tritonia report

BBP SPECIAL PUBLICATION 2006-003

Population outbreak of the Tritonia hamnerorum nudibranch,

an obligate grazer on the purple sea fan Gorgonia ventalina,

across Bermuda’s northern reefs in 2005

Thaddeus J.T. Murdoch

BREAM: Bermuda Reef Ecosystem Assessment

and Mapping Programme

Bermuda Biodiversity Project

May 8, 2006

Fig. 1: A photograph of a Tritonia hamnerorum nudibranch (Size = 15 mm).

BREAM Tritonia hamnerorum

TABLE OF CONTENTS

Executive Summary……………………………………………...2

Introduction………………………………………………………3

Discovery………………………………………………………….3

Regional Assessment……………………………………………...6

Conclusion of outbreak…………………………………………10

Sea fan mortality due to nudibranch predation……………….11

Sea fan distribution and population structure………………...13

Discussion and Conclusions…………………………………….18

Acknowledgements…………………………………………….. 19

References……………………………………………………….20

Appendix 1………………………………………………………21

Appendix 2………………………………………………………22

1


Population outbreak of the Tritonia hamnerorum nudibranch,

an obligate grazer of Gorgonia ventalina sea fans,

across Bermuda’s northern reefs in 2005

EXECUTIVE SUMMARY

On July 14th 2005 a large population of the nudibranch Tritonia hamnerorum was found to be at infestation levels on a collection of patch reefs on the Devil’s Flat reef complex. This species was never before recorded in Bermuda.

The nudibranch numbered in the many hundreds on some sea fan colonies, and was feeding on the sea fans, which caused damage and death to many of these important sessile animals.

Surveys carried out over the course of the summer determined that the outbreak was patchily located over a ~20 km2 area along the Northern rim and outer patch reefs of the North Lagoon, but not observed nearer to shore, nor along the Western, Southern nor Eastern rim reefs.

Highest levels of the outbreak occurred at the buoyed Marine Protected Area at Eastern Blue Cut (EBC), and across Devil’s Flat (DF), causing mortality to roughly half of the sea fans on some areas.

Re-assessment on November 18th, 2005 at the EBC and DF outbreak sites found that the populations were no longer present, presumably due to the colder temperatures prevalent as winter approached.

The BBP team will watch out for another outbreak of the nudibranch in the early summer of 2006, when sea water temperatures increase again.

Population assessments of sea fans indicate that these soft corals are most abundant in shallow outer reef areas, which may account for the high densities of the nudibranchs in the same area.

2


Introduction

On July 14th, 2005, a large population of the nudibranch Tritonia hamnerorum Gosliner and

Cheselin 1987 (Fig 1, Appendix 1) was discovered on the Bermuda reef platform. T.

hamnerorum is a small, white or beige, dendronotid nudibranch that feeds exclusively on the

purple sea fan Gorgonia ventilina. While the nudibranch has been recorded in the Caribbean

since 1987, this report represents the first definitive record for the species in Bermuda..

The presence of T. hamnerorum in Bermuda is of concern, as high densities the nudibranchs

have been observed to rapidly remove tissue and cause the partial or total mortality of sea fans in

the Florida Keys (Cronin et al. 1995). Often filamentous algae then colonize the dead sea fan

skeleton. T. hamnerorum has been observed previously across the northern Caribbean, in

Honduras, the Cayman Islands, the Bahamas and the Florida Keys. In Florida it was found that

the organism is capable of reaching the very high densities on individual sea fans of over 1700

individuals per colony.

Bermuda is noted for its high abundance of soft coral fauna. Purple sea fans (Gorgonia

ventalina) are one of the predominant forms of sessile biota found in some areas of the Bermuda

reef platform. Sea fans filter the water column for plankton and particulate matter, create critical

habitat for several species, and provide food for the Flamingo Tongue gastropod (Cyphoma

gibbosum; Neal and Pawlik 2002)

Discovery

The presence of the nudibranch was first noticed on July 14th, 2005, by BBP intern Caitlin Lustic

while assisting Thad Murdoch, BREAM Project Coordinator, BBP, in a pilot study on the

assessment of a collection of patch reefs located in the Devils Flat reef complex (Fig 2). It was

observed that many of the purple sea fans on a patch reef were dead or dying and also infested

with the nudibranch. There were large numbers of nudibranchs on many of the healthy sea fans

as well (Fig 3). Subsequent visual assessment of all of the patch reefs within a ~100 m area

confirmed that many sea fans on some patch reefs were also colonized by nudibranchs to a

varying degree, while other patch reefs were apparently devoid of them (Fig 2).

3


Fig 2. An aerial photograph of the Devil’s Flat reef complex, and the location of the first set of

visual surveys of the distribution of the T. hamnerorum nudibranch.

Fig 3. Underwater photographs of infested and dead sea fans at Devil’s Flat.

4


Fig 4. Digital photograph of juvenile nudibranchs on a piece of sea fan, taken under a

microscope. Each nudibranch measures 3 mm in length.

The heavily-affected patch reef site was marked with a buoy and it’s latitude and longitude

recorded [32° 24’ 52.47” N; 64° 50’ 35.73”W]. Also UW digital photographs of sea fans were

taken by TJTM, and samples of adult and juvenile nudibranchs collected. Collected specimens

were subsequently photographed while alive (Figs 1, 4), and then fixed in alcohol and archived

at the Bermuda National History Museum. Soon after the initial discovery of the nudibranch, a

range-extension report was sent in to the “Sea Slug Forum”, which is an online nudibranch

website hosted by the Australian Museum. The report, which is reproduced in Appendix 2, can

be seen at:

http://www.seaslugforum.net/display.cfm?id=14424

5

http://www.seaslugforum.net/display.cfm?id=14424


Regional assessment

In order to ascertain the extent of the distribution of the novel nudibranch species, surveys of

reefs located across the Bermuda reef platform were carried out by snorkel or while on SCUBA

over the rest of the summer 2005. During the snorkel surveys all colonies of sea fan located at

shallow reef sites were visually assessed during a timed 5-minute period for the presence and

relative density of the nudibranch. Sites were ranked on a scale from 0-5, with 5 indicating that

many sea fan colonies were heavily affected by T. hamnerorum, and 0 indicating that no

nudibranchs were observed on any sea fan. Additionally, during the assessment of reef sites for

the 2005 BREAM-AGRRA surveys, the presence and relative density of the nudibranch on sea

fans observed at any time over the hour-long dive was also recorded.

By October 2005 a broad expanse of the reef platform had been assessed for the T. hamnerorum

nudibranch, and the extent of its spread mapped to GIS (Fig 5). The nudibranch appeared to have

been limited to the northern rim and inner set of faros located to the north of the North Shipping

Channel). The western-most sighting occurred at Chub Cut, and the easternmost sighting was 3-

km west of North Rock. None were seen at the North Rock protected area. The largest area of

densely colonized sea fans appeared to have been on Devils Flat, where three separate patches

with high densities were observed.

Interestingly, when the patch reef with high densities was visited during days with winds over

15 kts and choppy sea state, no nudibranchs could be seen on any sea fan. However, when the

patch reef was re-visited on the next calm day sea fans were again re-infested by nudibranchs.

Additionally when one disturbed a sea fan with many nudibranchs on it by shaking or wafting

water past the fan, many nudibranchs fell off and then floated downstream, not sinking, but

suspended in the water column. It appeared that the nudibranchs could be transported across the

sandy areas between patch reefs in this manner. Also it appeared that since they were easily

dislodged by weak water currents, they left the surface of the sea fans on days with rough

weather, and presumably crawled into the reef matrix until the weather improved. However, we

could not find the nudibranchs hiding in the reef on rough days, even on heavily populated patch

reefs.

6


Fig 5. A map of the Bermuda reef platform illustrating the maximum distribution of Tritonia

hamnerorum nudibranchs detected during the summer of 2005. At sites where nudibranch

densities were characterized as “Rare”, less than 100 nudibranchs were observed on less

than 10 sea fan colonies, within a 5 minute search period. Sites characterized as

“Abundant” had greater than 100 nudibranchs and more than 10 sea fans were seen to be

affected within a 5 minute interval.

Of greatest concern was the observation of large numbers of nudibranchs on many sea fans at the

Marine Protected Area at Eastern Blue Cut on July 30th and again on September 10th. The

nudibranch was particularly abundant along the northern face of the sand hole located at the

northern side of the EBC tunnel, in which mooring buoy labeled “A” is seen in Fig 6, below.

Along this area many sea fans were heavily grazed and suffering from partial or total mortality

7


(Fig 7). Fewer nudibranchs were seen at the areas popular with SCUBA divers on the southern

side of the same sand hole or along the wall of reef on the eastern side of the tunnel.

Fig 6. A map of the Eastern Blue Cut Marine Protected Area. Gray areas represent reef substrate

and white areas represent sand-covered sea floor. Labelled circles represent the location

of mooring buoys.

8


Fig 7. High densities of nudibranchs can be seen as white objects on the sea fans in this UW

photograph, taken at Eastern Blue Cut during the apparent peak of the infestation event.

9


Conclusion of outbreak

On November 18th, 2005, sea fans were re-assessed for the nudibranch at both Eastern Blue Cut

and at the patch reef at Devils Flat where they were first discovered in high numbers. No

nudibranchs could be found at either location, even though it was a calm, waveless day. It is felt

that the colder temperatures and rough seas that are characteristic of the autumn season and that

occurred in the weeks prior to the survey contributed in the disappearance of the nudibranch (Fig

8).

Fig 8. Sea surface temperatures for the area near Bermuda during the year 2005. The time series

were extracted from a 50 km wide area monitored by satellite and located less than 1 km

SE of the island. The arrow labeled A indicated the temperature on July 14th, the first day

the presence of the nudibranch was observed, . The arrow labeled B indicates the >3°C

cooler temperatures Bermuda experienced on Nov. 18th. Chart modified from the Coral

Reef Watch SST series char, NOAA:

http://coralreefwatch.noaa.gov/satellite/current/sst_series_bermuda_cur.html

10

http://coralreefwatch.noaa.gov/satellite/current/sst_series_bermuda_cur.html


Sea fan mortality due to nudibranch predation

We assessed sea fan size, as height, nudibranch presence, and relative condition of 15 sea fans on

the heavily infested patch reef in Devil’s Flat by SCUBA (Figs 9, 10). The index of relative

condition was recorded as the relative proportion of the area of each sea fan that appeared dead,

according to six categories (0; <10, <25, <50, <75 and 100% dead).

0

1

2

3

4

10 20 30 40 50 60 70 80 90 100

Size class (height) of sea fan

Sea

fan

co

un

t

Uninfected

Infected

Fig 9. Size class and presence or absence of nudibranchs on 15 large sea fans on the heavily

affected patch reef at Devils Flat.

Of the 15 sea fans examined, five (33%) were infected and 10 (66%) were not. Infected sea fans

tended to be smaller than the average size overall of the sample. However, even sea fans that

were not found to have nudibranchs present at the time of examination did show signs of partial

or total mortality that appeared consistent with grazing scars created by the nudibranch. Only

11


one sea fan show no sign of tissue loss, 80% showed at least 25% tissue loss, and 60% showed

signs of loss of at least half the tissue of the whole sea fan colony. Assuming the source of partial

mortality was the nudibranch, and since no other cause was found, it appears that the Tritonia

nudibranch was having a dramatic effect on these sea fans.

0

1

2

3

4

5

0 10 25 50 75 100

Relative percentof tissue lost

Sea

fan

co

un

t

Fig 10. Relative condition of 15 sea fans located on the heavily infested patch reef at Devil’s

Flat.

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Sea fan distribution and population structure

As the nudibranch appeared to be the source of high levels of mortality to sea fans, we also

examined how sea fan density and population structure varied at intermediate spatial scales

across reefs, and at a larger scale across the North Lagoon of the Bermuda reef platform, so that

we could better understand where sea fans were most abundant and why. These pilot studies

represent the first time that the abundances of sea fan, or any soft corals, have been assessed

across large areas of the reef platform.

Intermediate scale - Sea fan distribution and population structure within faros

The change in relative abundance of different size classes of sea fans relative to distance from

the edge of a faro reef complex was investigated with a pilot study. Faros are large reefs

characterized by a ring of coral and reef substrate surrounding a shallower sandy basin within

which are often scattered smaller patch reefs. Hard and soft coral cover often appears higher on

the outer ring of the faro than on the patch reefs within the faro, even if macroalgae cover is low

and the relative cover of bare substrate is high. Assessment of the relative abundance of sea fans

of different size classes would allow for the first examination of whether sea fan density was

lower within faros, and whether this reduction in density was due to recruitment limitation, or

increased mortality of adults, or both processes.

Four transects were placed at three sites located differing distances from the southern outer edge

of the Crescent Reef faro, located in the North Lagoon (Fig 11). The size and abundance of sea

fans was measured as described in the pilot study above.

The overall abundance per square meter at each site are plotted below in Fig 12. Sea fans were

more abundant at the outer site near open water and least abundant at the inner site which was

close to the center of the faro. Lower abundances within the faro could be due to reduced

recruitment, reduced growth or enhanced mortality due to stress or disturbance. While growth

rates cannot be determined from the size-frequency data, recruitment and mortality can be, as

described below.

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The average abundance of each size class of sea fan at each site is plotted in Fig 13. Recruitment

of sea fans is apparently higher at the outer edge of the faro, and lower within the faro, as

determined by the relative abundance of the smallest size class at each site. However, since

abundance of sea fans peak at the outer site at smaller sizes than at the two inner sites, it appears

mortality is also higher at the outer sites relative to sites within the faro. The central and inner

sites have very similar levels of recruitment and population sizes of the 5-25 cm size class.

However, the inner site has lower densities of the larger 25-50 cm and >50 cm size classes

relative to the middle site, implying higher relative mortality to sea fans of these sizes at the

inner site. This may be due to a reduction in the amount of suspended matter available for

feeding at increased distances from the edge, or to declining water quality closer to the center of

the enclosed body of water within faro. Additional analysis of water quality and growth rates of

each size class at each site would be required to more fully explain which processes are driving

the patterns of abundance of each size class of sea fans across the faro.

Fig 11. Location of the three sites at Crescent faro in the North Lagoon. Sites vary in distance

from the outer edge of the faro, with the “Outer” site located closest to the edge and the

“Inner” site near the center of the faro reef complex.

14


0

1

2

3

4

5

6

7

8

9

10

0 50 100 150 200 250 300 350

Distance from edge (m)

Den

sity of sea fan

s per m

2

Fig 12. Average abundance (+/- SE) of sea fans at three sites on Crescent faro in the North

Lagoon.

0

1

2

3

4

5

6

<5 5 to 25 25 to 50 >50

Size class of sea fans

Se

a f

an

ab

un

da

nc

e m

-2

Outer

Middle

Inner

Fig 13. Average abundance (+/- SE) of sea fans of four different size classes at three sites on

Crescent faro in the North Lagoon

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Large scale - Sea fan density across the North Lagoon

The change in the relative density of sea fans was also assessed from data collected in 2000 with

videographic techniques. Four video transects measuring 25-m long by 40-cm wide were filmed

on the tops of each of six patch reefs separated by 1-3 km and located across the width of the

North Lagoon, from North Shore, Devonshire out to the rim reef near Snake Pit MPA. Each zone

was characterized by a differing degree of sedimentation as measured by water clarity as Secchi

depth. All sea fans visible on each video transect were counted, and the average number of sea

fans per square meter (+/- SE) at each site calculated (Fig 14).

Densities of sea fans were found to be highest at the site roughly 11 km from shore, and to

decline rapidly in density half-way across the platform at between 4 and 6 km from shore. The

two sites nearest to shore, and the southern shipping channel had approximately 10% of the

density of sea fans as the outer sites. Increased mortality nearshore due to declining water quality

closer to shore, or reductions in the rate of recruitment of the planktonic larvae of the sea fans

may be causing this pattern, but require further study. The next section examines whether

differences in the abundance of a range of size-classes of sea fans at a nearshore and offshore site

can illuminate the processes limiting sea fan densities nearshore.

0

1

2

3

4

0 2 4 6 8 10 12

Distance from North Shore (km)

Se

a f

an

co

lon

ies

m-2

Fig 14. Average density (+/- SE) of sea fan colonies at six sites located at increasing distances

from shore across the North Lagoon of the Bermuda reef platform, as determined from

16


counts from 4 videographic transects filmed on the tops of patch reefs located at each

survey site.

Large scale - Sea fan population structure at a nearshore and offshore reefs

Comparisons of relative abundances of different size-classes of sea fans may provide an

indication of the source of the decline in sea fan densities with proximity to shore, discussed

above. In a pilot study we quantified the density of sea fan’s of various sizes at two reef

locations: a patch reef close to shore at a site proximal to Government House, and the southern

outer edge of the faro at Devils Flat which is far from shore. All sea fans within size categories

of: < 5 cm, 5-25 cm, 25-50 cm and > 50 cm tall were counted within 12 transects measuring 10-

m long by 50-cm wide at each reef. Assessing the abundances of sea fans of different sizes

allows for the examination of the structure of the sea fan population and the production of

models of the future persistence of the population.

The results of the population assessment of sea fans is presented in Fig 15. A total of 60 colonies

were counted in 24 transects located over the two sites. Sea fans were much more abundant

offshore at the Devils Flat faro than at the patch reef near Government House. Additionally, at

the Government House patch reef sea fans peaked in density at the middle size classes while the

sea fans at Devils Flat peaked in density at the largest size class. The distribution of sizes of sea

fans at Government House implies low levels of recruitment as well as high levels of disturbance

which limit the abundance of the larger size classes of sea fans. At Devils Flat recruitment was

also fairly low, but large sea fans were abundant, implying low levels of disturbance allow the

soft corals to live for long periods of time in order to reach such large sizes.

17


0.00

1.00

2.00

3.00

4.00

<5 5 - 25 25 - 50 >50

Size Class

Ab

un

da

nc

e m

-2

Govt House

Devil's Flat

Fig 15. Average abundance (+/- SE) of sea fans of four different size classes at a reef near

Government House near North Shore, and at Devil’s Flat which is located far from shore.

Sea fans are more abundant at the Devil’s Flat site for all size classes, and show a

population structure that indicates low levels of mortality. Sea fans at the Government

House site show a decline in the larger size classes relative to the 5 – 25 size class,

indicating high mortality in larger sea fans at this site.

Discussion and Conclusions

T. hamnerorum has been observed in Florida to exhibit very patchy patterns of distribution and

density across areas at scales ranging from centimeters to kilometers (Cronin et al. 1995). The

patterns of distribution we observed across the Bermuda platform are similarly of a very patchy

nature. Additionally, all individuals on a single sea fan often appeared to be of similar size and

thus presumably age, implying that they either settled on the sea fan colony at once, or that they

came from eggs laid on the sea fan by an adult. On many sea fans we observed only a single

large adult, often in the act of laying what appeared to be strands of eggs. Additionally, large

numbers of very small nudibranchs could be seen feeding in small concentrated patches on many

of the sea fans. These observations support the conclusion that adults disperse across reefs and

then lay eggs on un-colonized sea fans. The young produced then appear to stay on the same

colony to feed and spread across the sea fan.

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No further surveys were done after November 18th 2005. Since the nudibranchs were first seen in

July, when temperatures were over 27°C it may be that another infestation event will occur in the

next summer, after the water warms up in May. However, in the other countries where the

nudibranch has been seen, population outbreaks appear to be episodic, generally rare, and

followed by long time periods with no further sightings. If as similar pattern of occurrence

happens in Bermuda it may be many years before another population of T. hamnerorum is seen.

Preliminary analysis indicates that sea fans are at highest relative densities on shallow reefs far

from shore, and that densities decline both towards the center of individual reef systems and

across the lagoon from offshore reefs to nearshore reefs. Recruitment limitation and reductions in

water quality may cause these patterns, but further research is needed.

Acknowledgements

Field assistance was enthusiastically provided by the Bermuda Biodiversity Project interns

Caitlin Lustic, Mike Colella, Eva Salas, Alandra Batson, Alicia Wanklyn, as well as Mark

Outerbridge of the BBP and Anson Nash of the Department of Conservation Services. I am

particularly grateful to the Friends of the Bermuda Aquarium and the Ernest E. Stempel

Foundation for supporting this research, and to the Bermuda Government’s Ministry of the

Environments, Department of Conservation Services for providing the necessary logistical

support.

19


References

Cronin G. Hay ME, finical W, Lindquist N (1995) Distribution, density and sequestration of host

chemical defenses of the specialist nudibranch Tritonia hamnerorum found at high densities

on the sea fan Gorgonia ventalina. Marine Ecology Progress Series 119(1-3): 177-189

GoslinerTM, Ghiselin MT (1987) A new species of Tritonia (Opisthobrachia: Gastropoda) from

the Caribbean Sea. Bulletin of Marine Science 40(3): 428-436.

O’Neal W, Pawlik JP (2002) A reappraisal of the chemical and physical defenses of Caribbean

gorgonian corals against predatory fishes. Marine Ecology Progress Series 240:117-26.

Strong, A. E., C. B. Barrientos, C. Duda, and J. Sapper, 1997. Improved satellite techniques for

monitoring coral reef bleaching. Proc 8th International Coral Reef Symposium, Panama City,

Panama, p 1495-1498.

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Appendix 1. Species description for T. hamnerorum on the Sea Slug Forum website.

21


Appendix 2. Description of the initial discover of T. hamnerorum in Bermuda on the Sea Slug

Forum website.

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