Question

Passage:
In 1977, oceanographers aboard the research submersible Alvin explored the Galápagos Rift, an active spreading center on the eastern Pacific seafloor. Anticipating only barren volcanic basalt at depths exceeding 2,500 meters, researchers instead encountered dense, localized ecosystems clustered around hydrothermal vents. These geothermally heated fissures discharged mineral-laden fluids enriched with hydrogen sulfide ($H_2S$) into the near-freezing abyssal waters. The discovery challenged the foundational ecological paradigm that all primary biomass production strictly depends upon solar radiation via photosynthesis. Subsequent biochemical analyses revealed that these benthic communities are sustained by chemolithoautotrophic bacteria, which oxidize inorganic sulfur compounds to fix inorganic carbon into organic molecules.

Among the most physiologically striking organisms inhabiting these vent fields is the giant tube worm, Riftia pachyptila. Reaching lengths of up to two meters, Riftia lacks a digestive tract, mouth, and anus at maturity. Instead, it relies entirely on intracellular bacterial symbionts housed within a specialized, highly vascularized internal organ termed the trophosome. The tube worm's distinctive bright red plume, rich in specialized hemoglobin molecules with an unusually high affinity for both oxygen and hydrogen sulfide, extracts dissolved gases directly from the surrounding water. These hemoglobins bind sulfide reversibly and with high affinity, simultaneously preventing sulfide toxicity to the host's own respiratory chain while transporting the substrate to the trophosome. Inside the trophosome, the endosymbiotic bacteria utilize the energy released from sulfide oxidation to synthesize organic nutrients, which are subsequently translocated to host tissues.

While sulfur-oxidizing symbioses represent the dominant trophic adaptation at basaltic ridge crests, subsequent deep-sea explorations at ultra-slow spreading centers—such as the Mid-Atlantic Ridge's Lost City hydrothermal field—revealed an alternative geochemical regime. At Lost City, serpentinization reactions between seawater and mantle peridotite produce alkaline, hydrogen- and methane-rich fluids at significantly lower temperatures (40°C to 90°C) without the high concentrations of hydrogen sulfide characteristic of basalt-hosted vents. In these alkaline environments, microbial communities rely primarily on methanogenesis and hydrogen oxidation, demonstrating that deep-sea primary production encompasses diverse chemotrophic pathways that operate independently of solar input and volcanically derived sulfur.

Statement: According to the passage, the mature giant tube worm (Riftia pachyptila) possesses neither a mouth nor a digestive tract, depending instead on internal endosymbiotic bacteria contained in an organ called the trophosome.