.) and the fish Nomeus gronovii.

• Mutualistic Case Studies:     * Goby Fish and Blind Shrimp: The blind shrimp (Alpheus djiboutensis) digs the burrow while the Goby (Cryptocentrus lutheri) acts as a sentinel, warning of danger.     * Mutualistic Cleaning: Removal of parasites from larger fish by cleaner species.     * Boxer Crab (Lybia tesselata): Carries sea anemones in its chelipeds (front claws).         * Crab Benefits: Defense provided by the anemone's stinging tentacles; modified legs are used for feeding since claws are occupied.         * Anemone Benefits: Increased mobility for finding food and access to scraps from the crab's meals.

Overview of Marine Parasitology

• Key Definitions:     * Klaus Rohde (2005): A close association of two organisms where the parasite depends on and derives benefit from the host.     * Robert Poulin (2006): An organism living in or on a host, feeding on it, showing structural adaptation, and causing harm.     * Oxford English Dictionary (2008): An organism obtaining nutrients at the expense of another species, which it may directly or indirectly harm.

• Study Methodologies: Includes visual observations, dissections, histology, molecular approaches (PCR), immunological approaches, and CT scans.

Parasite Population Metrics and Life Cycle Strategies

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

• Life Cycle Types:     1. Direct Life Cycle: The parasite passes from the definitive host (where adult stages occur) to a larval stage and back to a definitive host without intermediate hosts.     2. Indirect Life Cycle: Involves one or more intermediate hosts ($L1, L2$) and sometimes a paratenic (transport) host before reaching the definitive host.

Case Studies in Marine Parasitism: Trematodes and Acanthocephalans

• Cymothoa exigua: An isopod (crustacean) that parasitizes the Spotted Rose Snapper (Lutjanus guttatus), often replacing the host's tongue.

• Trematodes (Flukes): Typical complex life cycle involving:     * Definitive host: Vertebrates (adult parasitic stage).     * Egg: Released into the environment.     * Miracidium: Free-living stage.     * 1st Intermediate host: Molluscs (Sporocyst and Redia parasitic stages).     * Cercariae: Free-living stage.     * 2nd Intermediate host: Invertebrates or vertebrates (Metacercariae stage).

• Trematode Ecological Impact: Metacercariae in the 'foot' of cockles prevent the cockle from burrowing deep into the sediment. This leads to increased colonization of the shell by limpets and anemones, altering the infaunal diversity of the sediment and impacting the whole community.

• Acanthocephala (Spiny-headed worms):     * Anatomy: Characterized by a proboscis with hooks, neck, trunk with body spines, and lemniscus.     * Manipulation of Host Behaviour: Parasites like Pomphorhynchus laevis alter the phototactism (response to light) of their intermediate hosts (e.g., Gammarus pulex).     * Mechanism: Changes in brain serotonin levels in the host are linked to altered phototactic scores (Score $0$ vs Score $1$), making the host more vulnerable to predation by the definitive host (fishes or seals).

• Human Health Impacts: Consumption of raw fish can lead to infections such as:     * Chlonorchiasis (Liver Fluke): Causes jaundice, liver enlargement, and gastrointestinal issues.     * Anisakiasis: Causes sudden, severe abdominal pain.     * Diphyllobothrium (Tapeworm): Causes irritability, numbness, muscle weakness, and abdominal discomfort.

Manipulative and Feminizing Parasites: Rhizocephala and Microsporidia

• Rhizocephala (e.g., Sacculina sp.):     * Host: Crabs such as Carcinus maenas.     * Infection Process: The cyprid larva forms a kentrogon, which injects a "vermigon" into the host. The parasite then grows an internal network (interna) and an external reproductive sac (externa).     * Effects on Host:         * Extensive damage to endocrine organs, Central Nervous System (CNS), and gonads.         * Hypertrophy and degeneration of the androgenic gland in males.         * Sterilization and Feminization: Infected males may develop female-like wider abdomens.         * Behavioral Modification: Hosts may exhibit "egg mimicry," where they care for the parasite's externa as if it were their own egg mass.

• Microsporidia:     * Hosts: Amphipods (e.g., Gammarus, Echinogammarus).     * Vertical Transmission: Parasites are transmitted from parent to offspring via gametes. Because eggs are larger than sperm, transmission is usually maternal.     * Feminization Strategy: Since males are an evolutionary dead-end for the parasite, Microsporidia (Nosema granulosis, Dictyocoela duebenum) convert genetic males into phenotypic females.     * Results: These feminized males function as females and produce eggs that continue the transmission of the parasite, significantly skewing the sex ratio of the host population.

Questions & Discussion

• Question: Why are associations between autotrophs and marine invertebrates restricted to shallow waters?

• Answer: Algal symbionts require sunlight for photosynthesis to produce nutrients for themselves and the host; light penetration is limited in deeper marine zones.

• Question: How might algal associations come about?

• Answer: Possible mechanisms include ingestion of algae without digestion or the evolutionary integration of specific organelles (chloroplasts) into host cells.