Know thine anemone: peculiarities of iNaturalist's observational data

In my time curating the Cnidaria observations on iNaturalist, I’ve noticed some peculiar aberrations in the participation rate of certain regions.

I’m going to attempt to illustrate what I mean by highlighting the family Actiniidae. These are the most common and diverse sea anemones in coastal habitats, meaning that these are easily observed by casual users, thus not requiring any scuba equipment. I’ve chosen to examine a mix of regions, both in terms of ecology and population. The following chart indicates the number of observations per month.

South Africa / New Zealand / Australia / Japan / SE USA / Europe / West USA / India / Singapore / Hong Kong / New Caledonia

119 / 444 / 559 / 022 / 037 / 0286 / 3730 / 104 / 011 / 009 / 012 January
123 / 264 / 292 / 029 / 022 / 0379 / 3801 / 098 / 008 / 009 / 010 February
140 / 185 / 287 / 025 / 058 / 0438 / 4015 / 111 / 003 / 020 / 006 March
949 / 302 / 309 / 008 / 033 / 0758 / 4726 / 061 / 012 / 067 / 010 April
791 / 445 / 229 / 017 / 051 / 0954 / 4653 / 070 / 012 / 038 / 006 May
109 / 191 / 227 / 017 / 080 / 0912 / 8243 / 033 / 026 / 031 / 020 June
134 / 251 / 246 / 012 / 066 / 1593 / 5268 / 016 / 031 / 047 / 003 July
131 / 294 / 319 / 014 / 062 / 1783 / 3630 / 038 / 024 / 020 / 011 August
152 / 348 / 380 / 004 / 027 / 0948 / 2362 / 054 / 010 / 023 / 024 September
288 / 416 / 491 / 003 / 027 / 0804 / 2490 / 114 / 001 / 026 / 021 October
175 / 315 / 415 / 005 / 026 / 0345 / 3641 / 191 / 008 / 020 / 026 November
208 / 388 / 566 / 023 / 049 / 0318 / 3816 / 140 / 013 / 009 / 015 December

South Africa (12 species; tropical & subtropical): South Africa has an interesting mix of tropical and subtropical species (even becoming somewhat temperate around Cape Town, with species like Bunodosoma capensis). However, only 4.6% of the observations come from the tropical and subtropical region (i.e. Aliwal Shoals to KwaZulu-Natal). There is a relatively low, but steady, participation throughout the year, but with a MASSIVE uptick during the City Nature Challenge (CNC), largely thanks to Cape Town and Durban. I tend to get very bored identifying this fauna between April and May, when I’m hit with a large volume of the same 4 species.
4.65 observations per observer / 16.93 observations per identifier
Actiniidae : Actiniaria ratio = 83.5%

New Zealand (14 species; subtropical): Very consistent participation, at roughly twice the volume as South Africa, with minimal seasonal variation and a minor uptick during CNC.
5.82 observations per observer / 24.67 observations per identifier
Actiniidae : Actiniaria ratio = 73.6%

Southern Australia (16 species; tropical & subtropical): Similar to New Zealand, but without any bump from CNC.
4.25 observations per observer / 16.11 observations per identifier
Actiniidae : Actiniaria ratio = 79.4%

Japan (23 species; tropical, subtropical, & temperate): This data also includes South Korea, as well as the tropical fauna of the Ryukyu Islands, minus 2 observations from Hokkaido that didn’t fit within the map box. Minimal participation throughout the year. CNC is a non-factor.
2.98 observations per observer / 9.94 observations per identifier
Actiniidae : Actiniaria ratio = 66.7%

SE USA (5+ species; tropical & subtropical): Texas to North Carolina, with a few Bahamian observations that snuck into the map box. Participation is minimal throughout the year—slightly more than Japan, but about half the volume as South Africa.
1.69 observations per observer / 5.06 observations per identifier
Actiniidae : Actiniaria ratio = 45.4%

Europe (22 species; subtropical & temperate): This is the Northeast Atlantic and Mediterranean, minus Iceland and the Canary Islands. Participation is similar in volume to New Zealand and Australia during their respective winters, but MUCH higher during the summer. Europe clearly likes to holiday.
2.60 observations per observer / 13.95 observations per identifier
Actiniidae : Actiniaria ratio = 73.2%

West USA (17 species; subtropical & temperate): Vancouver to San Diego. Y’all are really into sea anemones over there. There’s little seasonal variation, and no obvious CNC bump, but the CalCoast BioBlitz is evident in June. Does that sound correct? Honestly, very surprised that the CNC bump doesn’t show itself here.
3.55 observations per observer / 27.48 observations per identifier
Actiniidae : Actiniaria ratio = 89.8%

India (11 species; tropical): Most observations (91%) are from a relatively small stretch of the coast, between Mumbai and Goa, and another 4.6% by Visakhapatnam on the east coast. The geographical breakdown is intriguing, as Kolkata and Chennai are poorly represented. The former sits on a river delta, which is poor habitat for actiniids. I’m not sure if there’s an ecological explanation for Chennai. Monsoon season is April-September, which is clearly evident in the data, but otherwise the volume is moderate, similar to South Africa, without any CNC bump.
6.82 observations per observer / 14.10 observations per identifier
Actiniidae : Actiniaria ratio = 80.9%

Singapore (9 species; tropical): Participation is, per capita, quite high, and nearly equal to (88%) Japan’s total volume. Climate plays a notable factor. During the cooler, drier periods associated with the Southwest Monsoon, observations increase. The downturn in October is also associated with a warming intermonsoonal period. CNC doesn’t see an uptick, likely due to the inopportune weather.
3.95 observations per observer / 12.23 observations per identifier
Actiniidae : Actiniaria ratio = 15.2%

Hong Kong (14 species; tropical & subtropical): Participation is, per capita, quite high, and nearly double (178%) Japan’s total volume. During their relatively mild winter, observations decrease noticeably, and there’s a slight uptick during CNC.
3.46 observations per observer / 8.17 observations per identifier
Actiniidae : Actiniaria ratio = 46.4%

New Caledonia (8 species; tropical): Participation is, per capita, quite high, and nearly equal to (91%) Japan’s total volume. In fact, if we factor in all of Actiniaria, New Caledonia has 227% as many observations as Japan and South Korea combined, despite having just 1.5% of their population! There’s little indication of any seasonal or CNC variation.
7.45 observations per observer / 20.5 observations per identifier
Actiniidae : Actiniaria ratio = 26.8%


I began this investigation with a couple major questions.

  1. Does the enormous drop in observations seen in South Africa following the City Nature Challenge occur in any other region?
    Answer: nope
  2. How does the volume of observations from Japan compare to other regions?
    Answer: surprisingly low.

But in putting together this data, I was struck with some other curious findings…

  • Europeans love to photograph sea anemones during their summer holiday. Seriously, you don’t see this trend anywhere else. Is this what they do in Europe on their holiday?

  • Also interesting is how different the total volume of observations is when comparing the Western and Southeastern USA. Clearly this relates to the abundance and diversity of species in these regions. Coastal ecosystems are replete with sea anemones out West, but they’re mostly a non-factor in the Southeast. But if we compare the Southeast to other corners of the globe, I think there’s a general lack of participation, particularly if we exclude the tropical Florida fauna from this equation. Coastal ecosystems of the Southeast are still very poorly represented on iNaturalist.

  • I want to give a special shoutout to the Indian iNaturalists (particularly @pradip @shaunak @ajamalabad), who’ve done such an exemplary job of documenting their fauna, both in terms of quantity and quality of their observations. Plenty of interesting discoveries have been made here, including some major range expansions documented for the first time. It’d be great if all coastal ecosystems had such a dedicated community of naturalists.

  • There are some ecological peculiarities revealed when comparing the faunal ratio of the family Actiniidae to the order Actiniaria. This family is generally the dominant sea anemone group in intertidal habitats, especially in cooler regions, but the data shown here is also artificially influenced by the current (and largely outdated) system of classification used for this group. For instance, in Singapore, this ratio was a meager 15%, but this is largely due to the fact that some of the more common actiniid-like anemones (e.g. Heteractis, Stichodactyla, Phymanthus) have been questionably placed into separate families, all closely related. If we factor this in by comparing the superfamily:order ratio, we get a more typical 77.2%. This is true throughout the tropics, which is why we see a similar phenomenon in New Caledonia, but there’s also a sociological factor at play. In India, where most observations are from tidepooling (rather than scuba or snorkel), the observations skew away from certain non-actiniids found at greater depths.

  • Another special mention is warranted for the small but dedicated group of iNaturalists in New Caledonia (particularly @damienbr @pl_stenger @juju98 @clairegoiran @jeanro), who have done such a tremendous job documenting their coral communities. The average of ~8 observations per observer puts to shame most other regions. Compare this to the 1.69 average seen in the Southeast USA, where the #2 most prolific observer trails the #8 most prolific observer in New Caledonia, despite that region having only 51% as many observations. Though to be fair, the fauna is significantly less diverse in the Southeast.

  • I’d be curious to see a modal breakdown of observations per identifier for these regions, but I’m far too lazy to crunch the numbers. I suspect the mean data shown here is largely influenced by a very small number of users, but this number probably varies based on the user base of each region. Myself and @phelsumas4life are by far the most prolific in this respect, accounting for nearly all identifications in some areas (looking at you, SE USA). One way to compare the relative quality and efficacy of the local identifiers is to see how many have accounted for at least 5% of their local identifications (i.e. dividing user identifications by total observations). For the purposes of this, I’ll expand the data to include all of Actiniaria: South Africa (5/209) / New Zealand (7/181) / Australia (6/301) / Japan (3/31) / SE USA (2/168) / Europe (4/750) / West USA (4/1907) / India (4/89) / Singapore (4/39) / Hong Kong (4/60) / New Caledonia (3/19). Thus, of the 209 users who have identified South African sea anemones, only 5 have been active enough to identify at least 5% of the total number of South African sea anemone observations. The only regions where I’m not the leading identifier are the West Coast (ranked a pedestrian #13) and New Zealand (#2, behind @tony_wills).

  • Lastly, Japan. I’ve always been perplexed by the relative lack of iNaturalist engagement from this country. One could be forgiven for thinking there might be some sociological explanation for this—maybe the Japanese simply don’t interact much with nature? But nothing could be further from the truth. Japanese divers are some of the most prolific in the world at documenting their marinelife, they simply don’t do it here. It’d be great to see this participation rate improve through a bit more outreach. I really can’t emphasize enough how anomalous this is.

Anywho, that’s all I’ve got. Hope you enjoyed the data, nerds. I’d love to see more analysis like this from the statisticians at iNaturalist.


We need more identifiers. I am still chewing thru a backlog for Rest of Africa, not only from CNC. We had the Great Southern Bioblitz too (that would be your October bump for South Africa)

I mean, a lot of Europeans will visit the shores (north and south) in summer. July and August are school holidays. But in winter even the mediterranian shores do not have so many visitors - even less of them will dive into the sea (e.g. Italys waters will be at around 10-15 °C or even lower in winter). I think it is a very understandable trend.


Nice analysis! One (fairly obvious point) is that you would expect to see a much larger seasonal variation in areas that have more extreme winters.

There’s little seasonal variation, and no obvious CNC bump, but the CalCoast BioBlitz is evident in June. Does that sound correct? Honestly, very surprised that the CNC bump doesn’t show itself here.

I believe that. My impression is that CNC is not as big a thing here as much (any more) and that the tidepool community tends to be more focused on tide related events. Be interesting to see if this carries across non-marine observations…

Could habitat vs where people tend to be make a difference for SE US? I’ve vacationed on the coast down there much of my life and at least for the area I have in mind I’d have to go looking pretty carefully since they seem to all be small and drab, and hard structure accessible to the public is pretty much just pier pilings and jetty rocks.

In a broader sense, if I wanted to extensively survey species on “my” beach in the southeast, I think I’d need a cast net, a shovel and sieve for sand dwellers, and a lot of miles walked for shells. That’s quite a bit of planning and work compared to other modes of collecting observations, say taking photos as one hikes. reckon in coastal areas with tidepool or rock habitat taking observations there is much more like going on a hike.


That is certainly a major factor. The West Coast sea anemones are mostly larger, showier species associated with tidepools. The Gulf Coast and Southeastern US sea anemones are primarily small, drab species favoring soft sediments or manmade structures.

In terms of species-level biodiversity, Southern California, Northern California, and the Pacific Northwest (each distinct marine ecoregions) each have 25–30 coastal species. The Gulf Coast and the Southeast each have ~10 coastal species. But if we break that down into total observations…

Southern California: 13,034
Northern California: 23,210
Pacific Northwest: 19,999

Gulf Coast: 154
Southeast: 464

Vastly different. We can also directly compare one introduced species common to both oceans, Exaiptasia diaphana, which is very much a drab, inconspicuous species. It has 179 observations in Southern California, versus 6 & 14 observations in the Gulf and Southeast, respectively.


Do you suggest anyone to get into European waters in winter or spring? In fall sea is still warm (till late October maybe), but most people get to shores in summer. In winter Black sea can be +8, without a suit noody would come in it for anemones or anything, it’s really cold. Plus all of the participation is linked to certain people, not everyone can photograph an underwater organism, so amount of users you’re looking at is limited, and some of prolific ones definitely have life and they can have more or less interest and ability to observe one specific group.
June-August are school holidays, so that to plays its part, and as most people can’t afford going to tropical islands, at least not each year, on their job holidays they will travel the closest to the sea/ocean they can.

My guess is that the social dynamics of “going to the sea shore” have a lot to do with it. Lots of coastal N. California lacks the kind of beach where the whole family can safely swim/paddle and build sand castles, etc. so the people visiting are visiting for the nature, and more likely equipped to photograph it.
The people on the US Atlantic beaches are likely more traditional vacationers, and they’re arriving as a family group and paying for parking next to a sandy beach. If you do not want the sandy beach, you are sad vacationing there in summer time, but you might watch birds in winter.
The people on the European beaches are vacationers, but their beach spots are more general vacation spots, and more walkable, so the member of the family who wants to photograph nature rather than sit on the beach can easily break away and do so.

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I sometimes meet people who started as enthusiastic and well-equipped photographers, then grew an interest in nature that starts with “nature as subject matter” (but of course can develop well beyond that). My perception is that a lot of the people who fit that template are German, and in some cases you may be looking at a map of “where Germans go on holiday.”

Americans with their lesser amounts of vacation time and steeper income inequality seem to have different travel priorities.


i suspect this explains much of it. the areas that joe_fish has delineated as Pacific Northwest, Northern California, and Southern California include huge human populations more or less right next to the ocean, with easy access to other spots along the ocean via the Pacific Coast Highway and other routes.

what is defined as the Gulf Coast doesn’t have nearly as many humans right next to the Gulf. Houston is probably the biggest city along the Gulf Coast, and i have to drive more than an hour (if the traffic is good) to get onto a barrier island and to a location that gives me access to the Gulf of Mexico. New Orleans is a big city right next to the water, but i think it’s mostly river delta nearby. then you get over to Alabama and the Florida Panhandle, and i feel like you have you get yourself onto the Gulf side of a barrier island in this area to have a reasonable chance at finding anemones.

in my few forays along the Gulf Coast, i don’t recall coming across anything that is like a tidal pool. the only anemones i’ve come across were washed up on the beach. they looked like nondescript brown globs stuck on shells and only revealed their brightly colored tentacles when i took one down to the water.


I don’t know much about anemones, but I’m pinning an imaginary medal on joe_fish for the title of this topic. Well done!


This is my anemone (it means wind flower)

Sea anemone comes second

Why more Sea Anemone observations on the west coast than the east? Lots of reasons!

In Oregon, and I suspect the other west coast states, the area between the tides is public, no matter what the ownership of the adjacent land above the tides. In eastern states, the intertidal can be private.

Western shores include a LOT of rocky land. The North American tectonic plates are moving west and have plowed up what were volcanic islands and pushed up coastal mountains. Rocky shores have tide pools and conspicuous anemones. Also, lava from what it now the Yellowstone Hotspot flowed all the way to the coast when the hotspot was in eastern Oregon. Eastern North America is not particular volcanic. It’s shores run to sandy beaches, salt marshes, etc., but not such large an area of rocks. Sea Anemones are larger and more colorful on rocks than in sand, and probably more numerous.

The gulf stream keeps the eastern seaboard a lot warmer than the west, where we’re influenced by currents coming down from the Arctic. Swimming in the ocean by Oregon for more than about 10 minutes can kill you. We love to go to the coast, walk the shores, and observe nature, but we don’t sit sunbathing on the sand.

So, different anemones, different terrain, different temperatures, different laws, different relationships with nature at the shore. Nice to see the clear impact on anemone observation rates!


Interesting! Just started to include low-tide observations from this area (north coast Occidental Mindoro, Philippines). Such opportunities depend very much on the weather and time of tides. There may be an extremely low tide (-0.40m) but impossible to go out because the surf is too rough. A few times per year, we are lucky and then will spend hours exploring - see the photo below.


Which doesn’t stop owners from enforcing their claims to it.

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It seems that folks may not always recognize anemones at low tide. Last year, I was walking along the packed sand line out to the main tide pools during a minus tide. A woman walked back towards me shaking her head, saying sadly, ‘it’s dead. It’s all dead out there. “.

I was taken aback as we were literally standing among a large colony of thousands sea anemones. So, I showed her the anemones, which were mostly closed and looked like green lumps ( they were above the tide line). She just had not known what to look for. I passed a long a few pointers, and she continued along the shore, now with a smile on her face taking pictures of the anemones and other tidal creatures.


In Oregon, it causes immediate push-back if the adjacent landowner tries to claim it. Of course, he can prevent movement over his property to shore. Oregonians are very tough about claiming our public shoreline.


When I was a (failed) zoology student my chosen project was recolonisation on a rock on Camps Bay beach. Cold Atlantic Ocean. Project failed because I didn’t realise that despite carefully going at low tide each week … I still couldn’t access my rock later in the year, against the waves!


On the whole, that pattern holds. It is intriguing, though, that if you go far enough north – New England and the Canadian Maritimes – then you can find shores as rocky as those on the West Coast. Given that these, too, are on the westward-moving plate, it is less straighforward to understand how they formed.


Another factor here in viewing attached invertebrates inter tidally is that at least where I live, the Salish Sea and Pacific surrounds, seasonality plays a large part of enjoyment. From May onward to August it becomes increasingly harder to see what is buried under the Fucoids. I much prefer exploration in the winter months at night with a flashlight/headlamp and low tides or better yet spring tides. We get mixed semidiurnal tides which differs from just the semidiurnal tides that the Atlantic Coast gets.