Rank Ordering of Species Sensitivities to Chemicals: Discrepancies between Behavioral and Lethal Responses in Freshwater and Marine Invertebrates

Locomotor behavioral
responses are increasingly recognized as sensitive
and ecologically relevant indicators for assessing aquatic organism
exposure to contaminants. They often occur at concentrations lower
than those causing physiological damage, yet their consequences for
organisms’ fitness and population dynamics remain unclear.
Moreover, it is still uncertain how these behavioral endpoints align
with traditional toxicological apical endpoints used to assess species
sensitivities in risk assessment. This study evaluates the correlation
between thresholds triggering locomotor reactions with LC50 values
across 19 freshwater and 12 marine invertebrate species exposed to
the insecticide imidacloprid and metal copper. Behavioral responses
were measured via videotracking during pulse exposure assays, while
96h acute tests provided species-specific LC50 values for copper.
Results reveal strong discrepancies: some highly sensitive species
showed no locomotor changes, whereas others that were tolerant to
lethality displayed strong movement responses. For instance, freshwater
annelids and gastropods responded markedly to imidacloprid, while
marine shrimp exhibited hyperactivity under low copper levels. These
findings indicate that relying solely on lethality-based data may
underestimate the fraction of species affected by contamination. Integrating
behavioral endpoints into population risk assessment frameworks is
therefore crucial to better protect aquatic biodiversity.