Some of the most beautiful and conspicuous animals we encounter in the plankton images are leptocephalus larvae. Eels, bonefish, ladyfish, and tarpon form a diverse superorder of fishes (Elopomorpha) that all begin their planktonic lives as leptocephalus larvae, which are characterized by long and laterally compressed bodies that are almost completely transparent. This type of larva is known by scientists to be relatively primitive, meaning that leptocephalus larval forms have been utilized by fishes for millions of years – probably since the Cretaceous period.

In this image, the you can see the backbone and the myomeres along the body. On the left side, the mouth of this larva is open.

Tarpon and bonefish larvae are slightly modified from the classic leptocephalus form. Unlike the eel leptocephali, tarpon and bonefish larvae are thinner vertically and have a well-developed caudal fin, which they use to swim.

Partial image of the tail region of a tarpon, bonefish, or ladyfish.

Most leptocephalus larvae (sometimes referred to as “leptos”) have a large set of teeth that protrude outward. For a long time, the diet of leptos was a complete mystery because their guts appeared to be empty. We now know that they rely on particulate matter, also known as “marine snow”, as their primary food source (Miller et al. 2013). Our in situ imaging work has revealed that leptos are quite abundant, particularly in the northern Gulf of Mexico, and have a variety of complex behaviors that may help them survive in a realm of intense predation pressure. We recently published a paper presenting evidence that the morphology of these leptos indicates that they likely serve as Batesian mimics of cestid ctenophores, which are strikingly similar in appearance (Greer et al. 2016). By resembling an animal with low nutritional value that predators avoid (the cestid ctenophore), the leptos gain protection from predators. The only perceivable morphological difference is that leptos have a head and a dark backbone with myomeres (muscle strands) that are visible when viewed through a shadowgraph imager, while cestid ctenophores lack these features. For a fish predator relying on vision in a complex ocean light environment, it may not be able to tell the difference, which could explain why predation rates on leptos are much lower than expected (Miller et al. 2015).

Cestid ctenophore

Leptocephalus larva – note the similarity in morphology to the cestid ctenophore above.

Leptos display complex behaviors that can make them difficult to identify in the images (and likely has a similar effect on predators). This one is curled up. The main cues that this is indeed a lepto are the visible myomeres, and the head and tail are meeting on the left and right sides of the body, respectively.

Leptocephalus larvae are fascinating, and we are just beginning to unlock some of the secrets of their biology and ecology through imaging. By combining the data produced from imaging them in their natural environment with other techniques to assess diets and predation pressure, we hope to learn more about these animals and what processes impact their survival during their time in the plankton. After all, some of these leptos eventually grow up to do this:

Source: tampayacht.com

References

Greer AT, Woodson CB, Guigand CM, Cowen RK (2016) Larval fishes utilize Batesian mimicry as a survival strategy in the plankton. Mar Ecol Prog Ser 551:1-12

Miller MJ, Chikaraishi Y, Ogawa NO, Yamada Y, Tsukamoto K, Ohkouchi N (2013) A low trophic position of Japanese eel larvae indicates feeding on marine snow. Biol Lett 9:20120826

Miller MJ, Dubosc J, Vourey E, Tsukamoto K, Allain V (2015) Low occurrence rates of ubiquitously present leptocephalus larvae in the stomach contents of predatory fish. ICES J Mar Sci 72:1359-1369

During the CONCORDE summer field sampling campaign, we had the good fortune of towing the ISIIS through a unique oceanographic feature called a “thin layer.” Thin layers are dense aggregations of phytoplankton or zooplankton that have a vertical dimension of less than 3 meters and have concentrations of at least 2-3 times the water column average abundance. They can span hundreds of meters to several kilometers in the horizontal dimension. This series of images shows how the plankton concentrations dramatically change from just underneath to above the thin layer. The hairlike objects are phytoplankton called diatoms, but there are also lots of zooplankton in the vicinity of the layer. Here you can see the barrel shaped doliolids, which are grazers of phytoplankton, as well as copepods and chaetognaths.

Image taken just below the thin layer

Within the thin layer

Just above the thin layer you can see lots of marine snow particles and a chaetognath.

Further above the thin layer, the plankton concentrations are not as intense.

Keep in mind that all of these images were taken within a few meters of each other. It is amazing to see firsthand how dramatically the ocean environment can change, and how these organisms respond this variability. Is this something they normally experience? The next steps are to use the image data to quantify the changes in abundances and look at the different mechanisms that may have caused this thin layer to form.

The more I learn about stomatopods (also known as “mantis shrimps”), the more fascinated I become with these true marvels of evolution. From the image data we have processed so far from the CONCORDE fall and spring field campaigns, it is pretty clear that stomatopod larvae were quite abundant in the nearshore waters of the northern Gulf of Mexico. As you can see from the images below, they are really cool looking animals – even Hollywood took notice, as stomatopods served as the inspiration behind the alien queen in the movie “Alien.”

Stomatopod larvae imaged in the northern Gulf of Mexico on October 30, 2015

The first thing you notice when you see the images of the larvae are those giant stalked eyes protruding from their heads. The eyes, which appear to be well developed in the larvae, are one of the most remarkable adaptations of the adult stomatopod. Unlike mammals and cephalopods, which have a camera like eye (single lens projecting an image onto a retina), stomatopods have a highly specialized compound eye, and each eye often moves independently from the other. They also have 12 visual pigments, giving them a huge range of detectable wavelengths, from infra-red to ultraviolet – much more than other invertebrates (Land & Nilsson 2012). In comaprison, humans only have three visual pigments (red, green, and blue), and the stomatopods can even detect different types of polarized light. Have you ever heard of circular polarized light? Well, they can see that too! The males have certain parts of the body can reflect circular polarized light in both right and left hand directions. The band across each eye has 4 rows of ommatidia that can see in color, and two rows (one on each end) detect polarized light (Chiou et al. 2008).

Image source: http://www.abc.net.au/catalyst/stories/3280489.htm Also, check out the interview with Dr. Justin Marshall who has been studying mantis shrimps for almost 20 years

Another impressive thing about stomatopods is their method of capturing prey. Different species of stomatopods use one of two different attack methods: smashing and spearing. You can tell what method it uses based on the morphology of the stomatopod appendages. The smashers have clubbed appendages that they flick outwards to smash their prey, and the spearers have pointy appendages that they can use to puncture their prey. The force created by these attacks is extremely powerful for their size. In fact, the movement of the appendages is so quick that it generates a large cavitation bubble. The phenomenon of cavitation happens when something in the water moves so fast that an empty space is created that the water cannot fill quickly enough. When a boat propeller rotates at a fast rate, it generates tiny cavitation bubbles. It is quite rare for an animal as small as the stomatopod to generate enough force to cause cavitation.

References:

Chiou T-H, Kleinlogel S, Cronin T, Caldwell R, Loeffler B, Siddiqi A, Goldizen A, Marshall J (2008) Circular polarization vision in a stomatopod crustacean. Current Biology 18:429-434

Land MF and Nilsson D-E (2012) Animal Eyes (2nd ed.) Oxford University Press

Things are going well onboard the R/V Point Sur as we finish up the first leg of the CONCORDE fall research expedition (www.con-corde.org). Although the water column has been well mixed from the recent windy conditions, we are seeing a lot of structure with regards to the biology. As we move along the transects, we see distinct changes in the plankton community. Here are just a few examples of the types of organisms we are seeing.

This looks like something out of an alien movie, but it actually a stomatopod larva. Stomatopods are also known as mantis shrimps and, like many ocean animals, have egg and larval stages in the plankton. Adult stomatopods can give one of the most powerful strikes of any animal for its size. They use their club-like appendages seen in the image to bash their prey, stunning or killing it before consumption. Stomatopods, with their large compound eyes, also have some of the best visual acuity for any invertebrate.

Today we also found this larval squid (~1 cm in size). It was imaged right next to two similarly sized squid larvae, so we can only assume that they were spawned from the same parents or spawning event (i.e., they are members of the same cohort). Many larval animals may aggregate in this fashion to protect themselves from predators. Even though aggregating together may not reduce each individual’s chance of encountering a predator, the schooling behavior probably has a benefit because predators may become confused by the many potential targets in close proximity.

This last image is one that is truly exceptional. It shows a large scyphomedusa (jellyfish) that covers almost the entire image. The field of view on the ISIIS is about 13 cm, and we can estimate that this individual is at least twice that size. It is likely a moon jelly (Aurelia spp.), but the most remarkable thing are the fish larvae aggregating underneath it. We can see four larvae that appear to be jacks or butterfish that seem to be using the moon jelly for shelter or protection. Jellyfish are often thought of as predators of fish larvae, but images like this one show that that interpretation may be a bit simplistic. A few types of fish larvae are known to use jellyfish for protection because some larval fish predators avoid eating jellyfish due to their stinging nematocysts and low nutritional value. It is extremely rare to document this kind of behavior, especially the large number of larvae aggregating under one moon jelly.

Recently the University of Southern Mississippi purchased a brand new In Situ Ichthyoplankton Imaging System (ISIIS) for use as part of a research consortium known as CONCORDE (www.con-corde.org). The goal of the consortium, which involves many different universities and governmental organizations, is to better understand potential physical and biological pathways of oil in river-dominated coastal ecosystems, such as the northern Gulf of Mexico.

USM researcher Kevin Martin helps to lower the ISIIS onto the Pt. Sur

In order to better understand the ecosystem, we must first get an idea of how the different planktonic organisms are distributed in time and space. The ISIIS is a good tool to do this because it will ultimately provide a high resolution spatial map of different plankton taxa, which can be used to complement many avenues of oceanographic research. Currently, we are aboard the R/V Point Sur, deploying the imaging system in the nearshore environment, identifying and mapping the different types of plankton in the highly productive northern Gulf of Mexico. Because the weather has and continues to be quite windy, we are generally seeing a well mixed water column, but there still a great amount of biological diversity. Throughout the course of the research expedition, we will post updates of the different organisms being found. Here is one image of a phyllosoma larva, which is the larval form of lobsters. This is possibly a slipper lobster, which is known to have a larval stage of over 9 months! These organisms must undergo incredibly long and dangerous ocean journeys in their first few months of life, as they traverse the coastal waters that are full of predators. We were lucky enough to come across this one:

Phyllosoma larvae are almost completely transparent and have remarkably long larval stages. This one is only ~2 cm long but could eventually develop into an adult lobster if it survives the perilous journey in the plankton.

For more updates on the research being done on the Point Sur, please follow Heather Dippold’s CONCORDE blog at www.con-corde.org/blog. There you can see information about the different instruments and people involved in studying various aspects of oceanography in the Gulf of Mexico. Also, stay tuned here to see different types of plankton we are finding in the images!