Stuck belly up on a coral reef: a sea urchin’s nightmare becomes reality 

by post-doc Noelle Lucey

Published this week in Frontiers in Marine Science:  

click here to see the article

Oceans are warming, becoming more acidic and devoid of oxygen. We wanted to know how hot, hypoxic and acidic one of the most exposed, well-flushed, active coral reefs in Bocas del Toro, Panama got last year. Temperatures reached 32.7°C (or 90.9°F), which would make most of us start sweating or look for air conditioning. When temperatures were at their highest, both oxygen and pH (a measure of acidity), were at their lowest. This was particularly evident at night, when pH and DO plunged to startlingly low levels. While severe, at least on the reef, these conditions were fleeting, not lasting more than four hours at a time. This relationship between temperature, oxygen, and pH wasn’t just found on the reef, but throughout the larger surrounding area. 

The reef at Hospital Point where we conducted
the study

But do the severe conditions that occur on the reef, all at the same time, have any negative impact on animals that live in them- even in short 2-hour doses? The well-exposed coral reef we measured the extreme conditions on is also one of the best places to collect the sea urchin, Echinometra lucunter. On first glance this is just a black, spiny sea urchin that is far to common to even consider putting your foot down on the reef. If the surf isn’t too rough, you can notice that they are hiding in every crevice and reef crack, happily getting pounded by the waves. When the sunlight hits them perfectly, they shimmer with bright purple and red colors.

We pried these sea urchins out of their hiding places with teaspoons and brought them the Bocas Research Station to expose them in the laboratory to the same severe hot, hypoxic and acidic conditions they felt on the reef for two hours. Once they were exposed to these different stressors, both singularly and together, we flipped them upside down. Normally, when a sea urchin is flipped on its back uses its spines and tube feet to immediately turn itself back over. It’s a sign of healthy, happy individual. If they don’t right themselves quickly, they might get beaten up by the surf or eaten by reef fish, or both. Finding their hiding spot and wedging into the reef will help them stay alive. 

Eileen sorts animals and puts the "babies" back
into the water (because we only did
experiments with adults) 

It was much harder for urchins to right themselves after being in deoxygenated water for two hours. While they lay upside-down, some with limp tube feet, most survived and were returned to the reef. We repeated this experiment a second time, after keeping them in the lab for a week at different pH, and again found no negative effect of low pH. However, urchins were incapable of finding their footing and flipping right-side up after low oxygen conditions, as well as hot temperatures.  

 

Surprisingly, it appears that even the tropical coastal environments that get plenty of wave action experience severe conditions, and these are pushing the limits of marine organisms that live in them. The lack of oxygen and high temperatures are instant bad news for reef animals. Not to mention, these severe conditions are occurring more frequently and for longer periods of time, upping the stress levels experienced by important reef organisms like sea urchins. 

This work would not have been possible without the support and funding from the Smithsonian Tropical Research Institution, the Bocas del Toro Research Station and MarineGEO.

Lucey, N., Haskett, E., Collin R. 2020. “Multi-stressor extremes found on a tropical coral reef impair performance.” Frontiers in Marine Science. DOI: 10.3389/fmars.2020.588764

Noelle Lucey, Antonia Hergwig (a STRI volunteer), and
Eileen Haskett head back to the Bocas Station after
collecting animals

The identity of this unusual sea creature was a mystery to biologists for 30 years.

The key to figuring it out was to think globally, not locally.

Mystery has surrounded these tiny animals since they were first discovered in the Gulf Stream.  Easily identified as sea star larvae (i.e., babies) they are some of the most abundant animals in certain plankton samples. Not only are samples “crawling” with them, but they show evidence of cloning themselves, something that was previously not known to occur in sea star larvae.  Using DNA sequences scientists have searched for 30 years for a match between these distinctive larvae and adult sea stars from the Gulf of Mexico and the Caribbean.

We collected some of the mystery larvae as part of project to describe and identify the larvae living in Panamanian waters. Like the prince with Cinderella’s slipper, we initially could not find a match to our DNA sequences either.  Using the Barcode of Life Database, we discovered a match to an unpublished sequence from the Indo-Pacific. No information other than the name of the scientist who generated the sequence was public, Gustav Paulay. Luckily (in this case anyway), the community of biologists working on marine invertebrates is small and I knew Gustav. After comparing the sequences, we agreed that the sequences from the Caribbean larvae match the sequences of a rarely seen sea star from the Indo-West Pacific, Valvaster striatus. 

This sea star appears to range across almost all of the Pacific, but it lives deeply hidden inside coral reefs, making them difficult to find without literally taking the reef apart.  

Adults have never been reported in the Caribbean, but they must presumably be lurking here, out of sight, shedding eggs and sperm into the water column to generate these famous larvae…. Or… maybe the larvae are cloning themselves, maintaining dense populations without the need for adults.

 

Only time (and more research) will tell.

Read our original publication at:  https://www.journals.uchicago.edu/doi/10.1086/710796

Collin, R., D. E. Venera-Pontón, G. Paulay, and M. J. Boyle.  2020.  World Travelers: DNA Barcoding Unmasks the Origin of Cloning Asteroid Larvae from the Caribbean.  Biological Bulletin.  doi/10.1086/710796

Do Slipper Snails Really Brood in the Mantle Cavity

Slipper snails brood their offspring.  They produce eggs enclosed in transparent capsules and they keep these covered by the shell.  The scientific literature is full of statements that they "brood in the mantle cavity".  This is not accurate.

What is the mantle cavity?
The mantle is characteristic of molluscs and is basically a skirt of tissue formed by the dorsal body wall  that covers the visceral mass.  In squid it is the part that is used as squid rings.  In clams and snails it is the tissue that underlies the shell.

Head-on view of a Crepidula

The mantle cavity is defined as the space enclosed by the mantle and  includes the gills, anus, osphradium and gonopores.  In slipper snails this space is large, to contain the extensive gills needed for filter feeding.  It extends from the front margin of the shell, over the head, and gradually tapers all the way to the posterior end.

Slipper snails do not brood their egg capsules in this space.

Crepidula atrasolea brooding.  The eggs are orange and
can be seen through the plastic the snail has attached to

Where do slipper snails keep their eggs?
In the CollinLab we keep slipper snails in plastic cups.  In this way we can see when they produce eggs and we can collect the embryos or larvae at the age or stage we need.  Looking a the snails in this way it is very clear that the egg capsules are deposited under the snail, not above the head in the mantle cavity, but below the head.  The mother attaches the stalks of the capsules to the substrate beneath her neck.  As far as we know there is not formal anatomical name for this space.  In publications we say that slipper snails brood the egg capsules "between the substrate, the neck and the propodium".

Head-on view showing the location of the eggs relative to the mantle cavity

Lateral view showing the eggs relative to the mantle cavity