Most surveys of marine life dramatically underestimate the total number of species because they miss animals burrowed in soft sediments, living within coral reefs, and in other hard-to-sample places like those deeper than diving-depth, says a study led by researchers from the Oregon Institute of Marine Biology and the Smithsonian Tropical Research Institute. Sampling sea water to detect the larval stages of marine animals may be a way to fill in the gaps. In a 17-year study of ribbon worms, one group of elusive animals that are hard to collect, comparisons of larval (baby) ribbon worms collected from sea water with adults found in the reefs at the same sites, showed very different diversities.  Sampling both life stages could double estimates of the number of ribbon worm species.

“If you have the babies, the adults must be somewhere around,” said co-author Rachel Collin, staff scientist at the Smithsonian Tropical Research Institute in Panama. “Our study focused on the nemerteans, a single group of understudied marine organisms, but the implications are far-reaching, because so many marine organisms have planktonic larval life stages.”

Nemerteans, a.k.a. ribbon worms, pass through a larval stage as they mature. Just as butterflies metamorphose from fuzzy caterpillars to hard-shelled pupae to flamboyant, winged adults, nemerteans spend their young lives as microscopic larvae drifting in sea water, but end up as adult worms burrowing in sediments or hiding in cracks and crevices.

In this study, the team identified both adult ribbon worms and larvae from sites in the Northeastern Pacific (Oregon, USA), the Caribbean (Bocas del Toro, Panama) and the Eastern Tropical Pacific (Bay of Panama, Panama). At low tide, they searched for adults under rocks and in sediments, and they collected animals from samples of coral rubble, algal mats, seagrass and kelp holdfasts while scuba diving. At the same sites, they collected planktonic larvae from the sea water. Watch this set of videos created with a grant from the U.S. National Science Foundation (NSF), about how to collect nemerteans and their larvae.

Ribbon worms’ beauty and grace may be why they are named for a Greek sea nymph, Nemertes, but more often than not biologists can’t distinguish closely related species, especially at the larval stage, based on their looks. Instead, they read their DNA. Each species has a unique DNA sequence, and like the supermarket tags that identify different products, DNA barcodes can be used to identify species.

After sequencing more than 1000 individuals—both larvae and adults—the team found 308 species in total: 101 from Oregon; 149 from Bocas del Toro, Panama; and 61 from the Bay of Panama. Many species were new to science.  By comparing the number of species collected as adults with the number collected as larvae, and the number that were found in both life stages, they showed that if they had only collected adults, they would have missed as many as 60% of the species in an area.  

“Many of the adults we find do not have larval matches either,” said Svetlana Maslakova, Associate Professor at the Oregon Institute for Marine Biology, who led the study, “The conclusion of our study is that each life history stage contributes its own subset of species to biodiversity counts, and the subsets are only partly overlapping.”

About 1300 species of ribbon worms have been discovered so far, but this number is probably only a tenth of the true number of nemertean species on Earth. While a dozen or so species live on land, and another dozen or so in fresh water, most live in the oceans, and many are found in warm tropical waters in places where researchers seldom visit. Some ribbon worms can be a nuisance to commercial fisheries by eating crab eggs and clams, or killing fish in the nets with their toxic secretions. Others earned the nickname “pythons of the sea” for their habit of swallowing large prey whole. In fact, the longest living animal recorded on Earth was an individual of the nemertean species Lineus longissumus, that washed up on a Scottish beach and measured 54 meters (177 feet) in length. The smallest nemerteans only grow to be as long as a dash (-).

In addition to the nemerteans, crabs, sea stars, sea urchins—in fact, most marine organisms—have larvae, so this team’s approach might significantly increase the documented diversity of many groups.  In fact, previous work by Collin’s lab in Panama found that very few of the larvae in sea water samples could be matched to adults using DNA barcoding. This is why they came up with the idea to actually test the hypothesis that sampling larvae can tell you which adults are present in the area.

“It's wild,” said Collin, “even when you have been sampling adults for years, it seems that you are still likely to miss a lot of species that could easily be collected with just a dip of the net. It really shows the importance of not being so adult-centric and sampling more than just one life-stage.”

Maslakova S et al. 2022. Sampling multiple life stages significantly increases estimates of marine biodiversity. Biol. Lett. 18(4): 20210596. https://doi.org/10.1098/rsbl.2021.0596