A study by recent PhD graduate Emily Richardson and her supervisor Dustin Marshall found that the larvae of a filter-feeding marine worm move from being feeding generalists to specialists as they develop.

We know that many organisms experience dietary shifts as they grow bigger or after undergoing dramatic changes in body shape and structure (e.g. metamorphosis). But what happens across larval stages? Some species can increase in size by several orders of magnitude during larval development, and yet, few studies have looked at how diet changes within the larval phase.

Emily and Dustin tested how fundamental ‘niche’ (or, in this case, the ideal diet in the absence of competition or other environmental influences) changes during larval development. Their filter-feeding study species Galeolaria caespitosa is included in a group overlooked in classic niche theory — filter feeders were once assumed to have no diet changes during their life cycle. Overall, the study targeted two knowledge gaps of niche theory: whether diet changes

1. across larval stages and
2. for filter feeders.

They assumed that smaller larvae would be limited to feeding on the smallest phytoplankton (i.e. algae) species, whilst larger larvae would eat more algae and enjoy a wider range of species. In other words, the fundamental niche would broaden as the larvae grew and were able to eat a wider range phytoplankton species of variable sizes.

This was not what they found. They offered the three different larval stages four different algal food species that vary in size. Each algal species was offered on its own so that there was no choice of food available and feeding rates were calculated from the difference in algal concentrations at the start of each run and after six hours of larval feeding.

The smallest larvae ate all four algal species in approximately equal proportion, although they did consume slightly less of the biggest species, which is roughly the same size as the mouth gape. As the larvae got bigger, they consumed large amounts of the two medium-sized algal species and barely touched the smallest species, despite its availability in high concentrations. Overall, small larvae had a broad niche that ultimately narrowed across the stages, because medium-sized cells made up the majority of the diet for large larvae.

While the bigger larvae were not constricted by algal size they may have concentrated on a narrower range of algae because it is more nutritious at the stage of development where they are preparing to settle and metamorphose into adult worms. Alternatively, the medium sized algae may have represented the most efficient food source, in that capture success was maximized in relation to encounter rate.

While this study doesn’t elucidate the reasons for this narrowing of the fundamental niche in developing larvae, it does suggest that the relationship between fundamental niche and body size may be more complicated than classic theory suggests.

A previous study from our lab has shown that benthic filter feeders have the potential to impact on phytoplankton communities through their feeding; the same may be true of filter feeding larvae.

This research was published in the journal The Biological Bulletin.