Chaetognaths- The truth about Arrow Worms

During our first plankton lab, we noticed some interesting worm-like creatures with vicious looking teeth. Under the microscope, we were able to observe some features that helped distinguish this creature as an arrow worm, also known as a Chaetognath (keet oh nath). They are not really worms in the traditional sense, in that they do not have segmented bodies. They are actually unlike any other set of marine animals, so they are their own phylum (group). There are about 150 species of chaetognaths known, all of them are marine and can be found in oceans worldwide.

Chaetonagth head.  Image Image M Onuma, HIMB

Arrow worms are zooplankton, small animal plankton (drifters of the sea). Unlike many zooplankton, which may be the young stage of familiar reef animals such as fish, seastars or even corals, arrow worms are holoplankton, they stay the same and remain plankton for their entire life.

One interesting fact about arrow worms is that they swallow their prey whole. Since they are clear, you can often see what they ate in their gut when looking at them under a microscope. Their main source of food is copepods and they find their prey in quite an amazing way. According to an article by Hiroaki Saito and Thomas Kierboe, Feeding rates in the chaetognath Sagitta elegans: effects of prey size, prey swimming behaviour and small-scale turbulence , chaeotognaths sense the vibrations from their moving prey and then attack them.

Arrow worm head.  Image M Onuma, HIMB

Group of arrow worms.   Image D Gazerwitz and J Dulin, HIMB

Arrow worms play an important role in the food web.  They are zooplankton predators that eat other zooplankton, which is an important initial step in the ocean food chain. The arrow worm in turn becomes food for fish larvae, which either grow up or are then eaten by larger marine animals.

Image from Climate Kids at NASA.gov

These very cool, clear worms are not well studied. If you would like to know more details about arrow worms, you can visit our sources:

 http://education.nationalgeographic.com/media/reference/assets/plankton-revealed-3.pdf


http://crev.info/2004/09/arrow_worms_miss_the_mark_in_darwin146s_tree/

http://www.adfg.alaska.gov/static/species/speciesinfo/_aknhp/arrow_worm.pdf

http://plankt.oxfordjournals.org/content/23/12/1385.full


Aloha,


Dana Gazerwitz and Jason Dulin 

What can you learn from a dead turtle?

Aloha,

Turtle carcass--M Heckman image

Over the last weekend, we noticed a bit of a smell along the shore by the Corrosion Lab site. A dead turtle carcass had washed up, so like any good biologists, we went to look. It was upside down, without a head or flippers and with what appeared to be parts of the shell missing.

The obvious conclusion would be that a tiger shark had been feeding on it and when we sent in the pictures that Larry had taken, Devon Franke from NOAA concurred that some of the scrape marks looked to be tooth marks.

Under side of turtle--Larry Stamey photo

So, did this animal get killed and eaten by a tiger shark? After all, tiger sharks eat turtles, but what if the turtle died from some other cause and then was just a convenient floating meal? I know that I prefer my food to be dead before I eat it. It is so much easier when it is not still trying to run away.

I think people have the impression that tiger sharks kill and eat more sea turtles than they probably do. Our new intern Kenzie Gauck went looking for information on this issue. She found a paper by W.N. Witzell in which an examination of 201 mid-Pacific tiger sharks revealed only 31 with turtle remnants in their stomachs.  So 15% of the sharks had recently eaten turtle - a reasonable amount, but not a huge number. They did not measure how much of the turtle was in the stomach, how digested it was, or so on, just whether it was present or not. A study done on dietary habits of tiger sharks here in Hawaii, by Lowe, et. al. showed that of 281 tiger sharks examined, only sharks over 7.5 feet in length had any turtle in their stomachs and that the proportion rose to 15% when they examined sharks over 9.8 feet in length. Turtle shell and parts might stay in the stomach longer than other food types, leading to an over reporting of  turtle compared to other items that might move through the system faster and be eaten more often, but I am just speculating here. Someone can set me straight if they know more.

We know that once turtles were protected by Federal legislation back in 1978, it took decades for their numbers to grow to the point that we actually commonly see them now - and they were never protected from sharks eating them during this time, only people. I suspect that the real issue for turtles is us.

We decided to flip the turtle over and look for any additional damage on the top of the shell before bagging it up for NOAA. By this time the decomposition had really set in. The nice shiny scutes had all fallen off. The main parts of the shell were starting to come apart, and the smell was actually not so bad. Leon picked up a bit of scute with some barnacles on it.

Below is an image of the top of the animal when we finally flipped it over. The shiny scutes are gone, only the bony plates below remain. You can see some additional breakage up near the center of the shell.

Green sea turtle carcass with possible boat strike injury, washed up at Moku o Lo'e.  M Heckman/HIMB photo

There is the possibility that the break is the result of a boat strike, but the shell is so decomposed that we will never know. I would think that with all of the power boats speeding around the bay, boat strikes must happen fairly often. In fact, the 1998 Green Turtle Recovery Plan lists boat collisions in Hawaii as a "primary threat" to green turtles. 16 years later with more turtles and certainly no less boats, it is hard to imagine that this is still not an issue.

We will never really know what came first, but I put my money on a boat strike, followed by a nice meal for a local shark, then the finishing smorgasbord for a whole host of very well fed crabs.

Sad for the turtle, it looked to be a good size. This reminds us to go slow and watch for turtles in the Bay.

Aloha,

Mark


Thanks to Intern Kenzie Gauck for the following links.

Selective Predation on Large Cheloniid Sea Turtles by Tiger Sharks (Galeocerdo cuvier):
http://www.sefsc.noaa.gov/turtles/PR_Witzell_1987_JapJHerp.pdf

Recovery Plan for U.S. Pacific Populations of the Green Turtle (Chelonia mydas):
http://www.nmfs.noaa.gov/pr/pdfs/recovery/turtle_green_pacific.pdf

Ontogenetic dietary shifts and feeding behavior of the tiger shark, Galeocerdo cuvier, in Hawaiian waters
University of Hawaii at Manoa. Christopher G. Lowe, Bradley M. Wetherbee, Gerald L. Crow, Albert L. Tester Environmental Biology of Fishes (Impact Factor: 1.31). 09/1996; 47(2):203-211. DOI:10.1007/BF00005044

HIMB in the News--Designer Reefs

Coral reefs are known to be beautiful underwater landscapes that serve purposes far exceeding simple tourist attractions.  Reefs are natural barriers of the sea that help reduce exposure to waves, floods, and storms on the shore line and mainland.  However, this delicate ecosystem is in distress.  Due to climate change and ocean acidification, many coral reefs face a drastic population crisis.  The ocean’s temperature is increasing at a more rapid rate along with a decrease in the acidity level.  These changes are due to an increase of carbon in the atmosphere.  The higher the temperature and the lower the pH of the ocean, the harder it is for the coral to survive.

The reefs of Ofu Island in American Samoa are a natural laboratory for studies of coral heat resistance.

Recently, however, there have been discoveries made that show a coral’s ability to acclimate to temperatures outside of its comfort zone.  It has been found that some coral are able to alter their physiology in order to live in warmer waters.  Since the evolution process takes years to establish useful adaptations, this acclimation process acts as a middle man between the organism’s current state and its future evolved state.  An article in Nature calls it a coral's “first line of response” to these environmental changes. 

But, the rate of evolution and the changes occurring in the environment are so out of sync that these “first line of responses” may not be enough to save the coral alone.  This is where Designer Reefs come in.  Here at the Hawaii Institute of Marine Biology, Dr. Ruth Gates is looking into cross-breeding coral reefs.  Essentially, she is planning to take the reefs that have already adapted to the changing environment and incorporate those specimens into the reefs that have yet to acclimate.

Some people remain skeptical about human involvement in the natural process of evolution.  Skeptics believe that when you interfere, you are disrupting the natural processes of the ecosystem, but as Dr. Gates states, they will not be introducing foreign DNA into these coral populations.  Also, human impacts are some of the main reasons coral are in this precarious situation  Therefore, this research is trying to remedy those effects and give the coral a fighting chance in a rapidly changing environment.

Take a look at this excellent article highlighting HIMB's Gates lab - Click on: Could 'Designer Reefs' Save the Ocean from the Huffington Post

Or see this broader article in the journal Nature: Corals use multiple tricks to adapt to hotter seas

Kenzie G.

More about ocean acidification:

http://ocean.nationalgeographic.com/ocean/critical-issues-ocean-acidification//