Cryptic Stonefly Morphotypes above and below a Dam
During a stream biomonitoring project, students collect 120 stonefly larvae from two reaches of an Ohio creek: Reach A above a low-head dam and Reach B below it. Using a key, they sort larvae into two morphotypes: ‘tufted’ (three gill tufts per abdominal segment) and ‘smooth’ (two tufts). Tufted larvae dominate above the dam; smooth larvae dominate below. A commercial lab sequences a 658-bp COI fragment from 12 larvae per morphotype. Within each morphotype, sequences differ by 0.0–0.3%; between morphotypes, they differ by 4.6–5.1%. The students also find one intermediate larva with three tufts on one side and two on the other. Their teacher proposes listing the two morphotypes as separate species in a local conservation plan. A local angler argues they are ‘the same bug’ because they look alike and the dam is artificial.
1. Apply the biological, morphological, and phylogenetic species concepts to the two stonefly morphotypes. Which concept(s) support separate species, and which do not?
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The biological species concept cannot be applied directly because the two forms are separated by a dam and no mating tests exist; if they interbreed where they meet, they are one species. The morphological concept weakly supports separation: gill tuft number is subtle, and one intermediate exists, suggesting hybridization or phenotypic plasticity. The phylogenetic concept strongly supports separate lineages because 4.6–5.1% COI divergence is much greater than the 0.0–0.3% within morphs. But one mitochondrial gene is not a species diagnosis. Overall, the data support two candidate species or management units, not a settled listing.
2. Interpret the COI divergence values. What additional evidence would you require before recommending separate species status?
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The within-group divergence of at most 0.3% and between-group divergence of about 4.8% form a barcode gap, consistent with reduced gene flow. But COI is maternally inherited and can be affected by endosymbionts, incomplete lineage sorting, or historical isolation without current species status. The intermediate larva argues for nuclear markers such as SNPs or microsatellites, larger samples from both reaches, quantified morphology, and ecological data. Before listing, I would want evidence of stable multilocus divergence or reproductive isolation, not just one mitochondrial fragment. Voucher specimens and sequence deposition would also make the finding verifiable.
3. The angler argues the dam is artificial, so the two forms are ‘the same bug.’ How should the teacher respond in terms of conservation units?
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The teacher should explain that a species or conservation unit is not defined by whether the barrier is natural. A dam can create reproductive isolation and divergent selection, and cryptic diversity is common in stream insects. However, the angler is right that appearance alone is weak evidence. The practical response is to treat the two reaches as candidate management units while gathering nuclear and ecological data. A precautionary conservation plan can protect both forms without asserting a formal species split. The artificial origin affects management options, such as dam removal, but it does not by itself erase biological divergence.