Trophic egg

A trophic egg, in most species that produce them, usually is an unfertilised egg because its function is not reproduction but nutrition; in essence it serves as food for offspring hatched from viable eggs. The production of trophic eggs has been observed in a highly diverse range of species, including fish, amphibians, spiders and insects. The function is not limited to any particular level of parental care, but occurs in sub-social species of insects as well as in Leptodactylus fallax, a species of frog known for its close parental care.

Parents of some species deliver trophic eggs directly to their offspring, whereas some other species simply produce the trophic eggs after laying the viable eggs; they then leave the trophic eggs where the viable offspring are likely to find them.

The mackerel sharks present the most extreme example of proximity between reproductive eggs and trophic eggs; their viable offspring feed on trophic eggs in utero.

Despite the diversity of species and life strategies in which trophic eggs occur, all trophic egg functions are similarly derived from similar ancestral functions, which once amounted to the sacrifice of potential future offspring in order to provide food for the survival of rival (usually earlier) offspring. In more derived examples the trophic eggs are not viable, being neither fertilised, nor even fully formed in some cases, so they do not represent actually potential offspring, although they still represent parental investment corresponding to the amount of food it took to produce them.

Trophic eggs are not always morphologically distinct from normal reproductive eggs; however if there is no physical distinction there tends to be some kind of specialised behaviour in the way that trophic eggs are delivered by the parents.

In some beetles, trophic eggs are paler in colour and softer in texture than reproductive eggs, with a smoother surface on the chorion. It has also been found that trophic eggs in ants have a less pronounced reticulate pattern on the chorion.

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