The cause, a parasite, more specifically a trematode, Scaphanocephalus expansus. This species belongs in a family called Heterophyidae and so they are referred to as heterophyid trematodes. As far as I know there is no common name for this parasite. Trematodes are a group of flatworms, phylum Platyhelminthes. Once placed in the class Trematoda but now designated as lower grouping, typically the order Trematoda, and along with other worms, placed in class Rhabditophora. Trematodes are also referred to as flukes because they are, well, flat. Fluke comes from the Old English “Floc’ or German “flach” meaning…yes you guessed it, flat. Or perhaps the Norse “fluke” for the flounder, a flat fish.
FYI: Fluke also refers to:
A chance occurrence. A whales tail. An Anchor arm. or perhaps even, a sudden gust of wind. WTF, really?
At least the whales tail, which is flat, connects. But still, English is a stupid language. Here is a bit more on flatness: A sole is also a flat fish, and also the flat part of a shoe, and soleus refers to a flat muscle in your calf. At least these all relate to the word flat, BUT why so many names for flat; well because English has stolen from so many other languages that’s it now a morass of sluck. See, English is so mixed up you didn’t even catch that I made up the word sluck.
Okay, back to the fun: A basic Trematode looks like this
Back to those fish with black spots, the black spots are caused by the encysting stage, called metacercariae, of the parasite. The black spots, according to (Overstreet and Hawkins 2017), help attract osprey predators.
We should look at a brief life cycle of parasites. Parasites typically do not spend their lives on a single host: Parasites in this group typically follow this pattern:
1. Parasite (Miracidium stage) imbeds in snails.
2. Swimming larvae (Cercaria) emerging from the snail burrow into the fish (or fish eat infected snails, but this is unlikely since the parasite is under the fish skin).
3. Birds eat the infected fish. The parasite matures to an adult (metacercariae), and, living in the small intestine (where it is housed—since it was eaten) produces eggs that can be shed easily (how convienient). If those eggs end up in water they morph into miracidium larvae and find a snail; then we are back to snails and the cycle repeats.
This parasite might not be typical and its life cycle may be different. Speaking of different, its external anatomy is different, the ‘basic’ trematode is like a flattened football, with small anterior and posterior ends. Here is an image of the adult parasite with its expanded anterior end.
And here is the potential life cycle.
With a life style like that, it’s no surprise that the parasite had been found in the past in a number of fishes, and a few bird species. These include the Nassau grouper (Epinephelus striatus), The Many-bar Goatfish (Parupeneus multifasciatus), The Gray Snapper (Lutjanus griseus) and even an elasmobranch, related to sharks, the smalltooth sawfish (Pristis pectinata). The birds, fish eaters, are the White-bellied Sea Eagle (Haliaeetus leucogaster), and the Osprey (Pandion haliaetus). That parasite does get around. In fact the first description of the parasite was from an Osprey, their definitive host. Osprey, fish hawks, tend to house lots of parasites, I suspect fish have lots of parasites and osprey, feeding on fish, get them. One Florida study found 14 different parasites species in just five osprey.
In the 2019 study (both 2019 studies below are from the same research group) researchers noted that black spots were particularly abundant on the ocean surgeonfish, Acanthurus tractus. Once they started to look closer they discovered an additional nine species that had spots, and thus housed the parasite. Fishes include the Bar Jack, Doctorfish, Orange-spotted Filefish, and Redband Parrotfish. One individual Redband Parrotfish, Sparisoma aurofrenatum, was infested with more that 300 worms, yikes. The parasitic worm is small, but not tiny; the metacecariae stage can be as long as 3 mm.
Ocean Surgeonfish eat algae, and detritus too. They eat inorganic stuff too, this apparently helps to grind their food after eating. Shells are found in the stomach too but this is thought to be part of the detritus they eat. Redband Parrotfish eat many algae, sea grasses, and coral bits. With the common food source, algae, it is possible the source of infection is a small snail clinging to algae.The fish are more likely to be infected by burrowing larvae but it’s possible that the fish eat, perhaps unwittingly, a snail.
Your job, go snorkel the Caribbean and find out. Rough job, bring sunscreen.
The most well-known trematode problem, of course, effects humans: Schistosomiasis, also known as snail fever because intermediate stages are found in freshwater snails. Schistosomiasis causes problems in its definitive host, humans (bummer). The parasites houses in the intestine or urinary tract. The problems it causes include pain, diarrhea, blood in stools urine, liver or kidney damage, even infertility. Be wary folks, although Schistosomiasis is primary known as a tropical disease the changing climate of the globe has allowed this scourge to expand into heretofore unavailable regions.
Black spots on fishes from freshwater fish can be caused by another the parasite, Posthodiplostomutn nanum. It’s also known from marine and freshwater aquarium fishes, and is caused by other parasite species. So overall if you see black spots on fishes it’s probably caused by a parasite but not necessarily this parasite.
Final End note:
The scientific name, Scaphanocephalus expansus (Creplin, 1842), the whole official name includes the (Creplin 1842) part; this means that the species was described by Creplin in 1842. It’s sometimes hard to find out information about the folks who have described a species, that is the case here. I cannot find a copy of the journal he published the description in, but I suspects its in German, most of the things I found about FCH Creplin were in German. In my searches I have found that he described many species from at least 1825 until 1852, they are, or mostly are, parasitic worms. So Ole Creplin liked to root around in fish and bird guts (and cats, his studies are cited in veterinary manuals for cats). He was a physician and studied worms and parasites particularly (Helminthology). He was the master of parasitic worms in his time. Plus he was a looker
Sources and Readings
Elmer FE, Kohl ZF, Johnson PTJ, Peachey, RBJ. 2019. Black spot syndrome in reef fishes: using archival imagery and field surveys to characterize spatial and temporal distribution in the Caribbean. Coral Reefs.https://doi.org/10.1007/s00338-019-01843-3
Faust E, and Nishigori M. 1926. The Life Cycles of Two New Species of Heterophyidae, Parasitic in Mammals and Birds. The Journal of Parasitology, 13(2), 91-128. doi:10.2307/3271705
Kohl ZF, Calhoun DM, Elmer F, Peachey RB, Leslie K, Tkach V, Kinsella JM, Johnson PT. 2019. Black-spot syndrome in Caribbean fishes linked to trematode parasite infection (Scaphanocephalus expansus). Coral Reefs 38: 917-930.
Mulero S, Rey O, Arancibia N, Mas-Coma S, Boissier J. 2019. Persistent establishment of a tropical disease in Europe: the preadaptation of schistosomes to overwinter. Parasites and Vectors 12: 379-389.
Overstreet RM, Hawkins WE. 2017. Diseases and mortalities of fishes and other animals in the Gulf of Mexico. Habitats and Biota of the Gulf of Mexico: Before the Deepwater Horizon Oil Spill. Springer, New York, New York, pp 1589–1738
Randall JE. 1985. Food Habits of Reef Fishes of the West Indies. Studies in Tropical Oceanography 5: 665—847.