Associations between hydrozoans and scleractinians

of the Maldives Archipelago

Could hydrozoans save corals from global change?

BACKGROUND

Why did I study the association between Zanclea sp. hydrozoan and scleractinian corals?

In 2017, Montano et al. showed that signs of disease, bleaching and corallivory are significantly lower in coral colonies hosting hydrozoans than in hydrozoan-free ones. As further research is needed to understand completely the benefits and costs of both parts, it still cannot be called a symbiosis itself.

The study of this association has a relatively brief history, as the first advanced research was published in 2010 (Pantos & Bythell). The progress of understanding why these two organisms coexist is slow, due to the very small size of the hydrozoan polyps (length < 1 mm), which makes their detection very difficult in the field.

REFERENCES

Montano, S., Fattorini, S., Parravicini, V., Berumen, M. L., Galli, P., Maggioni, D., ... & Strona, G. (2017). Corals hosting symbiotic hydrozoans are less susceptible to predation and disease. Proceedings of the Royal Society B: Biological Sciences, 284(1869), 20172405.

Pantos, O., & Bythell, J. C. (2010). A novel reef coral symbiosis. Coral Reefs, 29(3), 761-770.

MY CONTRIBUTION

I'm Irene, a marine biology student passionate for coral reef conservation.

In this Professional Practice, I collaborated with the Zoology Research Group inside the Department of Life and Environmental Sciences (DiSVA) of the Polytechnic University of Marche (UNIVPM), Italy.

OBJECTIVE OF THE PROFESSIONAL PRACTICE:

Finding non-random patterns in the occurrence of the association between hydrozoans of the genus Zanclea sp. and scleractinian corals, which could be a first step for upcoming restoration and conservation plans for coral reef ecosystems.

The student: Irene de Prado

The sampling before the Professional Practice

In May 2019, my mentor Camilla Roveta and her research partner Torcuato Pulido travelled to the Maldives Archipelago to photograph the 1600 corals that I analyzed for my Professional Practice.

Both tutors helped me through the whole learning process, while giving me all the independence to work on my own.

Not only they both acted as mentors, but also as good friends being there for anything. This is an acknowledgement in appreciation for their kindness and support.

Camilla Roveta (my mentor) and Torcuato Pulido (my second mentor)

Sampling location

2 central Maldivian atolls (Ari and South Malé), Republic of Maldives, Indian Ocean
8 locations (4 ocean and 4 lagoon reefs)

Samples

For each location: 2 linear transects (one at 5 m and the other one at 10 m depth), taking 100 photographs of individual corals per transect

TOTAL: 1600 photographs of scleractinian corals

Variables

PREDICTORS

Reef type (ocean or lagoon)
Depth (5 m or 10 m)
Coral family
Coral genera
Corallite structure
Colony morphology

RESPONSE

PRESENCE/ABSENCE of association with Zanclea sp.

HOW I ANSWERED THE RESEARCH QUESTIONS:

PART 1.

7 CORALLITE AND COLONIES.pdf

20 CORAL GENERA.pdf

PART 2.

REMARKABLE RESULTS

Where do corals and hydrozoans live together?

Figure 1.

The absence of Zanclea sp. hydrozoan individuals living on top of scleractinian corals was predominant over the presence for all the corals sampled in the study, independently of the reef type and depth (Fig. 1).

Corals living in lagoon reefs at 10 m depth have the highest percentage of association in comparison with 5 m depths and ocean reefs. The lowest percentage of association corresponds to ocean reefs at 10 m depth.

Supporting the results illustrated in Figure 1, a significantly lower probability of association was found in deeper areas of ocean reefs.

(logistic regression, p = 0.049)

2. 1. Which coral genera do hydrozoans prefer?

Scleractinian coral genera with a significantly higher probability of hosting Zanclea sp. hydrozoans:

Porites sp.

(logistic regression, p = 0.0016)

Pavona sp.

(logistic regression, p = 0.0003)

Montipora sp.

(logistic regression, p = 0.0005)

2. 2. Which corallite structure do hydrozoans prefer?

Scleractinian corallite structures with significantly higher probability of association with Zanclea sp. hydrozoans:

*no significant results were found for cerioid corallites.

Thamnasterioid corallites

(logistic regression, p = 0.014; Pavona sp.)

Plocoid corallites

(logistic regression, p = 0.022; Diploastrea sp.)

2. 3. Which coral colony morphology do hydrozoans prefer?

Scleractinian colony morphologies with significantly higher probability of association with Zanclea sp. hydrozoans:

Encrusting colonies

(logistic regression, p = 0.001; Galaxea sp.)

Massive colonies

(logistic regression, p = 0.001;

Goniastrea sp.)

Semi-massive colonies

(logistic regression, p = 0.01; Porites sp.)

DISCUSSION AND CONCLUSION

Although many of the results are significant, models show a relatively low power of the test, indicating that they did not manage to capture all the variability explaining this association, possibly due to pseudo-replication (Logistic Regression requires independence of variables). The relatively low number of observations and other predictor variables that were not considered in this study could have also affected the results.

Therefore, in order to increase the power of the test, this research group is working on the development of more complex models to analyze this database, namely Generalized Linear Mixed Models (GLMM).

In addition, further in-depth studies are necessary where the sampling size is increased and more predictor variables are included:

temperature
salinity
oxygen concentration
pH
level of bleaching in corals
mass bleaching event occurrence
corallivory

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