Elysia Survey Results in Bahia de los Angeles

 
Map of locations surveyed, indicating those at which Elysia or potentially suitable habitat were found during 2016 and 2018 field seasons.

Here’s the last bit from the work in Bahia de los Angeles this past summer, combined with a little data from previous summers. Along with the molecular assays to determine their food plant, and feeding assays to figure out whether being in the light makes slugs taste bad, we have been looking for slugs at various places around the bay.  Although searches were as thorough as possible, it is not only possible, but likely, that we missed some slugs.  At the field station, where sampling was the most exhaustive, surveys of the same areas could turn up no slugs one day and more than a dozen the next.

The quick summary is that Codium, which we have confirmed as one of the slugs’ food plants, can be found throughout the bay.  Some sites have more than others, and each site may have a different collection of Codium species, but the presence of Codium does not seem to be the limiting factor in the presence or absence of Elysia.

Field Stations: Casa Caguama and Vermillion Sea

Bay in front of the Vermillion Sea and Casa Caguama field stations. Left: view north from from Casa Caguama. The red roof and whale skeleton at the Vermillion Sea station can be seen in the background.  Center: Gorgonian and Padina on rocky bottom in front of the Vermillion Sea station.  Right: Codium in front of Casa Caguama station.

As noted above, the intertidal and subtidal zones in front of the field stations were sampled the most thoroughly.  I encountered my very first E. diomedea in front of the Vermillion Sea station, back when Ocean Discovery was using it as a base.  In subsequent years (2014 – 2018), we have been using the Casa Caguama station, just a few doors down.  We have performed dozens of surveys, involving many hours of snorkeling, and have found the slugs’ abundance to vary considerably from day to day.  A few generalizations may be possible, for example that they tend to cluster around growths of Codium in the area just north of the station, but the variation is difficult to explain.  Dozens can be found in one hour of snorkeling one day, and none the next, even though location, time, and tide appear identical.  They are generally difficult to find in tide pools at low tide, except when they are plentiful.   Our experience at the field stations, where we have had the opportunity for repeated and prolonged sampling, indicates that slugs will probably turn up at locations with suitable habitat, even if we have not found them yet.   

Cabeza de Caballo

Cabeza de Caballo (large island to left) and Gemelito Oeste (small white island at right).  Taken from Casa Caguama field station July 8, 2016

Cabeza de Caballo, or simply “Cabeza,” is one of the most prominent and easily identifiable islands in the bay.  We performed one survey at Cabeza  in the summer of 2016.  The rocky bottom at the north end of the island is covered with a deep growth of the brown alga Padina, fed by significant guano production from birds on the cliffs above.  The middle portion is more sparsely vegetated, but there are small growths of Codium.  We did not find any slugs at any location on the island, however.    

Coronadito

Coronadito.  Left: a student in the 2016 Photobiology group explores a large growth of Sargassum.  Middle photo: typical rocky bottom, with hydroids (bushy, lower left), gorgonians (purple, branching, center), and diverse algae species.  Right: a portion of sandy bottom, with loose colonies of red algae forming “rhodoliths.”

Coronadito, at the far northern end of the bay, is a favorite snorkeling spot.  Although we have often visited the site during field trips over the years, we only systematically surveyed it in summer of 2016. There were small growths of Codium in rocky areas, but it was not particularly abundant.  No slugs were found.

Gemelito Este

Gemelito Este.  Right: the east end of the island, seen from the water.  Middle: the bottom is covered by a diverse mix of brown and green algae.  Left: a large gorgonian among the algae.

The Gemelitos, or twins, comprise a pair of small islands just south of Cabeza de Caballo.  There is a particularly nice spot for skorkeling at the east end of the island.  The bottom is entirely rocky, and covered in a diverse mix of algae.  During the first survey in 2016, no slugs were found, but the group had better luck in 2018.  

La Mona

La Mona.  Left: view of the shore from the water, looking south toward the beach at El Rincon.  Middle: Rocky bottom, covered with a turf of mixed algae.  Right: Promising Elysia habitat, consisting of Codium in a cleft in the rock.  

La Mona is a small inlet on the side of the bay north of El Rincon, opposite the town of Bahia de los Angeles.  The rocky shore slopes down to a sandy bottom.  During a survey in 2016, we found plenty of Codium, and one slug was kicked up by a student’s fin.  

Playa la Gringa

Playa la Gringa.  Left: 2018 Photobiology students before the survey.  Middle: Bottom with mixed algae.  Right: Large colony of Codium.  

Playa La Gringa is in the area surveyed originally by Bertsch from 1992 to 2001.  He found that E. diomedea was the most abundant sea slug at Playa la Gringa and the nearby site of Cuevitas.  By the end of his study, the abundance of Elysia had dropped precipitously and inexplicably.  During our surveys in 2016 and 2018, we found plentiful life on the sea floor, including Codium, but no sign of Elysia.  Because the site is easily accessible and, in fact, a beautiful place to snorkel or dive, it will be worth sampling it in the future.  

Other Sites

In 2018, a student found 11 E. diomedea during a “beach fun” day at Hotel Villa Bahia, which is north of town.  No photos, unfortunately.

At the island of La Ventana, there was a lush growth of Codium all around the south end of the island in 2018, but no Elysia.