Final answer:
A light-sensing eyespot is ineffective for an obligate saprobe because saprobes depend on organic matter, not light. An alternative organ for a saprobic protist would be a chemoreceptor, allowing it to detect chemical signals in its environment which aligns with its chemoheterotrophic nature.
Step-by-step explanation:
A light-sensing eyespot would be ineffective for an obligate saprobe because saprobes require organic matter to survive rather than light. Saprobes, such as fungus-like protists, thrive in environments where they can absorb nutrients from nonliving organic matter, such as dead organisms or their wastes. In many aquatic ecosystems or places like hydrothermal vents, light availability is limited or nonexistent, rendering light-based photosynthesis impractical or impossible.
Therefore, rather than relying on a light-sensing mechanism, saprobes can benefit from having a chemoreceptor. Chemoreceptors allow these organisms to sense and respond to chemical signals in their environment which is crucial for finding food sources in dark environments like the bottom of lakes, ponds, or deep ocean layers. Saprobes are typically chemoheterotrophs, meaning they derive their energy from chemical sources and their carbon from organic compounds, not requiring light for survival.