Tiny marine larvae survive by turning carnivorous

The marine worm is a small predator.

The tiny blobby babies are less than a millimeter long, and they devour the tiny crustaceans and other prey that live in plankton.

The observations suggest a new lifestyle option for living in the ocean. Scientists usually think of plankton-dwelling larvae either growing by grazing on the plankton or relying on the egg's yolk reserves to become adults. Carnivory appears to be a third strategy.

One of the big surprises is that these nondescript wormlets are very efficiently taking down some of the fastest prey items out there. He and his colleagues describe their findings in two new papers. The March issue of Invertebrate Biology contains both of them.

The discovery began the way many do, via a combination of Procrastination and Brilliance. Von Dassow is based at the Oregon Institute of Marine Biology in Charleston. Sometimes, when he needs a break, he walks down to the marina to collect a sample of seawater and then takes it back to the lab.

He said that if you peer into a bowl for long enough, you will see something interesting.

Von Dassow had a hunch. His wife is an expert on ribbon worm development. She had long suspected that they were feeding on something in the plankton, but they didn't seem interested in algae. Her lab had conducted DNA sequencing experiments that suggested the insect might be a predator.

Von Dassow spotted a worm sucking on a larger crustacean in a bowl of plankton-dwelling larvae. These worms look like ruffly ribbons or tubes as adults, and they are top predator in sea floor food webs. They have small blobs covered in hairs.

They don't have many of the features biologists associate with a plankton-feeding lifestyle. As he watched, the creature took all the meat from its victim and left an empty shell.

The researchers found many species of marine worm in hunting behavior. Being based at the field station allows scientists to collect and observe the wild organisms that they study day after day.

The marine worms they studied were ribbon worms and flatworms. They ran tests to see if the worms could grow to adulthood on a diet of prey. They took the samples and compared them to known species of adult worms.

The team observed hundreds of these events and captured many with a video camera or a series of still photos. They showed a variety of hunting strategies. The pelagic predators don't seem to be pursuing prey through plankton because they lack more than the most rudimentary of sensory organs.

Some of the worms may be acting like marine spiders, trapping prey with nets of mucus. They create strands that act like a web.

Some larvae use proboscis to stun their prey. Others grab onto the prey, find a crack, and then swallow the flesh within. The studies show the range of these tiny larvae.

A lot of work is needed to get adequate nutrition from harvesting algae. Eggs are a big investment for parents and they are especially tasty to predators, making them vulnerable to attack. This alternate strategy could be worth the effort for some marine animals.

Von Dassow said that instead of building specialized body parts for catching lots of tiny meals, why not just eat someone who went to the trouble to do so.

Von Dassow said that marine worm larvae are an important part of the marine food web.

When people think about how plankton works, and how it connects to life cycles and food webs, it is important to know how things are making a living there.

There are close-to- home implications for Oregonians in the story. The Dungeness crab is a pillar of the Oregon fishing industry. The adult worms live on adult crabs and prey upon the crab's eggs, while the parasites lay their own eggs among the crab's brood.

After dispersal, hatchling come back to crab hosts as adults. The study helps fill in a previously unknown link in the life cycle of that parasites. Knowing what the crabs are eating during the dispersal stage could help researchers better understand the population.

More information: George Dassow et al, Pelagic larval polyclads that practice macrophagous carnivory, Invertebrate Biology (2022). DOI: 10.1111/ivb.12361

Hoplonemertean larvae are planktonic predator that capture and devour active animal prey. There is a book titled "10.1111/ivb.12363."

Journal information: Invertebrate Biology Citation: Tiny marine larvae survive by turning carnivorous (2022, March 4) retrieved 4 March 2022 from https://phys.org/news/2022-03-tiny-marine-larvae-survive-carnivorous.html This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.