By Amanda Sills

As a recent graduate working in the de Mutsert Fish Ecology Lab, one of my primary responsibilities has been sorting and identifying ichthyoplankton samples collected as part of long-term monitoring studies in tributaries of the tidal freshwater Potomac River.  A large portion of my master’s thesis was devoted to developing an identification guide and functional dichotomous key for commonly found ichthyoplankton in these tidal freshwater habitats.

Here, I provide an overview of this important growth and developmental stage for fishes, as well as an introduction to the, sometimes tedious, process of ichthyoplankton identification.

What are ichthyoplankton?

Ichthyoplankton are the larval development stages of fishes. This stage is marked from the time an egg is fertilized to the point where a fish begins juvenile metamorphosis (where a fish has developed almost all adult anatomical features but remains sexually immature). Because larval fish are part of the planktonic community, they cannot swim strong enough on their own to control their location – often relying on currents and tides to carry them to nursery habitats where they can thrive.

Typically, larval fishes are divided into two growth stages, yolk-sac and post yolk-sac larvae. Most larval stage fishes will be underdeveloped when they hatch, lacking functional mouths, eyes, digestive systems, and fins. Because they are incapable of capturing their own prey during this time, a portion of the yolk is retained on the body after hatching, dubbing the term yolk-sac larvae. You can think about this yolk-sac as an energy drink for the fish, providing highly nutritious energy for the fish to use in developing features, like mouths or fins, that will enable them to catch their own food and maintain position in the water column.

Post yolk-sac stage larval fish have completely absorbed the yolk-sac retained after hatch, and are self-sufficient in finding their own food in the environment. While many ichthyoplankton at this time have developed fins, they are still not strong enough swimmers to move out of the area where the currents or tides have carried them.

 

Why are ichthyoplankton important?

Ichthyoplankton have been utilized in research in a variety of different ways. For example, simply looking at the abundance of ichthyoplankton in a tow sample can be used as an indication of the amount of spawning occurring in a system. If ichthyoplankton have been regularly collected, examining changes over time in the species composition of samples could be indicative of changes occurring in environmental parameters or available habitat areas in the ecosystem of interest. Because fish in their early developmental stages are in most cases less resilient than older conspecifics, they are frequently utilized in ecosystem monitoring.

How can ichthyoplankton be identified?

Because larval stage fishes can look dramatically different from adult conspecifics, or even more developed ichthyoplankton of the same species, identification can be challenging. Several techniques, depending on budget, survey design or resources, can be used to classify larval stage fishes as far taxonomically as possible.

Up and coming in the realm of ichthyoplankton research is the use of genetics and DNA to identify samples. While these methods can be pricier, there are a large collection of studies, which have had great success employing such techniques.

Clearing and staining involve dissolving all of the tissues of a specimen, and staining the remaining skeleton a bright color for analysis. The bone structures of different families, genus, or species of fishes can differ, making this identification technique not only handy but artistic.

Lastly, ichthyoplankton can be identified manually using known morphology and meristic characteristics. Typically, these methods involve the use of a dissecting microscope and counting features like myomeres (bundles of muscle fibers), fin rays or pigmentation spots to classify specimens.

Tips for easier identification of preserved specimens.

I won’t sugar coat it – manually identifying ichthyoplankton can be a challenging and sometimes frustrating process, especially when you are first starting out. Myomeres can start to blur after a few long hours, and some specimens can be damaged or poorly preserved, distorting the anatomy essential for you to classify it past a family taxonomic level.

Here I’ve put together a few tips that I’ve picked up over the past few years identifying larvae, that I find can make the process a bit easier. Especially when dealing with large volumes of identifications, the littlest maneuver or tweak can make a difference in not only your accuracy but efficiency as well.

• Familiarize yourself with anatomy basics. This is helpful when working frequently off of dichotomous keys for identifications. If you have a solid understanding of where dorsal, anal, pelvic and pectoral fin rays should be located or the relative location of dorsal-lateral pigmentation spots, then you can work much quicker through your specimens without getting hung up on the location of anatomy you should be looking for.

 

• Line up a bunch of fish to measure at once. Over the past few years, I cannot put a total on the number of clupeids I have had to identify – waging a guess it would be in the thousands. What I’ve found is if my samples are loaded with similar species that can be broken in to size classes, instead of working one individual at a time I instead will line up several and work through the keys there. This allows you to knock out a bunch of repetitive tasks all at once. For example, need to take a standard length to total length ratio? Work through those specimens all at once instead of jumping around different size classes.
• Lighting is your best friend. Finding pigmentation spots and exact myomere counts is perhaps one of the trickiest aspects of larval identification. I’ve found that playing with different levels and positioning of light is ideal to locate faded supracaudal pigments or faint myomeres around the nape region. Underlighting, external lighting, or even working on a black background can really make a difference in what you can and cannot see, so be sure to play around with what works best for you and your microscope.
• Maneuvering is everything. It’s very easy for dorsal and anal fin rays and fin folds to get tucked under the body, making them difficult to see when you need them. Be sure to flip specimens around and over a few times to make sure important features aren’t actually hidden instead of absent.

• Keep your specimens hydrated. Lastly, whether you are working in ethanol, tap water or another type of preservative, your specimens can shrink or grow depending on how hydrated they are or what you are working in. This is important to consider if you aren’t aware, because it can mess up measurements or ratios of features. For example, working in ethanol, specimens can dry out if not careful, often resulting in damage or reduced ability to identify.