Testes investment and spawning mode in pipefishes and seahorses (Syngnathidae)

Externally fertilizing fishes are predicted to invest heavily in testes, because large numbers of sperm should be favoured by the high risk of sperm competition from sneaker males, and/or the dilution of ejaculates when shed into open water. Using museum specimens, we measured testes mass and body mass of 83 mature males, belonging to 21 genera of the family Syngnathidae (pipefishes and seahorses). In this family all species show paternal care, ranging in degree from eggs being attached to the skin of the male, to eggs being completely enclosed and nurtured within a brood pouch. The former, 'unprotected' group, is thought to have external fertilization, whereas in the latter, 'protected' group, males fertilize the eggs internally in their brood pouch. Smaller relative testes investment was thus predicted for genera that have protected compared with unprotected brood care. However, we found this not to be the case. Instead, all genera showed the same relationship between testes and body mass, regardless of brooding type. The possible implications of this surprising result are discussed, including the possibility that the mode of fertilization might have been misjudged for the pouchless syngnathids. (C) 2004 The Linnean Society of London.