Diversity of both grazers and habitats is key for healthy ecosystems

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Me working on the vertical rock wall that was the stage for this experiment. Behind me is a rock bench covered with living organisms and a retreating Pacific Ocean.

In order to maintain healthy ecosystems, we need to consider how environments change in relation to the organisms living in those environments. My colleagues and I recently published a paper showing how the varieties of both habitats and animals interact to speed the recovery of seaweeds on a rocky shore. Habitat and animal diversity were important on their own, but having a range of habitats was essential to promoting recovery of seaweeds when an important grazing animal species was removed from the community. Thus, a mix of habitats for organisms to utilize may provide a buffer against the loss of species. Maybe variety really is the spice of life.

Understanding the causes and consequences of biodiversity is a major motivation for ecologists, and these causes and effects may be related in important ways. The aspects of an environment that allow diverse communities to develop may also help explain how biodiversity influences essential processes in ecosystems, such as the ability of communities to recover after being disturbed. In our study, recovery meant that seaweeds grew back quickly after we removed them from small areas.

The stage for our study was a vertical rock wall high up in the intertidal zone at Bodega Marine Reserve. This location features a wide variety of life in very small areas, and much of this life is slow-moving or does not move at all. These aspects, along with steep environmental gradients where land becomes sea, have made rocky shores ideal systems for conducting experiments in the rough and tumble of nature for many decades.

Our cast of characters included stalwart barnacles, several varieties of snails (periwinkles and limpets), and a mélange of green and red seaweeds. These creatures interact with one another in a number of ways: seaweeds and barnacles compete for space on rocks, snails eat seaweeds, barnacles protect small seaweeds from being eaten by snails (they can’t reach between the barnacles), limpets can bulldoze young barnacles from rocks, and tiny periwinkles live inside dead barnacle shells. Given all of these interactions, it can be difficult to predict what will happen when we change something in the system, but this is exactly what excites me about ecology.

Here’s how we designed our experiment: we manipulated the cover of barnacles and the number of species of snails after removing seaweeds from small areas on the shore, and we tracked the recovery of seaweeds over the course of one year. We first set up areas in which we 1) left barnacles completely intact, 2) removed all barnacles, or 3) removed barnacles from only one half of the area. This last “half barnacle” treatment we considered to be more diverse because it contained two distinct habitat types. For every habitat type we then manipulated the number of snail species that were present: an intact snail community with periwinkles and two types of limpets, and three communities each with only one type of these snails (we removed the other snail types).

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The figure (above) summarizes the results for the seaweeds that grow slowly and tend to stay on the shore for long periods of time, so-called “perennial” seaweeds. The panel on the left shows the final percent cover of perennial seaweed in each barnacle and herbivore treatment, while the panel on the right shows cover of perennial seaweeds on each side of the areas in the half barnacle treatment. When the ribbed limpet was present, seaweeds recovered fastest in areas completely covered with barnacles likely because barnacles provided predation refuge from the ribbed limpet, which is the largest of the snails and a habitat generalist. However, when the ribbed limpet was removed (the rough limpet and periwinkle treatments) seaweeds recovered fastest in areas in which both barnacles and bare rock habitats were present. This happened because of the characteristics of the other snails that were present. The rough limpet tends to avoid barnacle areas (its shell actually grows to fit the shape of the rock surface!) so seaweeds were able to recover on the side with barnacles where it did not graze (see photograph). Tiny periwinkles, on the other hand, hang out near barnacles, but seaweeds recover faster there, too, because the barnacle-free side became covered with weedy seaweeds that choke out the perennials.

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One of the plots from our experiment. The top half has no barnacles and features several visible rough limpets, but the bottom half in full of barnacles and tiny periwinkles. Recovery of perennial seaweed was faster on the side of plots with barnacles.

The results of our experiment were complex and not easy to predict ahead of time based on our natural history knowledge, even though we worked in a relatively small and simple ecosystem. For me, this is much like changing your look when you only have a few articles of clothing at your disposal. A typical suit can look very different if you add a cowboy hat or a bolo tie. What if you threw some spandex into the mix? Chaos?

[NOTE: Originally published in The Aggie Brickyard in 2016]

Dia de los Bryos

Okay, I know my timing is a bit off for a reference to el Día del los Muertos, but something I saw recently made me think of the celebration that I associate most with skulls. I’ve always had a fascination with skulls, what they can tell us about evolution and adaptation, but also how they represent death or perhaps something ‘other,’ something less tangible, even sinister. Yet, skulls are one of the last remains left by vertebrate animals that people can easily put a name and a face to.

The skull I saw, however, did not represent death. True, it was made of calcified material, much like our bones are…

BryoSkull_close2

Small Membranipora membranacea colony growing on bull kelp. Bryozoans grow clonally, budding off new individuals from a founding zooid that settled as a larva from the plankton. In M. membranacea, the larva quickly develops into two zooids, so this species has two founders, or ancestrulae, which are the eyes of the “skull.” This colony is about 0.5mm wide.

…but this isn’t a skull at all. Rather, it’s an exoskeleton inside of which three tiny, genetically identical animals, known as zooids, reside. The animals are called bryozoans, and they are amazing, though diminutive, creatures. The name translates as “moss-animal,” and they are so-called because they grow as colonies that can coat hard surfaces, such as rocks, animal shells, and underwater plants.

The species is Membranipora membranacea, which is part of one of the coolest groups of bryozoans because it spends a long time (months1) feeding in the plankton as a microscopic swimming baby (It’s really called a cyphonautes larva, but I can’t resist). One outcome of this planktonic life history phase is that bryozoan babies can potentially spread over wide distances along coast before they ‘settle’ on a surface and undergo metamorphosis to start life affixed to a surface.

In the last week of November 2013, we had a huge influx of Membranipora settlers on kelp along the Northern California coast. My labmate had collected bull kelp (Nereocystis luetkeana) near Fort Ross, CA, to use in feeding experiments with urchins and abalone, and we noticed that the blades were peppered with tiny spots.

Bryo Skull_far

Numerous colonies on a small piece of a bull kelp’s blade. Note the colony with five zooids in the lower-right corner of the photo.

You can see in the photo above some colonies are slightly larger, which suggests that they settled earlier or grew faster. You can even see the faint outlines of colonies on the opposite side of the blade. Hundreds of colonies were scattered over the blades of a single kelp individual.

One of my professors, Dr. Eric Sanford, mentioned seeing the same thing on giant kelp (Macrocystis pyrifera) at Point Reyes around the same time. I find it fascinating to think about what will happen to all of these colonies. Will they be eaten by sea slugs? Will they overgrow one another? Will their kelp hosts get ripped off the rocks and wash ashore in a big storm?

Unfortunately, I haven’t gone back to see what the kelp looks like now, but you can bet that most of the animals that settled will die before reproducing. And, imagine how many of the babies were eaten or “lost” in the plankton! Fortunately for Membranipora membranacea, they produce many offspring and the fact the larvae spend so much time in the plankton may actually give them access to good food resources that can prepare them for a chance to settle on preferred substrates.

One last photo shows of the zooids extending its tentacles to feed (sorry it is a bit blurry):

The feeding structure, or 'lophophore', of the bottom zooid is extended

The feeding structure, or ‘lophophore’, of the bottom zooid is extended

References:

1Yoshioka, P.M. 1982. Role of planktonic and benthic factors in the population dynamics of the bryozoans, Mebranipora membranacea. Ecology 63: 457-468.

2http://www.photomacrography.net/forum/viewtopic.php?p=72231&sid=142f1a3db539b55a9419986601286718

Science Jargon Band Name of the Day: “Parasitic Castration”

I recently made a classic move in experimental marine ecology, scraping barnacles off of rocks. My main motivation was to set up a small experiment testing habitat use by snails, but I had another mission. I wanted to dissect a few barnacles to see if I could find any predators or parasites hanging out inside the barnacle shell (“test”). I’d been reading about a fly that only lays its eggs on barnacles, and when the larvae hatch they crawl inside barnacle tests to feed on the unprotected tissues and hide from waves (more on that in a later post, hopefully).

On this day, I found no fly larvae, but I did come across a cool parasite. At first glance, this grubby creature looked as though it could have been an insect larva. On closer inspection, the head, legs, and arrangement of segments made me think of a crustacean more than an insect.

Parasitic_Isopod(1)

This animal is most likely a female Hemioniscus balania, an isopod that lives as an parasite on barnacles. Isopods are familiar to many of us from their terrestrial incarnations such as woodlice (aka roly polies, pill bugs, or potato bugs), but they are an incredibly widespread and diverse group of crustaceans, occurring both on land and in nearly every environment in the ocean.

Parasitic_Isopod(2)

What makes this particular isopod so cool is its morphology, ecology, and life history. Its genus name, Hemioniscus, means “half woodlouse,” which I think derives from the fact that only the anterior (“forward”) portion of the animal resembles an isopod. When most isopods molt, or shed their old exoskeleton to reveal a new, slightly bigger exoskeleton underneath, they molt the posterior (“back”) half first and then their anterior half second.1 Somehow, this animal has incomplete molting of some kind that allows it to retain the typical isopod segmentation in front while turning into a giant blob in the back,2 a “mullet bug” of sorts (new common name, anyone?).

Parasitic_Isopod(3)

Young isopods are free-living males and look like typical isopods. They gain entry to barnacles and start to act as parasites. Eventually, they will turn into a bloated female like the one pictured above.3 Here’s where it gets crazy. They feed entirely off of the ovarian fluids of the barnacle, preventing the barnacle from making eggs and shunting a lot of the barnacle’s precious energy directly to the parasite. This is where the term “parasitic castration” comes from. Since barnacles are simultaneous hermaphrodites, they can still function as a male, but you can imagine this form of parasitism could have major impacts on barnacle populations.4

Reproduced from Blower and Roughgarden 1987

Reproduced from Blower and Roughgarden 1987

 

1- see Johnson W.S., E. Stevens, and L. Watling. 2001. Reproduction and development in marine peracaridans. Advances in Marine Biology 39:107–261.

2- see Williams, J.D. and C.B. Boyko. 2012. The global diversity of parasitic isopods associated with crustacean hosts (Isopoda: Bopyroidea and Cryptonicoidea). PLoS ONE 7:e35350.

3- see Crisp, D.J. 1968. Distribution of the parasitic isopod Hemioniscus balani with special reference to the east coast of North America. Journal of the Fisheries Research Board of Canada 25:1161-1167.

4- see Blower, S. and J. Roughgarden. 1987. Population dynamics and parasitic castration: a mathematical model. The American Naturalist 129:730-754.

Worshiping the Plankton in Caprellid City

While we often think of docks and piers covered in attached organisms like mussels and algae, there are also plenty of freely moving organisms like crustaceans and fish that benefit from the habitat provided by attached critters. Last month, I noticed that the submerged portions of the docks in Bodega Harbor were covered in a waving mess of tiny animals. These creatures were none other than caprellid amphipods of the species Caprella mutica, which is non-native in California but commonly found in harbors and estuaries. However, I had never seen such a dense collection of the animals in my life. Check out this video to see what I mean.

Most of the caprellids are engaged in body-waving motion, which is an active method for capturing food from the water. Their long antennae are covered in fine hair-like projections resembling a comb that can trap plankton and detritus. Many of the caprellids are hanging on to an arborescent (tree-like or branching) bryozoan called Bugula neritina (also non-native). That is the bushy purple stuff, which is actually a colony of many interconnected animals. Caprellids like to hang on to something, so it’s easy to see how the non-native bryozoan with all of its branches might benefit the caprellids by providing many “perches.” I wonder if the caprellid offers any benefit to the bryozoan, like preventing other animals from overgrowing it, or if they disrupt feeding of the colony’s tiny animals (“zooids”).

Here are a couple of other things to watch for in the video:

at 1:15, you can see that two caprellids at the bottom of the screen appear to be engaging in crustacean fisticuffs.

at 1:25, you will see a group of small caprellids filter feeding around the inhalant siphon of a native sea squirt (Ascidia ceratodes).

Key West at a Crossroads

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credit: Arnaud Girard @ thebluepaper.com

Next week voters in Key West, Florida will decide whether or not the Key West City Commission can conduct a Feasibility Study designed to determine the environmental and economic impact of widening the city’s main ship channel. Widening the channel has been deemed necessary by some locals (aka “conchs,” a nickname that refers to the overfished snail) in order to allow bigger cruise ships to make Key West a port of call. As a former resident of Key West – I lived there from first through third grade – and a concerned scientist, I have been following this story over the past few months.  Here’s my take on the issue.

Key West is a city that depends on tourism, so it seems natural that conchs would want to facilitate the transit of cash carrying visitors. Yet, much of this tourism relies on the fact that Key West is one of the most tropical places one can find in the US and its warm, shallow waters teem with life. The island city is located in the middle of the Florida Keys National Marine Sanctuary, which encloses the third largest barrier reef in the world and the only one in the United States. So, it’s also understandable that many conchs would oppose any action that would threaten an ecosystem that already bears its share of insults (overfishing, climate change, coastal development).

I guess the decision about whether to widen the channel or not has a lot to do with what locals want from their Key West. Do they want to their commercial district to consist of wall-to-wall trinket stores offering t-shirts that read “I’m with stupid,” or do they want to foster local artists and fisherman who offer a glimpse into what the Keys really are, a biodiversity hotspot­ and our closest connection to the Caribbean. Ok, to be fair the city already has enough trinket stores that walking down Duval Street is not so much fun, except possibly during Fantasy Fest.

So what exactly would the environmental impact of widening the Key West channel be? Aside from directly removing organisms that live on the seafloor, dredging – essentially shoveling out a deep passageway – would bring an incredible amount of sediment from the bottom of the sea into the water. Sure, a lot of sediment would be hauled away to be used for some other purpose (maybe to fill in a marsh or create a new island), but a lot of it would end up suspended in the water. This would decrease the amount of light reaching the bottom, reducing the ability of corals and seagrasses to photosynthesize. The sediments would eventually settle, but much of it would “land” in areas where corals reside and potentially bury them. The combined effects of blocked sunlight and a blanket of sediment could only harm the corals that form the foundation of an entire ecosystem. Moving forward, the bigger cruise ships will bring more people to Key West, which will undoubtedly put more pressure on coastal habitats through land, water, and energy usage.

To get a glimpse of what suspended sediments look like in Key West and how many local fishermen feel about the referendum and the ecosystem, check out these videos.

It’s amazing how much sediment is kicked up when big-ass cruise ships move through the channel, but imagine how much sediment would be lifted off the bottom if the channel was dredged. And, would even bigger big-ass cruise ships continue to kick up sediments as they chug into Key West Harbor after dredging?

I initially figured the referendum to be a sneaky way for the City Commission to get the channel dredged, perhaps by spinning the Feasibility Study results. But after reading a recent piece in a local newspaper, it seems like the referendum may be even more insidious (although I hope it ends up as a boneheaded move). It turns out (SURPRISE) that dredging in the Florida Keys National Marine Sanctuary is prohibited and it may not be possible to conduct a study even if the referendum passes. This either leaves the City Commission looking like they missed a major memo or like they are planning ahead. At this point, it really seems like the referendum is just a way to start the processes of lifting the ban on dredging by forcing a view of public support and economic incentive.

The Florida Keys and the surrounding waters are a national treasure and deserve protection from overdevelopment. Living there as a boy inspired me to study marine ecology and I go back as often as I can to explore and fish with old friends. But, the area is drastically different than it was even twenty years ago and thinking about what it will look like in the future is worrying at best. I totally understand that local businesspeople want to keep their businesses strong and continue to make Key West an incredible tourism hub, but there has to be a balance that takes into account both the flow of tourist dollars and the natural capital that makes the Keys what they really are. I hope the citizens of Key West realize this when they go to the polls next Tuesday.

Marine Organism of the Day – “The Alien”

Ok, I definitely won’t be able to post information about organisms I see every day, but here’s a start to what hopefully will become a fairly regular thing.

A few weeks ago, I visited a marina in Bodega Bay, California to collect organisms for an upcoming experiment. A local fisherman stopped me to ask what I was up to, and we chatted for a bit about my research and his livelihood. He was very knowledgeable about many of the organisms that I sought on the marina’s docks, and he wanted to point one out that he didn’t know much about. What he knew was that it was one of the few dock organisms that fared well after a recent harmful algal bloom that directly or indirectly harmed many organisms on the Northern California coast, including abalone and purple sea urchins.

What the fisherman did not know was what kind of organism it was; he referred to them as “aliens.” We walked together to a nearby boat slip and peered over the edge. What I saw was a purple, dome-shaped mass about 8 inches in diameter stuck on the dock, just below the water line. It was pock marked like a golf ball or the surface of the moon, and it jiggled in the gently flowing water. I reached down to touch it and found it had almost no structure. It was very much like a big ball of snot. I told the fisherman that I wasn’t sure what it was, but probably some colonial animal that feeds on material suspended in the water column.

I was annoyed that I didn’t have a better sense of what this critter was, so I went back for a closer look. I woke to a gorgeous day in Bodega Bay and headed out to the marina. The water was flat like a pane of glass, even offshore, and the sun was shining brightly. Many of the boat slips were empty, so the fishers must have been taking advantage of the lovely weather. I went back to the spot where I had seen the alien previously and laid face down on the dock to get a closer look.

First shot of "the alien"

First shot of “the alien”

I found the alien mass once again, but this time it was personal. The sun provided good lighting, the boat in that slip was out to sea, and I had my camera. I took a few minutes to just sit and watch. I noticed that there were some kinds of feeding structures protruding from the surface of the dome that seemed to spontaneously retract back into the heart of the colony.

I played around with this behavior a bit, poking and prodding at the colony’s surface. When I gently touched one feeding structure, it quickly darted to safety. When I touched a few structures at once, a handful retracted including some that I did not touch. Finally, I put my whole hand on part on the colony and shook it slightly. Nearly every feeding structure retracted in a rapid wave, reminiscent of some common audience participation played backwards.

I still did not know what this creature was, so I took some close-ups that would hopefully reveal more about its identity. I was fortunate to get a few shots in focus:

Partly zoomed in photo of Myxicola cf. aesthetica

Partly zoomed in photo of Myxicola cf. aesthetica

Macro shot of Myxicola cf. aesthetica

Macro shot of Myxicola cf. aesthetica

What I could tell immediately was that I was dealing with a worm, and specifically a polychaete worm. Based on the photos, I could see rings of fairly stout tentacles projecting from worms’ heads. This reminded me of feather duster worms, and gave me a strong inkling that these worms were members of the family Sabellidae. After talking with experts at Bodega Marine Laboratory (Eric Sanford and Jackie Sones), we determined that this worm might be in the genus Myxicola. Only two species in this genus are described in California, with one, Myxicola aesthetica, “often found in masses attached to ropes or other sunken objects; <50 mm long” (Carlton 2007). I may have found a match!

There are two curious characteristics about this organism that I want to share. First, the genus Myxicola is known for its ‘giant axon’, or nerve, that runs nearly the entire length of the worm’s body. This giant axon directly innervates the worm’s muscles and presumably aids in super-fast retraction of the unprotected feeding structures back into the tubes.

Having these outsized nerves makes these worms model organisms for the study of nerve function. Another type of organism that has a similarly large axon is squid. They use this giant axon in their jet propulsion system, presumably to aid in rapid predator escape (perhaps these similar traits represent convergent evolution toward a common function). One nice thing about using the worm as a model system over the squid is that the worms are sessile as adults and can be found near the coast throughout much of the year.

Myxicola

The two sets of diagrams show the structure of Myxicola’s nerve cord. The top figure shows the location of the nerve cord in the dorsal (belly) side of the worm and its physical structure. The bottom figure shows a transverse section cut through the nerve cord (like a cut across the belly to eviscerate prey or foe) . Note the feathery structures on either side of the nerve cord represent longitudinal muscles. Adapted from Gilbert 1975 and Nicol 1948.

The other interesting thing about Myxicola is that it can reproduce both sexually and asexually, and apparently can regenerate from a single segment (Berrill 1961)! In fact, when a new worm forms, it does not have any cephalic (aka. head) structures before it separates from its ‘parent.’ Asexual reproduction in annelids is not very common, and is generally relegated to smaller, colonial suspension feeders. Many aggregations of worms are actually a result of ‘gregarious settlement’ (larvae preferentially settle near members of their own species) rather than asexual reproduction. I would love to know more about this critter if anyone can drop some knowledge on me.

References:

Berrill, N.J. 1961. Growth, Development and Pattern. Freeman, San Francisco, CA, USA.

Carlton, J.T. (editor). 2007. The Light and Smith Manual: Intertidal Invertebrates from Central California to Oregon, Completely Revised and Expanded. University of California Press, Berkeley, CA, USA.

Gilbert, D.S. 1975. Axoplasm architecture and physical properties as seen in the Myxicola giant axon. Journal of Physiology 253:257-301.

Nicol, J.A.C. 1948. The giant nerve-fibres in the central nervous system of Myxicola (Polychaeta, Sabellidae). Quarterly Journal of Microscopical Science 89:1-45.