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“Python of the Sea” study highlights marine biodiversity underestimates

April 28, 2022

Most ocean life remains to be discovered. Because fish and many other animals that live in the ocean often have larvae or other, microscopic life stages that drift freely in ocean water, counting species by genetic barcoding of plankton samples adds to counts of species recorded as adults and is a highly efficient way to understand what lives in the ocean and how biodiversity changes as we modify the ocean environment.

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.”

Colorful adult ribbon worms like this animal from the family Lineidae, may be difficult to find when sampling marine life. Credit: Terra Hiebert.

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.

An adult nemertean, Notospermus stigmatus. Credit: Svetlana Maslakova.

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.  

A larval nemertean in the genus Hubrechtella. Credit: Svetlana Maslakova.

“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 (-).

Bocas del Toro Research Station hosts Tropical Taxonomy Training courses to share information about how to collect and identify tropical marine organisms. From left to right: Suzanne Fredericq, algae (seaweed) expert and professor of biology at University of Louisiana at Lafayette; Rachel Collin, STRI staff scientist and director of the Bocas del Toro Research Station; D. Wilson Freshwater, algae expert from the Center for Marine Science, UNCW; STRI videographer, Ana Endara; scientific illustrator, Meghan Rock; Maria Pia Miglietta, hydrozoa expert and assistant professor at Texas A&M University at Galveston; Nicole Moss, teaching assistant; Svetlana Maslakova, nemertean expert and associate professor at Oregon Institute of Marine Biology. Credit: Ana Endara, STRI.

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.”

Bocas Del Toro Research Station. Credit: Sean Mattson, STRI.

Maslakova S et al. 2022. Sampling multiple life stages significantly increases estimates of marine biodiversity. Biol. Lett. 18(4): 20210596.

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