Banded Killifish

Fascinating facts about the Banded Killifish

• A single banded killifish can lay up to 100 eggs in one spawning bout, and the parents abandon the clutch after it sticks to a plant.
• When predators loom, these fish swell their shoals from typical groups of 3-6 adults to larger gatherings, exploiting the dilution effect so each individual’s chance of being eaten drops dramatically. It highlights how social flexibility can outweigh the loss of personal feeding space.
• Males flaunt more than nine dark bars in front of the dorsal fin, a trait that lets researchers instantly tell sex and even subspecies, because the eastern form sports 9-15 anterior bars while the western shows far fewer. The pattern acts as a natural barcode for scientists tracking hybrid zones.
• Spawning occurs when water warms to 21-23 °C (70-73 °F) between June and mid-August.
• The largest recorded individual measured 12.8 cm (5.0 in) in Indian Bay, Canada, compared with an average length of 6.3 cm (2.5 in).

At a glance

About the Banded Killifish

The banded killifish (Fundulus diaphanus) glides through the quiet shallows of eastern North America with a sleek, elongate body painted in muted bluish-gray or olive tones, its dorsal side darker than the pale, yellowish belly. Thin black vertical bars, 12 to 20 in number, run along its sides, a pattern that shifts between sexes: females display distinct, often incomplete bars, while males carry a greater than nine count of silvery, tightly spaced bands before the dorsal fin, and in breeding season flash brilliant blue patches on the anal fin and a shimmering blue iridescence across the lower body. These modest fish, rarely exceeding 13 cm (5.1 in) total length, favor the shallow, vegetated margins of clear lakes, ponds, rivers and estuaries, where sand and gravel offer refuge for hatchlings and abundant aquatic plants provide both shelter and spawning sites. They are euryhaline yet predominantly freshwater dwellers, forming loose shoals of three to six adults, or larger groups of eight to twelve juveniles, balancing the safety of numbers against the competition for food. A surprising trait emerges when predators loom: the shoal swells, exploiting the dilution effect, while hungry individuals may abandon the group, prioritising foraging over protection, a dynamic dance of risk and reward that underscores their finely tuned ecological role.

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