Voyage to the Ridge 2022

May 14 - September 2, 2022

A Rarely Seen Sea Urchin Aggregation

During Dive 08 of the third expedition of Voyage to the Ridge 2022, remotely operated vehicle (ROV) Deep Discoverer (D2) was operating in deep waters near St. Croix when it recorded a rarely seen event — a large gathering of the sea urchin Conolampas sigsbei! At a depth of 411 or so meters (1,348 feet), pilots operating the ROV, along with scientists participating in the dive from both sea and shore, and members of the public joining the livestream, encountered a HUGE “crowd” of more than 35 individual Conolampas sigsbei on the seafloor, in what was most likely a mating aggregation!

On August 30, 2022, during Dive 08 of the third Voyage to the Ridge 2022 expedition, scientists were thrilled to come across a rarely seen aggregation of Conolampas sigsbei sea urchins. Video courtesy of NOAA Ocean Exploration, Voyage to the Ridge 2022. Download largest version (mp4, 168 MB).

Urchins reproduce via spawning, which entails emitting their sperm and eggs into the water and allowing the gametes to fertilize in the water column. There is generally no direct contact between the male and female urchins. However, in order to enhance the chances of fertilization, many species of urchins gather in groups, likely for sustained periods of time. Based on our observations, this aggregation included at least 35 individuals spread out over a distance of several meters.

Conolampas is a bit unusual for sea urchins. If one imagines an hemisphere-shaped animal that resembles a steamed bun plowing through the sand of the deep, that’s pretty much what Conolampas looks like! Conolampas belongs to a group known as the “irregular” urchins and has spines which are almost hair-like, much shorter than the shallow-water urchins with which one might be familiar.

This aggregation of sea urchins, with at least 35 individuals, was a rarely observed phenomenon seen during Dive 08 of the third Voyage to the Ridge 2022 expedition.
This aggregation of sea urchins, with at least 35 individuals, was a rarely observed phenomenon seen during Dive 08 of the third Voyage to the Ridge 2022 expedition. Image courtesy of NOAA Ocean Exploration, Voyage to the Ridge 2022. Download largest version (jpg, 1.1 MB).
Three individuals of a large group of sea urchins seen during Dive 08 of the third Voyage to the Ridge 2022 expedition. Note the debris “hats” directly on top of the body. Scientists commented that since each individual had debris in exactly this place, it was too consistent to be incidental.
Three individuals of a large group of sea urchins seen during Dive 08 of the third Voyage to the Ridge 2022 expedition. Note the debris “hats” directly on top of the body. Scientists commented that since each individual had debris in exactly this place, it was too consistent to be incidental. Image courtesy of NOAA Ocean Exploration, Voyage to the Ridge 2022. Download largest version (jpg, 1.3 MB).

This species lives on sandy bottoms in the Bahamian region at a depth of approximately 277-540 meters (909-1,772 feet). These animals live by digging and plowing through sand and sediment, leaving very distinctive, meandering trails behind them. The presence of MANY of these trails in the sand evidenced the movement of this group over the area!

Perhaps the most unusual aspect of this urchin social “meet n’ greet” is the fact that all of the observed urchins displayed some kind of debris on the apex of its body — a sort of ocean debris “hat”!! This seems to be a result of “collecting,” an unusual behavior among urchins which involves gathering and collecting debris, including rocks, shells, and other materials, and bringing them up to a specific place on the animal’s surface — in this case, specifically on the top of the body, reminiscent of a hat.

An image of a rarely observed gathering of the deep-sea urchin Conolampas sigsbei, as seen during Dive 08 of the third Voyage to the Ridge 2022 expedition. The black-and-white photo was edited with increased contrast to highlight the trails the sea urchins have made in the soft sediment.
An image of a rarely observed gathering of the deep-sea urchin Conolampas sigsbei, as seen during Dive 08 of the third Voyage to the Ridge 2022 expedition. The black-and-white photo was edited with increased contrast to highlight the trails the sea urchins have made in the soft sediment. Image courtesy of NOAA Ocean Exploration, Voyage to the Ridge 2022. Download largest version (jpg, 2.3 MB).

This urchin “meet n’ greet” was a singular and rarely seen event. There have been observations of shallow-water urchins aggregating to spawn, but for urchins which live at 411 meters depth, well below sunlit waters? This was observation of behavior previously limited to the domain of a very small number of scientists during an age when video was not as high quality, convenient, or dependable as it is today.

This event begs many questions! Do the urchins stay together after spawning is over? How long does the urchin group stay together? Are the urchins actually there for spawning? Is food also a factor? Do the movement tracks in the sand show any deliberate patterns?? And perhaps most interestingly… WHAT IS the deal with those little debris “hats” and why are all the urchins wearing them???

A view of a sea urchin group seen during Dive 08 of the third Voyage to the Ridge 2022 expedition.
A view of a sea urchin group seen during Dive 08 of the third Voyage to the Ridge 2022 expedition. Image courtesy of NOAA Ocean Exploration, Voyage to the Ridge 2022. Download largest version (jpg, 643 KB).
Remotely operated vehicle Deep Discoverer is seen approaching a group of sea urchins during Dive 08 of the third Voyage to the Ridge 2022 expedition. The small dots in front of the vehicles are sea urchins of the species Conolampas sigsbei.
Remotely operated vehicle Deep Discoverer is seen approaching a group of sea urchins during Dive 08 of the third Voyage to the Ridge 2022 expedition. The small dots in front of the vehicles are sea urchins of the species Conolampas sigsbei. Image courtesy of NOAA Ocean Exploration, Voyage to the Ridge 2022. Download largest version (jpg, 6490 KB).

References

Pawson, D.L. & Pawson, D.J. (2013). Bathyal sea urchins of the Bahamas, with notes on covering behavior in deep sea echinoids (Echinodermata: Echinoidea). Deep Sea Research Part II: Topical Studies in Oceanography, Volume 92, pages 207-213.
https://www.sciencedirect.com/science/article/pii/S0967064513000428

Young, C., Eckelbarger, K. J., & Eckelbarger, K. (1994). Reproduction, Larval Biology, and Recruitment of the Deep-sea Benthos. Columbia University Press.

By Chris Mah, National Museum of Natural History
Published September 1, 2022