the Editor says: In recent years, the topic of marine citizen science has been gaining increasing attention, and many intertidal observers and underwater photographers have been sharing their wildlife photos on various platforms. While photography is one of the more accessible entry points into marine citizen science, the long-standing pursuit of high artistic quality in underwater photography can sometimes result in images that lack the key features needed for species identification — a real shame for scientific purposes. Over the years of running our marine citizen science programme, we have frequently encountered the situations described below — always with a smile. This article aims to give every underwater photographer and nature observer some practical guidelines on how to photograph specific groups of organisms in ways that capture the features needed for reliable identification.
3 Essential Concepts Every Marine Citizen Scientist Should Know About Species Identification
Instructor, I just saw a blue fish — what kind of fish was it?
The question above is one we hear constantly at the water's edge. When learning to identify marine life, there are several key concepts to keep in mind:
Identification Concept 1: Colour
Vision is the primary sense through which humans observe the world, and colour is the most powerful piece of information it delivers. As a result, we very often use colour as a basis for describing or evaluating what we see during nature observation — phrases like "a fluorescent-blue fish," "a red-spotted nudibranch," or "a white eel with black spots." Colour, however, can also cause a great deal of confusion, especially when observing marine life.
First, because seawater absorbs light, the colours we see underwater are often not the organism's true colours. We therefore need to use a strobe or video light to restore accurate colour. With proper lighting, we can see the vivid hues of coral reef creatures — nudibranchs, shrimps, crabs, and all manner of tropical fish — and use their colours to narrow down possible species. That said, some species that live outside coral reef environments — such as open-water sardines or yellowtail amberjack, or sandy-bottom lizardfish and flatfish — may not display particularly vivid colouration at all.
Furthermore, many marine creatures are skilled at changing colour for communication, camouflage, courtship, and other behaviours. This is especially true of masters of colour change like the octopus, scorpionfish, and flatfish — in such cases, colour alone simply cannot be relied on for identification.
Identification Concept 2: Morphology
When colour is not enough to identify an organism, we begin to look at physical characteristics. Describing an organism's shape is the foundation of learning to recognise it. Rather than resorting to hand gestures, use detailed morphological terms to describe its appearance. Here are some common ways we describe fish body shape:
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"Elongated" — for moray eels, sea cucumbers
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"Round and stout" — for pufferfish, cowries
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"Flat" — for stingrays, flatfish
When photographing, it is best to capture the full body shape of the organism. Side-on, overhead, and head-on angles are all useful if possible.
For organisms that are difficult to identify from overall appearance, we need to look for subtler clues — structural features that do not change, such as the shells of crustaceans, the mantle of molluscs, or the scales and spines of fish. Size is also an important morphological characteristic. Take moray eels in the genus Gymnothorax: some species, such as the yellow-margined moray and the undulated moray, can reach up to 2 metres in length, while others, like the finstripe moray, are only a few tens of centimetres long.
Identification Concept 3: Behaviour & Habitat
Finally, a few small contextual clues can greatly help clarify species identity — for example: At what depth did you see it? Was the substrate sandy or coral reef? Was it solitary or in a group?
From the above, you can see that if you want to ask your instructor what species you just saw, try rephrasing your question like this:
Instructor: At about 5 metres depth on the coral reef bottom, I saw a group of oval-shaped, fluorescent-blue fish about 5 cm long. What were they?
Most people reading that description can quickly guess that the organism is likely the neon damselfish Pomacentrus coelestis, which is commonly found in shallow water. Why do we say likely? Because at this point we can probably narrow it down to the family, or even the genus — but to distinguish the exact species, more information is still needed!
Even if it is likely a neon damselfish, its colour can still change depending on the environment. Marine Encyclopedia – Northern Taiwan, P. 705
Ready to Level Up? Let's Take on Species Identification Challenges for Common Marine Life!
Congratulations — you've completed the beginner's course in species identification! Are you ready to move on to the advanced level with us? Some marine creatures change not only their colour but even their appearance depending on the environment, and the attachment of algae or debris to their bodies can make identification even harder. In such cases, we need to focus on the organism's hard skeletal structures — features that do not change with the environment, and are therefore the most reliable identification markers.
Species Group 1: General Fish
Because fish species are enormously diverse and identification features vary greatly between groups, we will start with some general guidelines for fish identification. The primary identification features for fish are the dorsal fin as seen from the side, and the body's lateral colouration and markings. This is why the fish illustrations in the Marine Encyclopedia are almost always shown in full lateral view — it is simply the easiest angle for identification.
Here we use two species of jack (family Carangidae) commonly found along the Northeast Coast as an example: the Derbio (Trachinotus blochii) on the lower left, and the often-confused Longfin trevally on the lower right. They can be distinguished by the second dorsal fin and the tail fin. The second dorsal fin of the two species differs — one is rounded and blunt, the other sickle-shaped — and the lower lobe of the tail fin of the Derbio is white, while that of the Longfin trevally is not. So, a good identification photo of a general fish species must capture a clear full lateral view!
Left: Derbio (P. 569). Right: Longfin trevally (P. 571).
Species Group 2: Seahorse
One of the underwater world's star creatures is undoubtedly the seahorse! Beyond the impossibly tiny pygmy seahorse, the species most people encounter is the larger-bodied seahorse. After every seahorse sighting, divers tend to declare with confidence: "I saw a yellow seahorse!" or "A yellow seahorse must be species X!" Unfortunately, seahorses are one of those creatures that change colour according to their environment, which means external colour is completely useless as an identification criterion.
A simpler identification method for seahorses involves counting the tail rings. The image on the left shows Hippocampus kelloggi (Kellogg's seahorse), which has 40–41 tail rings. The image on the right shows Hippocampus kuda (the spotted seahorse), which has 34–38 tail rings. If the tail rings cannot be counted clearly, you can also observe the prominence of the body rings along the trunk: H. kelloggi typically has sharper, more angular body rings, while H. kuda tends to have smoother ones. So, a good identification photo of a seahorse must show either the tail rings or the lateral prominence of the body rings!
Left: Kellogg's seahorse (P. 501), which typically has sharper, more angular body rings. Right: Spotted seahorse (P. 503).
Species Group 3: Scorpionfish (commonly called stonefish)
Scorpionfish commonly lie motionless on the bottom waiting for prey to pass, making them relatively easy for divers to photograph. However, capturing a full lateral view of a scorpionfish is not sufficient for species identification, because their body colour is highly variable and the skin flaps that mimic algae vary enormously between individuals. For identification, we need to move in close and look at the numerous bony spines on their faces — these innate structural features are much less subject to change, and therefore serve as important identification markers. For the best chance of a correct identification, try to get close-up shots of the head from both the side and above. So, a good identification photo of a scorpionfish must include a close-up of the face!
Left: Mozambique scorpionfish (P. 526), with elongated supraocular skin flaps resembling feathers. Right: Golden scorpionfish (P. 527), with small or even absent supraocular skin flaps.
Species Group 4: Lizardfish
Lizardfish are among the more difficult fish to identify — the distinguishing features between species are not obvious. The one feature that actually makes identification possible is the nasal flap. As ambush predators, lizardfish typically rest quietly on the reef floor and rely on specialised nasal flaps to enhance their detection of scent particles in the water, helping them sense the position of prey and predators alike. Different species of lizardfish have evolved distinct nasal flap shapes. So, a good identification photo of a lizardfish must show the nasal flap at the snout!
Lizardfish (family Synodontidae) are notoriously difficult to identify. The image above shows the variegated lizardfish. Marine Encyclopedia – Northern Taiwan, P. 461.
Species Group 5: Frogfish
Frogfish are remarkable masters of colour change underwater. Not only do juveniles and adults display different colouration, but they can alter their appearance at will to match their surroundings — making body colour a poor basis for species identification. The "simplest" way to identify a frogfish is by its illicium and esca (the fishing rod and lure). Different species have evolved distinct lure shapes to match their hunting strategies: some mimic a swimming shrimp to attract fish, while others mimic a worm drifting in the water, occasionally twitching the lure to draw prey within striking distance.
Although the lure is described as the "simplest" feature to use, it is unfortunately not always deployed. In such cases, look closely at the second and third dorsal spines: in some species they are swollen like a drumstick, in others they are paper-thin, and in still others they resemble a slender twig. To reliably identify a frogfish, it is best to photograph its face clearly — and if you can capture the lure in the shot, even better! So, a good identification photo of a frogfish must show the lure and the second and third dorsal spines!
The painted frogfish is a star species, but is often confused with other species. Marine Encyclopedia – Northern Taiwan, P. 471.
Species Group 6: Gastropods & Bivalves (Shells)
Molluscs with shells play an important role in marine ecosystems, yet most people photograph them only from above, which is often insufficient for species identification. Many shelled molluscs are covered in algae or debris as a form of camouflage, making it nearly impossible to see the shell's surface texture and shape from the outside.
The best way to identify a shelled mollusc is to flip it over and photograph the aperture (opening). Because the aperture is where the animal's mantle and foot pass in and out every day, it is usually kept smooth and clean, making it the most reliable identification feature for this group. A word of caution, however: some shelled molluscs — such as cone snails — are highly venomous, so exercise extreme care when turning them over. Once you have finished photographing, be sure to turn them back over and return them to exactly where you found them, to minimise disturbance to the animal. So, a good identification photo of a shelled mollusc must show the aperture on the underside!
Shell photos need to include a shot of the aperture to allow for identification. Marine Encyclopedia – Northern Taiwan, P. 184.
Species Group 7: Nudibranchs
Thanks to the community's enthusiasm for macro photography, nudibranchs are represented in marine databases by a far greater variety of images than most other organisms. While many nudibranch species have yet to be formally named, as long as you can photograph their gills and rhinophores (sensory tentacles), species identification becomes possible. So, a good identification photo of a nudibranch must show the rhinophores and gills!
The thecacera pacifica nudibranch, commonly nicknamed "Pikachu." Marine Encyclopedia, P. 225.
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