Methodological bias in the estimations of important meroplanktonic components from near-shore bottoms

Abstract

In spite of the many direct observations on larval release, the presence of lecithotrophic larvae of sponges and bryozoans has either been anecdotally or never reported in plankton studies. We collected plankton over an artificial rocky reef and on adjacent sandy bottoms in the NW Mediter- ranean and compared larval abundance and species richness of the major groups of invertebrates found over each of such substrates. Observations in the laboratory showed that formalin (4% in sea- water) irreversibly damaged most sponge larvae. To elude the effects of preservation and make it possible to culture larvae for later identification of juveniles, the samples were first identified in vivo and successively fixed. We compared larval abundance and species richness of the major groups of invertebrates found between samples observed in vivo and after preservation with formalin (a pos- teriori). The majority of the sponge larvae (93%) were only visible in the samples observed in vivo whereas abundance estimations of bryozoan and ascidian larvae were equally achieved with both methods. Estimations of the species richness were biased after sample preservation for sponge and bryozoan larvae. We found significant differences between the numbers of planktotrophic larvae observed a posteriori and in vivo. Sponge larvae were the most abundant plankton collected over the reef where the adults live (38% of the total larvae; maximum: 100 ind. m -3 ) and among the most abundant over the sandy bottom (13%). Bryozoan and ascidian larvae were scarce over both sub- strates (<1% of the total) even though they were more abundant near parental habitats than on the sandy bottoms. Gastropod and mussel veligers and spionid larvae were the most abundant plank- totrophic larvae. Early and late developmental stages of long-lived larvae such as those of barnacles and late stages of polychaetes and echinoderms were mostly collected over parental habitats (either the reef or sandy bottoms). From this report it appears that the importance of lecithotrophic larvae of sponges and bryozoans in near-shore meroplankton assemblages needs to be examined further.