Question

Passage:
In 1977, the discovery of hydrothermal vents along the Galápagos Rift revolutionized marine biology by revealing thriving ecosystems in the aphotic zone of the deep ocean. Prior to this, the scientific consensus held that all complex biological communities ultimately relied on photosynthesis. However, these vent communities were supported by chemosynthesis—specifically, the oxidation of hydrogen sulfide ($H_2S$) dissolved in the vent fluid. The most visually striking inhabitants of the Galápagos vents, the giant tube worms (*Riftia pachyptila*), completely lack a mouth and digestive tract. Instead, they rely entirely on endosymbiotic sulfur-oxidizing bacteria housed in a specialized internal organ called the trophosome. The worms' specialized hemoglobin transports both oxygen from the ambient seawater and hydrogen sulfide from the vent emissions directly to these bacteria, which in turn synthesize organic molecules that nourish the host.

While the Galápagos discoveries established the paradigm of sulfur-based chemosynthesis, subsequent explorations in the 1980s and 1990s along the Mid-Atlantic Ridge and the Gulf of Mexico cold seeps expanded the known metabolic repertoire of deep-sea symbioses. Researchers investigating the Snake Pit hydrothermal field on the Mid-Atlantic Ridge discovered dense beds of *Bathymodiolus* mussels. Unlike *Riftia*, these mussels possess a functional, albeit reduced, digestive system, allowing them to filter feed, but they derive the bulk of their nutrition from bacterial symbionts. Crucially, investigations led by microbiologist Colleen Cavanaugh demonstrated that the bacteria residing in the gills of these mussels do not utilize hydrogen sulfide; rather, they are methanotrophs, oxidizing methane ($CH_4$) gas bubbling up from the seafloor.

Further complicating this physiological landscape, later expeditions to the Logatchev hydrothermal field in 1994 revealed dual-symbiont systems. Certain species of *Bathymodiolus* were found to harbor both sulfur-oxidizing and methane-oxidizing bacteria simultaneously within their gill tissues. This physiological plasticity allows these mussels to exploit fluctuating vent fluid compositions, surviving in environments where the relative concentrations of hydrogen sulfide and methane are highly variable. By selectively regulating the populations of each symbiont type based on the immediate chemical environment, these dual-symbiont mussels exhibit a profound ecological adaptability not seen in the obligate sulfur-dependent *Riftia*.

The contrast between the rigid physiological specialization of *Riftia pachyptila* and the flexible metabolic strategies of dual-symbiont *Bathymodiolus* species underscores a broader ecological divergence within deep-sea chemosynthetic environments. *Riftia* typically colonize ephemeral vent sites characterized by robust, high-temperature, sulfide-rich emissions. Their rapid growth rates—often exceeding one meter per year—allow them to swiftly exploit these transient habitats before the vents clog or seismic activity diverts the fluid flow. In contrast, the dual-symbiont mussels are typically found at cooler, more stable diffuse flow sites or cold seeps, where chemical emissions are less concentrated but longer-lasting. The evolutionary pressure at these stable but chemically fluctuating sites has favored metabolic versatility over sheer growth speed. Consequently, while *Riftia* communities often experience catastrophic local extinctions followed by rapid colonization of new vents, the mussel beds persist as long-term anchors of their respective ecosystems, slowly adapting to the gradual waning of geothermal activity.

Based on the passage, which of the following statements about deep-sea organisms and their symbiotic relationships are explicitly supported by the text? Select all that apply.