Ecology Explained: Settlement & Recruitment

Marine ecologists specializing in larvae often use two similar terms seemingly interchangeably: settlement and recruitment. What do these terms mean?

We use the word “settlement” to describe the transformation between the planktonic larval phase and the sessile/benthic adult phase. A settler is any organism that is no longer floating free in the water column–it’s now either physically attached to a habitat (like barnacles attaching to a rock) or metaphorically attached (like a fish settling around a particular coral colony).

On the other hand, we use the word “recruitment” to describe the addition of newly settled organisms into the population that scientists or surveyors have measured. So, a recruit is an organism that a scientist has seen in a settled form. Hypothetically, the number of recruits equals the number of settlers. However, this is rarely, if ever, the case. Post-settlement mortality, sampling bias, and other factors practically ensure that the measured recruitment number does not equal the actual settlement number.

I couldn’t find a good Settlement vs. Recruitment diagram on the Internet, so I made my own!

In this diagram, we start out with twenty barnacle settlers on this rock face (yes, they are orange in real life!). But time has passed before our surveyor comes along to count this year’s new barnacles. Some barnacles have died between settlement and recruitment recording. So, our surveyor only finds 13 barnacle recruits! The number of recruits does not equal the number of settlers.

Time is critical when measuring recruitment. The less time between settlement and recording, the better, as fewer new settlers have died. However, it can be very difficult to reduce the time between settlement and when we measure recruitment. Factors such as tides, weather conditions, and funding can reduce scientists’ ability to minimize this bias.

Plus, real life isn’t as simple as this diagram! It’s easy to miss a barnacle or two (or three, or twenty) hiding in a crevice or in the shadow of an adult barnacle while sampling. Or, the area measured may not be indicative of larger-scale settlement patterns!

In order to separate these two concepts, we use the terms “settlement” and “recruitment” to reduce confusion and more accurately describe what we’re talking about!

Do you have a lingering question about settlement and recruitment? Ask it in the comments section and I will be happy to help as best I can!

PSA: Bryozoans are awesome!

We just covered lophophorates in In-Depth Marine Biology, which was great fun for me (I love lophophorates!). I’ve noticed that, somehow, many people haven’t heard of these wonderful animals. So I tasked my class with creating Public Service Announcements to inform the public about these wonderful creatures.

One student, Maria C., wrote a whole song about bryozoans! Take a listen below–it’s very catchy! (the video has no visuals, just sound)

It’s been stuck in my head for a while now! Even more amazing: it’s the first song she’s written! Incredible job, Maria!

Ecology Explained: The Intermediate Disturbance Hypothesis

The Intermediate Disturbance Hypothesis is one of the staples of ecology, especially marine ecology. The Intermediate Disturbance Hypothesis was first proposed by Connell, a well-known intertidal and general ecologist, in 1978 (See his article “Diversity in Tropical Rain Forests and Coral Reefs”). But what is it exactly? Let me explain!

Let’s start with this handy diagram from Wikipedia.

Source Link

As you can see, we have Level of Disturbance on the x-axis. That simply describes the level of disturbance present in the system. It could be any sort of disturbance…fires, hurricanes, waves, human or animal trampling, wind, sun, and so on. The level of disturbance increases from left to right. So, the area marked with I has less disturbance than area II.

Species Diversity on the y-axis is more of a general term. Sometimes this is “species richness,” which is a pure count of species present in the ecosystem. Sometimes this is actually referring to “species diversity,” which takes the number of species from species richness and combines it with how the species are distributed in the system. But that’s a subject for another blog post! All you need to know now is that species diversity in the system increases from the bottom to the top.

Now let’s look at those areas marked by Roman numerals.

Area I is first up. In that section, we have a low amount of disturbance, which results in an okay amount of diversity. Why is that?

Ecosystems typically have a successional pathway–a pattern across time of species that are present. Think about a forest directly after a forest fire. Early-regrowth forests are going to look totally different from established forests! Those established forests will most likely have one or more competitive dominants–the species that compete the best. That’s great for the competitive dominants because they do well in those established systems, but it’s not so good for diversity. The competitive dominants compete so well that there isn’t much room for other species.

(Sometimes you will see similar graphs that are a simple bell curve with Area I showing the same low, low diversity that Area III exhibits. This can also happen–if there is no disturbance in the system at all, the species diversity is going to be extremely low. But, from my perception, it’s probably not going to get as bad as Area III. It’s all up to interpretation.)

Area II looks great! There is an intermediate amount of disturbance and the maximum amount of diversity. In Area II, there is enough disturbance in the system to stop the competitive dominants from over-dominating. Organisms earlier in the successional pathway that are poorer competitors (but still play an important role) are able to survive. This results in the maximum amount of species diversity! Hence the name “Intermediate Disturbance Hypothesis!” This area is experiencing an intermediate amount of disturbance, but not enough to push it to Area III.

Area III is not a good place to be. Area III exhibits a very high amount of disturbance and a low amount of diversity. It’s pretty easy to understand why! Think of a coral reef that’s constantly being battered by huge hurricanes or a forest that has repeated, huge fires. The succession pathway barely even gets a chance to begin before another huge disturbance sweeps through. This will result in very low diversity.

So, the Intermediate Disturbance Hypothesis shows us that with an intermediate level of disturbance we can expect a high amount of diversity. With low and high levels of disturbance, not so much!

Do you have a lingering question? Ask it in the comments section and I will be happy to help as best I can!

The beginning of the academic year is upon us!

The beginning of the academic year is always exciting for me because it signals my return to two of my favorite activities: learning and teaching! I’m offering three courses through Athena’s Advanced Academy in the beginning half of the semester: Marine Mammals, The Rocky Intertidal Ecosystem, and In-Depth Marine Biology.

I love teaching all of my courses, but I am particularly excited to teach In-Depth Marine Biology. For one, it’s a year-long course–the first I’ve ever taught! We’re going to cover some of my favorite topics under the umbrella of marine biology. Plus, my students are wonderful and are already diving deep into the material!

I wanted to share a couple of my students’ work from the first week of In-Depth Marine Biology with you from a taxonomic classification assignment. Students were tasked with pretending that everyday objects were organisms and creating their own taxonomic system to classify these “organisms.”

I am pleased to report that students took this idea and ran with it! We had writing instrument classification…

By Aria (who correctly noted that she wouldn’t be able to tell if one of these organisms were a mimic! She created a whole timelapse video showing her experiment! This is a still from the video.)

Two different candy classifications…

By Bridget (who grouped candy in Genera and pointed out that we are missing observations over time–very thoughtful!)
This student went another direction with the taxonomy of candy (and included an amazing spreadsheet detailing each step!)

LEGO brick classification…

By Jonathan V. (who not only did a fantastic job showing the narrowing of his groups, but also arranged the taxonomic tree in the shape of a bearded anglerfish!)

And even pasta!

By Kai S. (who not only sorted by different morphological characteristics & ingredients, but also created this beautiful artistic depiction of Kingdom Pasta!)

And guess what?! There were more submissions too! The other submissions just had text, so they wouldn’t fit well in this format. But rest assured that they were all wonderful and creative too. As a class, we developed taxonomic classifications for board games, tea, and more!

I am so looking forward to seeing what my students create over the next academic year!

Gorgeous Geniculated Coralline Algae

I’ve been busy lately! My final year of my undergraduate degree wrapped up this spring, which has opened up time for a lot of awesome projects. I’m helping my local community college by scanning in the best examples of their algae collection with my mother’s awesome scanner! (Plus, I’ve been updating the labels so they’re more up-to-date!)

I just got to the coralline algae section of the collection and it is so cool to see these specimens in such detail.

Edit: The image is so good that WordPress can’t handle it! See the zoomed-in image below!

I actually need to create a new label for this one at some point. The name listed on the label for this species is Corallina gracilis f. densa, but this name is now a synonym of Corallina vancouveriensis. The need for a label update is not all that surprising since this specimen was collected in 1967 and scientific algae names change often!

The WordPress picture size limit doesn’t quite allow you to see how amazing these scans are, so here’s a close-up:

Awesome scanning power!

As you can see, coralline algae doesn’t have the typical slimy, floppy look that most algae forms have. That’s because coralline algae are calcifying algae! Just like reef-building corals, coralline algae use calcium carbonate to beef up their tissue and create a sort of “skeleton,” hence the name “coralline,” meaning “coral-like.”

This particular coralline algae species is geniculate–hence the title of this blog post! Geniculate corallines refer to coralline algae that has “joints,” which enable them to have some mobility, unlike coral skeletons. This term comes from the word root for knee!

Here’s a close-up under a microscope:

Those pink parts are called intergenicula, as they’re between the genicula. The intergenicula are the parts with calcium carbonate deposits. You can actually see a geniculum quite well in this picture–it’s the yellow stringy stuff between the first and second intergenicula from the left. There’s none of that tough calcium carbonate present in the genicula so they can bend!

This specimen is a different species, although I’m not sure which. This one has been bleached a little–a lot of the pink color has left. Like coral, coralline algae usually looks white when it’s been dead for a while. Bright pink samples were most likely collected from live organisms.

Here’s a stained slide of some genicula and intergenicula:

The blue parts are intergenicula and the pink parts are genicula!

That same stringy genicula texture persists here!

These genicula are very important for the coralline algae, since it enables them to grow tall without being inflexible and brittle. One problem with coral is that it is rather susceptible to waves and storms–the wave energy can smash the immovable coral. Geniculate coralline algae has the ability to move with the waves, reducing its susceptibility to wave energy.

One thing’s for sure: coralline algae is more than a pretty pink color!

The Uniqueness of Urchins: Tube Feet

One of the most common questions I get when teaching my students about urchins is: “Can they move?” The answer is: yes!

Urchins move around by utilizing tube feet powered by their water vascular system–a system that uses water pressure to extend and retract the tube feet. Sea stars more famously utilize a similar system to control their tube feet. The first step of the water vascular system is the madreporite–in this case, a large, porous plate at the top of the urchin.

See that heart-shaped, porous plate? That’s the madreporite!

It’s a little tough to see on this specimen, but there are ten apical plates surrounding that center hole in the test. Five apical plates have an eyespot and five have an gonopore for releasing gametes. One of those gonopore plates also holds a madreporite.

The madreporite is typically covered in skin while the urchin is alive and is rarely still present on dead urchin tests. The apical plates often break off from the urchin test and are easily lost. I was thrilled to find that this urchin test still had its madreporite!

Once water is brought in through the madreporite, it passes through a series of canals before reaching the tube feet. Tube feet are connected to ampullae, which help the tube feet extend and retract.

Imagine an eyedropper filled with water connected to a thin plastic bag. Squeezing the eyedropper pushes the water out into the plastic bag, extending it. Sucking the water back into the eyedropper retracts the plastic bag. That’s pretty much how tube feet operate! The ampullae are the eyedroppers and the plastic bags are the tube feet.

Here’s a video of tube feet in action that I took under a microscope:

If you look closely, you can actually see particles moving down on the left and up on the right in the closer tube foot. Those are coelomocytes–cells present in the water vascular systems of urchins. They mark the direction of fluid travel on each side of a dividing septum. An extended tube foot has constant flow to keep it extended.

You can actually see exactly where tube feet extend from by looking at the inside of an urchin test.

Those five groups of holes in wavy lines are the holes where tube feet poke out. In life, the ampullae would be lining those holes on the inside, each one controlling a tube foot. The groups of tube feet and groups of large spines alternate on urchin tests. If you look closely, you can actually see those holes on the outside of the urchin test too.

See those holes between the spine nubs?

And those are the basics of urchin tube feet! Next urchin post: how and what urchins eat!

The Uniqueness of Urchins: Spines

Sea urchins–purple urchins especially–have a bit of a bad reputation on the West Coast of the United States right now. They are currently forming and maintaining urchin barrens, which are kelp forests’ “evil twin” or alternative stable state. But that’s a complicated story for another time!

Even though urchins aren’t exactly universally loved, I am fond of them. It probably has something to do with my general love for marine invertebrates. Regardless, there’s lots to learn about urchins! Let me show you with the help of some specimens, starting with their spines.

This is one of my specimens–a dried purple sea urchin with its spines still intact. As you can see, several of its spines have broken off over time. They can move each spine individually to defend against attacks. If you “tickle” an urchin on its exoskeleton, its spines will respond in a matter of seconds.

For reference, here’s a smaller, living purple sea urchin:

The color has faded in the dead specimen–living urchins are a brighter purple.

Typically, dead urchins don’t retain their spines for long. The spines tend to fall off, along with the skin, revealing what’s known as the test. The test is the urchin’s main, circular exoskeleton.

A purple urchin test

The spines have sockets that connect to the ball-like bumps protruding from the test. This enables them to rotate each spine. Unlike arthropods, urchins grow their test and spines continuously–no need to molt!

Urchins exhibit pentaradial symmetry, which means that their bodies are radially symmetrical in multiples of five. Sea stars and other echinoderms are also pentaradial. This test shows how urchins have alternating stripes of spine-bump sizes.

The little holes in the test are for the tube feet to extend from. You’ll hear more about tube feet in a future blog post!

It’s important to mention that some urchins have venomous spines–particularly tropical species. Neither purple urchins nor the closely related red urchins have venom in their spines, so they are completely safe to touch. The only threat these temperate urchins pose to humans is the threat of getting spines stuck in your skin.

Urchins can drop spines when living for a variety of reasons, including disease, duress, or trying to dispose of a fouling organism. It’s possible for urchins to recover from dropping a few or many spines if given enough recovery time, especially if it has enough nutrients in the water and in its diet.

Those are the basics about urchin spines! Stay tuned for more urchin facts and pictures!

Investigating kelp in Minecraft (Activity for Middle School and Junior High students)

Since I teach online, I’m always coming up with new activities for my students to complete. I thought I would share one of the activities in my Underwater Forests: Kelp Ecosystems course I’m teaching this semester. We’re looking more closely at Macrocystis pyrifera this week, so I’m assigning a Minecraft activity where students will observe and experiment with Minecraft’s version of kelp. They are not only learning more about real life giant kelp, but they’re also practicing running experiments.

For my students who follow me here–spoilers!

Minecraft recently added kelp into the game, which gives us the perfect opportunity to investigate its properties!

Find a natural kelp forest in Minecraft. 

Plant a kelp individual in fairly deep water. How long does it take to grow a block higher? Time it! Try it again with a few more individuals. Is the time the same for all of them?

Plant a kelp individual in shallow water. How long does it take for it to grow a block higher? Time this one too and try it with several individuals! Does it take a shorter amount of time?

Compare this data to real kelp organisms. Do you think the time it takes for kelp to grow would differ between shallow and deep water? If so, how?

Finally, take a look at the surface of the water. Does the kelp keep growing at the surface? How does this compare to a real kelp forest?

Write down your results and thoughts in at least six interesting sentences. Good luck & have fun!

I’m looking forward to seeing what they discover!

Marine Miniature Ecosystem(s) Update #2 (A fast-paced update!)

Quite a lot has happened to the jar ecosystems since I last wrote. For one, the first ecosystem did not go as well as hoped.

As it turned out, there were a few more polychaetes than expected…as in, approximately five. I discovered them shortly after posting the first update.

There’s one now!

There were also two larger roundworms and several smaller worms that were extremely difficult to photograph!

One of the larger roundworms hanging out at the surface of the water.

Although I’d hoped to return the polychates to the ocean in the morning, there was unfortunately not enough dissolved oxygen for them to survive the night. After they died, the nutrient influx was massive and caused the ecosystem to collapse.

Some lovely nutrient sludge draped over rotting algae.

I still had another jar to fill with seawater, but I waited. My Marine Ecology course was starting in only a few days and I wanted my students to be involved in the jar creation process. In the best case scenario, our class jar would illustrate how ecosystems function and ecosystem processes. In the worst case scenario, it would illustrate how ecosystems are very difficult to get right and how we can learn from every experiment, regardless of whether it turned out correctly.

I polled the class and they decided to create the jar as follows:

  • With both coarse and fine sand
  • Water from “As far out into the ocean as Emma can reasonably wade”
  • Green intertidal algae and red intertidal algae
  • Mussel shell and a small rock as decorations

After a quick trip to the ocean, our jar was created!

A fresh start.

After a week and a half of slow deterioration, the class decided to add more algae and remake my first jar attempt with the same characteristics as before.

The algae in the class jar was looking a little worse for wear, but there were some signs of life–like this roundworm.

Back to the ocean I went. Once I returned, I rearranged algae and tried to aerate the jars a bit to get more dissolved oxygen in the water. My dorm room had definitely smelled better! I attempted to capture as much diversity as possible, since diversity generally increases ecosystem stability.

Old (2nd attempt) jar on the left, new jar on the right.

In exciting news for the class, our new jar had unknowingly brought home a new class mascot: a little nudibranch!

The class voted to name them “Peanut butter.”

Unfortunately, after a couple days, Peanut butter disappeared. We came to terms with the loss as a class with a moment of silence.

The original jar wasn’t doing well either, even with the addition of more algae. However, this was a great example of alternative ecosystem states for the class to learn from! It can be very difficult to push a jar ecosystem back into a different ecosystem state.

Fast forward a few weeks and both jars had seen better days.

(The jars were also moved home, since the quarter ended!)

Before moving back home for winter break, I noticed that there seemed to be a growing layer of white around the circumference of the jar. My students and I were very interested in what this could be, so I took a sample to school and took a look under the microscope.

Bacterial mat?

The consensus was bacteria, especially since I also saw paramecia (which feed on bacteria). There might be hope for life in these jars!

Today, the jars are doing about the same.

A picture after they’ve been stirred up a bit.

I’m looking forward to watching them as they progress in these alternative stable states!

Marine Miniature Ecosystem Update #1

I realized a dream of mine today– a dream to create my very own miniature marine ecosystem. As a marine ecology student, the idea immediately appealed to me.

I’d been researching the concept for months and finally had the materials to put it into action! So I drove up to Davenport (where it’s legal to collect algae) this morning and created my brand-new ocean-in-a-jar.

From this angle, you can see Ulva and a red algae I have yet to positively ID. It’s a little beaten up and might actually be two species growing together.

The red algae has some interesting animal life of its own. It has some sort of bryozoan species encrusting on it, which I’m still in the process of identifying. It also has a few tiny ostrich plume hydroids. I’m interested to see if they have survived/will survive. Although I was sad to take them from their habitat, I rescued this algae and its epibionts from the wrack line. If the bryozoans and hydroids aren’t already dead, they would’ve been shortly thereafter.

Here you can see the (empty) mussel shell with encrusting coralline algae.

I included this mussel shell and another mussel half-shell in an attempt to provide algae a place to settle and provide shelter for organisms.

Here you can see the salt residue on the side of the jar and some Cladophora columbiana.

The algae has already begun photosynthesizing, which I was extremely happy to see! You can see some of the air bubbles still attached to the Cladophora.

Earlier today, there was an interesting development.

The unmistakable bristled sides of a polychaete worm!

It seems that a stowaway came along! I rinsed off all the algae in the ocean, but it seems that this one is good at holding on.

Polychaetes are a type of annelid (segmented) worms. The term polychaete means many bristles, which are attached to the little extensions (parapodia) along its body. This particular polychaete uses its bristles and parapodia to “swim” through the water.

I decided that it was a little big to be in the jar so back to the ocean we went. Although it seems that this is some sort of clam worm, which does eat algae, it seemed like too much to have in a jar that was just starting out.

Ready for release!

The release went well and the little polychaete scurried away from the sunlight fairly quickly. When I returned, I was shocked to find another surprise waiting for me!

Another one?! (it’s hanging onto the red algae)

Apparently, one of these types of algae is a common hiding place for these polychaetes! Since this one is a little smaller, I’m going to see how it does in the jar for now. I can’t find it right now, so hopefully it’s found a good place to live.

I’m looking forward to watching the jar change and finding out what other organisms appear!

If you are interested in creating your own ecosystem, I highly recommend checking out the YouTube channel Life in Jars?, which offers tutorials, information, and entertaining jar updates:

Stay tuned for updates on this jar and on the next ecosystem I create…