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Keywords:

  • Caribbean;
  • colour polymorphism;
  • co-occurrence;
  • coral reef;
  • evolution;
  • fish;
  • hamlets;
  • marine;
  • sympatry

ABSTRACT

Aim  To test historical and current influences on the distributions of sympatric colour morphotypes in the coral reef fish genus Hypoplectrus.

Location  The Caribbean and surrounding tropical waters. These areas cover the entire distribution of the genus.

Methods  A large and extensive database of Hypoplectrus sightings was used to establish the distribution of colour morphotypes and test a long-standing hypothesis regarding their origin. First, we considered the evidence for the previously proposed ‘population centre’ hypothesis, which suggests that current morphotype distributions reflect past conditions where these colour forms evolved in allopatry. Using morphotype sighting data, the existence of clusters in occurrence and density was tested. Second, we examined whether the observed patterns of morphotype co-occurrence deviate from random expectations using null model simulations, within subregions of the distribution of the genus, to infer ecological influences on distribution.

Results  There is considerable variation in morphotype distribution, with even widespread morphotypes showing geographical clustering. There is also little evidence to suggest past or current geographical isolation, with only one of the 11 morphotypes (Hypoplectrus chlorurus) showing a density distribution that is consistent with the population centre hypothesis. Null model analyses show that variation in local morphotype co-occurrence is typically significantly lower than expected under random dispersal conditions.

Main conclusions  Our results strongly suggest that morphotype co-occurrence is not random, but there is no evidence to suggest a past allopatric radiation in Hypoplectrus colour. Current distributions are likely to be driven by competitive interactions and/or habitat preferences. Our study highlights the value of the Hypoplectrus species complex as a system for the study of speciation in the marine environment, and implies that these closely related morphotypes have ecological relevance rather than being simple colour variants of a single polymorphic species.