Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73(S1): e63697, enero-diciembre 2025 (Publicado Mar. 03, 2025)

Effect of temperature and salinity on the seagrass Halophila baillonii

(Hydrocharitaceae) under aquarium conditions

Carla Olvido van Barneveld Pérez*1; https://orcid.org/0009-0007-5643-5619

Jimena Samper-Villarreal2; https://orcid.org/0000-0002-7513-7293

1. Conservation Ecology, Groningen Institute for Evolutionary Life Science (GELIFES), University of Groningen, P.O.

Box 11103, 9700 CC Groningen, the Netherlands; carlavan2094@gmail.com (*Correspondence)

2. Centro de Investigación en Ciencias del Mar y Limnología (CIMAR), Ciudad de la Investigación, Universidad de Costa

Rica, San Pedro, 11501-2060 San José, Costa Rica; jimena.sampervillarreal@ucr.ac.cr

Received 05-X-2024. Corrected 09-I-2025. Accepted 24-I-2025.

ABSTRACT

Introduction: Halophila baillonii, also known as “clover grass”, is a rare seagrass species found in tropical waters

off the American continent. This is a small and ephemeral species classified as Vulnerable in the IUCN Red List.

Objective: To determine how variations in temperature and salinity affect this seagrass.

Methods: H. baillonii was collected in the southern Pacific coast of Costa Rica either by hand or with a corer

(8 cm diameter). Two experiments with three treatments each were carried out in aquaria. Each treatment was

applied to three aquaria, for a total of nine aquaria per experiment. The temperature treatments consisted of 23

°C (Low), 28 °C (Control), 33 °C (High), with a constant salinity of 25 over 51 days. Salinity treatments were

15 (Low), 25 (Control), 35 (High) with a constant minimum temperature of 28 °C over 31 days. Five plant

performance parameters were measured: 1) foliar shoot survival; 2) increase in the number of foliar shoots; 3)

horizontal rhizome elongation; 4) rhizome internodal length; and 5) leaf area.

Results: H. baillonii survival rates were higher when collected manually rather than using a corer. All plant

performance parameters were higher at 28 °C temperature (control). In contrast, variables of plant performance

were similar in all salinity treatments, except that the seagrass presented smaller leaves at higher salinities. Female

flowers were found towards the end of the experiments, being the first report of flowering of this species under

aquaria conditions.

Conclusion: H. baillonii has a wide salinity tolerance, thus enabling plant survival during dry or rainy seasons. In

contrast, H. baillonii appears to be more sensitive to lower and higher temperatures than 28 ºC. This is the first

study reporting the response of this threatened species to experimentally induced fluctuations of temperature

and salinity.

Key words: Eastern Tropical Pacific; Golfo Dulce; climate change; environmental factors; seagrass condition.

RESUMEN

Efecto de la temperatura y salinidad sobre el pasto marino

Halophila baillonii en condiciones de acuario

Introducción: Halophila baillonii, también conocido como “pasto de trébol”, es una especie poco común de pasto

marino que se encuentra en aguas tropicales del continente americano. Esta es una especie pequeña y efímera

clasificada como Vulnerable en la Lista Roja de la UICN.

Objetivo: Determinar cómo las variaciones en la temperatura y salinidad afectan a este pasto marino.

https://doi.org/10.15517/rev.biol.trop..v73iS1.63697

SUPPLEMENT

2Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73(S1): e63697, enero-diciembre 2025 (Publicado Mar. 03, 2025)

INTRODUCTION

Seagrasses are flowering plants that can

live and reproduce while submerged in brackish

and marine water. The “clover grass”, Halophila

baillonii Asch. is a small seagrass species and

its foliar shoot is composed of a cluster of

approximately four leaves on a vertical shoot

(van Tussenbroek et al., 2010). This seagrass is

dioecious, with female or male flowers found in

the centre of the leaf cluster (van Tussenbroek

et al., 2010). In the IUCN Red List, H. bail-

lonii is listed as a Vulnerable seagrass species

(Short et al., 2010). Overall, not much is known

about the relative abundance of H. baillonii.

It is mostly considered to be a rare species in

much of its range, mostly forming fragmented

populations in the Caribbean Sea and Atlantic,

with few populations in the Eastern Tropical

Pacific (ETP) (Magalhães et al., 2025; Samper-

Villarreal et al., 2018b; Samper-Villarreal, 2024;

Short et al., 2010).

On the Pacific coast of Costa Rica, H. bail-

lonii was previously found in Bahía Culebra.

However, that meadow disappeared after a

severe storm in 1996 (Cortés, 2001). Follow-

ing its disappearance there were no reports of

seagrass presence on the Pacific coast of Costa

Rica until 2010, when a monospecific meadow

of H. baillonii was found in Golfo Dulce, on the

Southern Pacific coast of Costa Rica (Samper-

Villarreal et al., 2014). Soon after, in 2011, a

larger seagrass meadow containing H. baillonii

was found nearby within Golfo Dulce (Sar-

mento de Carvalho, 2013). More recently, H.

baillonii was found on the Pacific coast of Costa

Rica at Bahía Potrero (Samper-Villarreal et al.,

2018a), El Jobo and Matapalito (Samper-Villar-

real et al., 2020), and Sámara (Samper-Villarre-

al et al., 2022). The biggest meadow including

this species on the Pacific coast of Costa Rica

is found at Playa Colibrí, Golfo Dulce (Samper-

Villarreal et al., 2018b). Water temperature

and salinity at Playa Colibri vary over time.

Furthermore, seagrasses in this meadow show

temporal variability of density and spatial dis-

tribution linked to changes in environmental

conditions (Barquero Chanto, 2018).

Our understanding of seagrasses in gen-

eral in the ETP is limited (Samper-Villarreal,

2024). Information on the ecology, reproduc-

tion, physiology and population dynamics of

H. baillonii in the ETP and within its dis-

tribution range is scarce (Samper-Villarreal,

2024; Samper-Villarreal et al., 2018b, Short et

al., 2010). Therefore, enhancing knowledge of

Métodos: Se colectó H. baillonii en el sur de la costa Pacífica de Costa Rica por medio de colecta manual o con

nucleadores (8 cm diámetro). Se realizaron dos experimentos con tres tratamientos cada uno en acuarios. Cada

tratamiento se aplicó a tres acuarios, para un total de nueve acuarios por experimento. Los tratamientos en el

experimento de temperatura fueron 23 °C (Bajo), 28 °C (Control), 33 °C (Alto), con una salinidad constante de

25 durante 51 días. Los tratamientos del experimento de salinidad fueron 15 (Bajo), 25 (Control), 35 (Alto) con

una temperatura mínima constante de 28 °C durante 31 días. Se midieron cinco parámetros del rendimiento de

las plantas: 1) supervivencia de los haces foliares; 2) incremento en el número de haces foliares; 3) elongación del

rizoma horizontal; 4) longitud internodal del rizoma; y 5) área foliar.

Resultados: Las tasas de supervivencia de H. baillonii fueron mayores cuando fueron colectadas manualmen-

te en lugar de colectadas con un nucleador. El rendimiento de las plantas fue mejor en condiciones de 28 °C

temperatura (control). En contraste, no hubo variación en el rendimiento de la planta en los tratamientos de

salinidad, excepto por hojas más pequeñas en el pasto a mayores salinidades. Se encontraron flores femeninas

en los acuarios hacia el final de los experimentos, siendo este el primer reporte de floración para esta especie en

condiciones de acuario.

Conclusión: H. baillonii tuvo una amplia tolerancia a la salinidad, así permitiéndole a la planta sobrevivir tanto en

la época seca como lluviosa. En contraste, H. baillonii parece ser más sensible a temperaturas menores o mayores a

28 ºC. Este es el primer estudio que reporta la respuesta de esta especie amenazada a fluctuaciones en temperatura

y salinidad en condiciones experimentales.

Palabras clave: Pacífico Tropical Oriental; Golfo Dulce; cambio climático; factores ambientales; condición de

pastos marinos.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73(S1): e63697, enero-diciembre 2025 (Publicado Mar. 03, 2025)

seagrasses in the ETP, particularly H. baillonii,

an understudied yet critical species, is essential

for advancing seagrass management and con-

servation efforts. Here, we assessed the effect

of temperature and salinity fluctuations on H.

baillonii plant performance under controlled

aquarium conditions. We hypothesized that

H. baillonii would have optimal growth under

temperature and salinity conditions similar to

those found in the field.

MATERIALS AND METHODS

Field collection site: Samples for this

experiment were collected at Playa Colibrí in

Golfo Dulce, on the southern Pacific coast of

Costa Rica (8° 40’ 12.43’’ N, 83° 26’ 35. 27’’ W).

The sample collection site was at a meadow

within the gulf, located at ~100 m from the

coastline and ~1 m depth at low tide. The Pacif-

ic coast of Costa Rica has a dry season (Decem-

ber-April) and a rainy season (May-November).

Samples were collected at the beginning of the

dry season (January and February).

Sample collection: Samples were collected

on two separate dates, January 27th and Febru-

ary 13th of 2020, using two different sampling

methods: (1) a corer and (2) manual collection.

Cores were collected using an 8 cm diam-

eter PVC corer inserted into the sediment to a

depth of 5 cm to extract the plants and associ-

ated sediment intact. The cores were placed in

plastic containers and filled with seawater from

the site. Manual collection entailed carefully

extracting the seagrass plants with roots and

rhizomes attached by hand and gently rinsing

them free of associated sediment while making

sure the rhizome contained an apical meristem.

Rhizome sections collected with either method

had a minimum of one and a maximum of ten

foliar shoots. Manually collected samples were

stored inside a sealed zip lock bag with paper

towel dampened with sea water from the site

to maintain humidity levels. Floral buds were

not seen in the foliar shoots at the time of

collection. Samples were transported inside a

portable cooler without additional temperature

controls to the facilities of CIMAR, University

of Costa Rica, San José.

Experimental setup: Each aquarium (39

cm long, 19 cm wide, and 20 cm high) was

filled with 10 L of artificial seawater (Red Sea

Salt), and when necessary, reverse Osmosis

De-Ionized (RO/DI) water was added to com-

pensate increases in salinity from evaporation.

The water was recirculated without blowing

additional bubbles into the aquarium with a

submerged 2W water pump (Aquatic Pond). A

thermostat with heater (Dophin Heater 50 W)

was used to maintain the water at a constant

temperature (Fig. 1). Every two weeks, 10 % of

the water was replaced with new artificial sea-

water, making sure the salinity was consistent

due to evaporation.

The aquaria for the temperature experi-

ment were filled with ~3 cm height of natural

sediment previously collected from seagrass

meadows from both coasts of Costa Rica,

cleaned with water and NaClO to sterilize the

sediment, and finally dried and sieved through

a 2 mm sieve to exclude any potential remnants

of macro-invertebrates, shells, and stones. For

the salinity experiment, the sediment height in

the aquaria was ~3 cm and consisted of unpro-

cessed sediment collected at the same time

and site as the seagrass samples. There were

nine aquaria in total for each experiment, and

one core-collected and one manually-collected

seagrass sample was planted in each aquarium

using randomly generated numbers.

During the acclimation and experimental

period, aquaria were monitored daily, measur-

ing salinity and temperature manually using

a portable ACT refractometer and a digital

aquarium thermometer. Additionally, a HOBO

data logger was introduced randomly in one

aquarium per treatment, logging temperature

at 30-minute intervals for at least one week

in each experiment. Water pH was measured

using commercially available water alkalinity

strips for aquaria and remained between 8.5

and 9.0 during the experiments.

Experimental treatments: Average

ambient conditions at the sampling site were

4Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73(S1): e63697, enero-diciembre 2025 (Publicado Mar. 03, 2025)

25 salinity and 28 °C temperature, measured

as part of a long-term seagrass monitoring

program at Colibrí based on four samplings

per year over the two years prior to sample

collection (Samper-Villarreal et al., In prep).

These average ambient conditions were there-

fore selected as the control values for each of

the experiments. A one-year monthly study,

between March 2016 and March 2017, at a

site within this meadow found that water tem-

perature was highest in July (31.6 ± 1.1 ºC)

and lowest in October (27.1 ± 0.05 ºC) (Barqu-

ero Chanto, 2018). As maximum temperatures

reported in the study were ~ 5 units above the

average, temperature treatment variation was

selected as ± 5 ºC compared to the control. In

the monthly study, salinity was highest in April

(33.8 ± 0.4) and lowest in October (19.6 ± 1.4)

(Barquero Chanto, 2018). As maximum salini-

ties were almost 10 units above average condi-

tions at the sampling site the salinity treatment

range was selected to be ± 10.

Temperature treatment: Aquaria were set

up indoors applying an artificial LED light of

1600 lumens (LED- MS60 16W) set to 12:12 h

light:dark cycle with air temperature at 23°C and

salinity at 25. Seagrasses were planted in each of

the nine aquaria and acclimated for eight days

under control conditions. Conditions in each

aquarium were modified according to each

treatment on day one of the experiment. The

experimental setup consisted of three tempera-

ture treatments (T): Control (CT 28 °C), Low

(LT 23 °C), and High (HT 33 °C). Thermostats

with heaters were used to increase water tem-

perature for treatments requiring temperatures

higher than 23 °C. Plants were exposed to the

treatments for 51 days. Temperature and salin-

ity variations between treatments during the

experiment is presented in Table 1.

Salinity treatment: Aquaria were set up

in a roofed area outdoors with natural sun-

light and a global radiation of 2 MJ m-2 (Insti-

tuto Meteorológico Nacional, 2020). Seagrasses

were planted in each of the nine aquaria and

acclimated for 10 days under control condi-

tions. Conditions in each aquarium were modi-

fied for each treatment on the first day of the

experimental time period. The experimental

setup consisted of three salinity treatments (S):

Fig. 1. Halophila baillonii under aquarium conditions. A) Experimental set up. B) Female flower at the center of the leaf

pseudo whorl of a foliar shoot.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73(S1): e63697, enero-diciembre 2025 (Publicado Mar. 03, 2025)

Control (CS 25), Low (LS 15), and High (HS 35).

Water temperature conditions were maintained

at a minimum of 28 °C with thermostat heaters.

Plants were exposed to treatments for a total of

31 days.

Seagrass response: Plant performance

was monitored five times a week. The plant

performance parameters were: 1) foliar shoot

survival; 2) increase in the number of foliar

shoots; 3) horizontal rhizome elongation; 4)

rhizome internodal length (length of rhizome

between two consecutive foliar shoots); and 5)

leaf area. Additionally, the number of leaves per

foliar shoot were counted for leaf area samples

and the number of flowering shoots in each

aquarium noted if present. Foliar shoots were

considered alive from the moment they began

to emerge from the sediment with pale green

leaves until the leaves lost their darker green

color, indicating leaf senescence. Foliar shoot

increase was the number of shoots that were

alive at the end of the experiment minus the

number of shoots that were alive at the begin-

ning of the experiment.

Rhizome elongation (mm week-1) and

internodal length (cm) were estimated by

taking subsequent perpendicular and scaled

photographs of foliar shoots of each plant.

The image analysis software ImageJ (Schnei-

der et al., 2012) was used to estimate rhizome

length between foliar shoots on subsequent

days. A negative rhizome elongation refers to

the change between subsequent days caused

by the death of the rhizome tissue previously

present and not compensated by growth of new

rhizome in that time period. Leaf area (cm2)

was measured once the experiments were final-

ized. Three shoots were randomly selected and

photographed perpendicularly flat over a white

surface with the leaves separated and a scale

added. Length, width and area were measured

for each leaf using ImageJ and the surface area

(cm2) per shoot was calculated.

Data analysis: The effect of salinity and

temperature on plant performance was ana-

lyzed using one-way analysis of variance

(ANOVA). When differences among treat-

ments were found, post hoc Tukey HSD was

used to identify variation between treatments.

Normality of the data was tested with the

Kolmogrov-Smirnov test and homogeneity was

tested with Bartlett. When data did not show

normality and transformation of data was not

possible the Wilcoxon non-parametric test was

used. All statistical analyses were done using R

v.3.5.1. (R Core Team, 2020).

RESULTS

Collection method: During the experi-

ments, hand-collected H. baillonii had higher

shoot survival rates (55 %) than those collected

with a corer extraction method (22 %) (Wil-

coxon test, P = 0.02).

Table 1

Water temperature and salinity conditions during experiments (nine aquaria per experiment, three aquaria per treatment)

and number of leaves at the end of the experiments per Halophila baillonii foliar shoot.

Experiment Temperature (°C) mean ± SD Salinity mean ± SD Number of leaves per shoot mean ± SD (n)

Temperature

Low (LT) 24.2 ± 1.1 28.9 ± 1.1 4 ± 0 (9)

Control (CT) 28.5 ± 0.8 28.7 ± 1.0 2 ± 0 (3)

High (HT) 32.9 ± 1.4 28.9 ± 1.1 3 ± 1 (9)

Salinity

Low (LS) 25.4 ± 2.7 16.8 ± 2.0 4 ± 1 (6)

Control (CS) 25.9 ± 1.6 25.1 ± 1.6 3 ± 1 (6)

High (HS) 26.2 ± 2.7 34.2 ± 2.7 4 ± 0 (11)

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Temperature experiment plant perfor-

mance: Rhizome elongation varied among tem-

perature treatments (ANOVA, F2, 80 = 70.2,

P < 0.001) and was highest in the CT treat-

ment (Fig. 2). Rhizome internodal length also

showed differences among treatments. Plants

at 33 °C (HT) had shorter internodal lengths

(4.2 mm) compared those at 28 °C (CT 15.3

mm) and 23 °C (LT, 16.4 mm) (ANOVA, F2, 24

= 7.2, P < 0.05). Foliar shoot increase (Fig. 2)

also varied among treatments (ANOVA, F2, 7 =

5.3, P < 0.05). Shoots increased in 50 % of the

rhizomes sampled in CT and LT, while they

only increased in 17 % of the HT rhizomes.

CT showed the highest increase in the number

of shoots, while HT showed the least shoot

increase. Similarly, surface area per leaf varied

among treatments (ANOVA, F2, 66 = 46.9, P <

0.05). CT had a higher leaf surface area, with

0.7 ± 0.2 cm2 compared to 0.4 ± 0.1 cm2 in HT

and 0.4 ± 0.09 cm2 in LT. There were also differ-

ences among treatments for leaf length (F2, 66 =

26.4, P < 0.05) and width (F2, 66 = 57.6, P < 0.05).

For both leaf length and width, controls dif-

fered from the rest of the treatments, while HT

and LT had no significant differences between

them. Thus, both leaf length and width were

larger at 28 °C (CT) than at 23 °C (LT) or 33 °C

(HT). Number of leaves per foliar shoot varied

among temperature treatments (Wilcoxon test,

P < 0.001). There were less leaves per shoot in

HT than CT yet the number of shoots analyzed

per treatment was imbalanced due to very low

number of HT shoots available (Table 1).

Salinity experiment: There were no dif-

ferences among salinity treatments in regards

to rhizome elongation (ANOVA, F2, 54 = 1.8,

P = 0.2), internodal length (ANOVA, F2, 32 = 1.2,

P = 0.3) or number of foliar shoots (ANOVA,

F2, 15 = 0.01, P = 1; Fig. 2). In contrast, there

was variation among treatments in regards to

leaf surface area (ANOVA, F2, 86 = 7.1, P < 0.05;

Fig. 2), leaf length (F2,86 = 6.2, P < 0.05), and

width (F2,86 = 15.3, P < 0.05). Leaf surface area

was 0.4 ± 0.1 cm2 in the control salinity treat-

ment (CS 25), 0.5 ± 0.3 cm2 in the low salinity

treatment (LS 15), and 0.3 ± 0.1 cm2 in the high

salinity treatment (HS 35). For both length and

width, there were differences between HS and

the rest of the treatments, while CS and LS did

Fig. 2. Halophila baillonii plant performance (mean ±

standard deviation, n = 3) per temperature and salinity

treatment a) Horizontal rhizome elongation (mm week-

1). b) Foliar shoot increase/decrease. c) Leaf surface area

(cm2). Note: Refer to Table 1 for temperature and salinity

values for each treatment.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73(S1): e63697, enero-diciembre 2025 (Publicado Mar. 03, 2025)

not differ between them. This indicates that leaf

length and width were smaller at higher salini-

ties; while leaf length and width were similar at

control and lower. There was no variation in the

number of leaves per foliar shoots among treat-

ments (ANOVA, F2, 21 = 1.9, P = 0.2; Table 1)

Flowering: It was noted that at the end of

the experiment when plants were placed again

at control temperature (28°C) and salinity (25),

50 % of the H. baillonii foliar shoots produced

flowers. Only female flowers were seen within

the aquaria (Fig. 1). No flowers were found

during the acclimation periods or when plants

were exposed to experimental treatment condi-

tions in either of the experiments.

DISCUSSION

This is the first study reporting the response

of the threatened species H. baillonii to experi-

mentally induced fluctuations of temperature

and salinity. We report that this species has

a wide salinity tolerance while it appears to

be more sensitive to temperatures below or

over 28 ºC. Other species of the Halophi-

la genus have been grown under controlled

aquaria conditions, studying, for example, the

flowering, thermal tolerance, and simulated

climate change conditions for Halophila stipu-

lacea (Forssk.) Asch. (McMillan, 1980; Moses

& Fredrick, 2017; Nguyen et al., 2020; Wessel-

mann et al., 2020). Furthermore, temperature,

pH, hyposalinity stress, and salinity reduction

has been studied for Halophila johnsonii Eise-

man (Gavin & Durako, 2012; Griffin & Durako,

2012; Torquemada et al., 2005). Likewise, light

deprivation, substrate, salinity, light, and CO2

enrichment effects in Halophila ovalis (R. Br.)

Hook. have been previously assessed (Bujang

et al., 2008; Longstaff et al., 1999; Sidik et al.,

2010; Wong, 2016). Halophila decipiens Ostenf.

was also grown in experimental conditions, dis-

covering it is the first seagrass known to need

exogenous vitamins (Bird et al., 1998) and the

role of light and salinity on seed germination

for this species (McMillan, 1988). The effect of

light on seed germination was also studied for

Halophila engelmannii Asch., as well as its high

salinity tolerance and flowering (McMillan,

1976; McMillan, 1987; McMillan & Moseley,

1967). To the best of our knowledge this con-

stitutes the first experiment under aquarium

conditions and to report flowering in aquaria

for H. baillonii.

Plants collected by hand had a higher

survival rate than those collected with a corer

in our study. This finding was unexpected,

because of the potential alteration to root and

rhizomes and their microbiome in manually

collected samples versus intact ones in cores

(Wang et al., 2021). The lower survival of core

samples is not likely due to a lower number

of apical meristems of the rhizomes in corer

samples. A previous study at a nearby site

within Golfo Dulce, which collected samples

using corers of the same diameter (5 cm) as

the ones used in this study, reported rhizome

growing tip densities of 1 630 m2, there were

also on average 10 rhizome sections per core,

half of which had foliar shoots, and one foliar

shoot per rhizome section on average (Samper-

Villarreal et al., 2014). Thereby, it is assumed

that both cored and manually collected samples

had growing rhizome tips and a similar num-

ber of shoots. Potentially, the transportation

method could have played a role in the sample

survival as manually collected samples were

stored in plastic bags with a wet tissue while

cored samples were transported covered with

water. However, the reason for differences in

sampling technique survival remain unclear at

this time and further study on the most effec-

tive sampling and transport methods for this

species is needed particularly given implica-

tions for potential restoration initiatives.

Treatment values were selected based on

the variation between average and maximum

temperatures and salinities reported at the site

prior to the beginning of the experiment (Bar-

quero Chanto, 2018; Samper-Villarreal et al.,

In Prep). The lower treatments (15 salinity and

23 °C temperature) fell slightly below reported

values at the time and to date (19 salinity and

26 °C temperature Samper-Villarreal, 2024).

H. baillonii plants performed best at the CT

8Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73(S1): e63697, enero-diciembre 2025 (Publicado Mar. 03, 2025)

temperature treatment (28 ºC) compared to

the other treatments. Nonetheless, plants still

survived at LT (23 ºC) and HT (33 ºC), indicat-

ing a certain tolerance for higher temperatures,

although its performance was poor. Seagrasses

can tolerate temperature changes (Thorhaug et

al., 1978) yet their tolerance to high tempera-

ture for long periods is less studied for tropical

seagrasses (Campbell et al., 2006). Additionally,

short-term exposures at seawater temperatures

as high as 40°C could cause irreparable effects

and damage seagrass physiology (Campbell

et al., 2006). Halophila stipulacea was able to

perform over a wide range of temperatures

(8 – 38 ºC) (Wesselmann et al., 2020), similar

to other tropical seagrasses (Campbell et al.,

2006). Other species such as Halophila ova-

lis, Zostera capricorni Asch., and Syringodium

isoetifolium (Asch.) Dandy showed low toler-

ance for short-period thermal stress (1 – 4 h

at 35 – 45 °C) (Campbell et al., 2006). Tem-

perature and salinity tolerance thresholds at

small incremental intervals in experimental

conditions for this species should be identified

and further understanding of environmental

conditions in the field for this species is needed.

Plant performance of H. baillonii appeared

to be mostly unaffected by salinity fluctuations.

Even so, the aquarium experiments conducted

here suggest that H. baillonii tolerates the low

salinities used better than it does the higher

salinity conditions it was exposed to in the

experiment. The high salinity treatment (HS

35) plants were exposed to in this experiment

was 10 units above average salinity at the site

(CS 25); however, it is important to keep in

mind that average sea water salinity is roughly

35. H. baillonii was been reported in salinities of

40.6 ± 1.5 in semiarid conditions in Brazil (Bar-

ros et al., 2014), yet most seagrasses are thought

to be more sensitive to hypersaline conditions

(Adams & Bate, 1994; Biebl & McRoy, 1971;

Doering & Chamberlain, 1999; Kamermans et

al., 1999; Ogata & Matsui, 1965; Van Katwijk et

al., 1999). Diminished performance for other

seagrass species has also been described for

hypo- and hyper salinities (Kamermans et al.,

1999; McMillan & Moseley, 1967; Walker, 1985;

Walker & McComb, 1990). H. baillonii is also

found naturally in salinities of 25.1 during the

rainy season and up to 32.8 in the dry season in

Honduras (Carrasco & Caviedes, 2013; Carras-

co & Caviedes, 2015).

In this study, the response of H. baillonii to

temperature and salinity fluctuations was only

studied during a short time (51 and 31 days

respectively) and under experimental aquaria

conditions. Therefore, H. baillonii response in

the field may vary and further studies are need-

ed. The plant’s performance varied between the

two experiments. This may be due to poten-

tial differences in light intensity between both

experiments, as the temperature experiment

had consistent artificial lighting while the salin-

ity experiment had natural fluctuating lighting.

This experimental aquarium study pro-

vides a first estimate of horizontal elongation

rate for this species to the best of our knowl-

edge. The highest average horizontal rhizome

elongation for H. baillonii in this aquarium

study was 137 cm year-1. This appears to be

within the range of horizontal elongation rates

reported for other Halophila species. H. ovalis

has the highest horizontal elongation rate for

the genera growing 356 cm year-1. H. decipiens

has a horizontal elongation of 215 cm year-1

while Halophila hawaiiana Doty & Stone rhi-

zomes grow 89 cm year-1 (Marbà & Duarte,

1998). At optimal control conditions, the inter-

nodal length of H. baillonii in this study was

15.3 ± 6.2 mm. Other Halophila species were

found to have an internodal length between

10 and 17 mm from samples collected in their

natural habitat (Marbà & Duarte, 1998), com-

parable to the ones in this study.

During the experiments, only female flow-

ers were found in this aquarium study. Recent

studies have indicated that H. baillonii may be

a recent introduction into the Eastern Tropical

Pacific (Van Dijk et al., 2023). On the Pacific

coast of Costa Rica, only < 1 % of 1 300 shoots

had flowers and all flowers found were female

(Samper-Villarreal 2025). It is thought that H.

baillonii may be solely propagating by clonal

growth on the Pacific coast of Costa Rica. This

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73(S1): e63697, enero-diciembre 2025 (Publicado Mar. 03, 2025)

could explain why 100 % of our samples that

flowered were female.

This study reports the successful culture

of H. baillonii in aquaria under controlled

conditions and may help understand how the

plant will behave in the future, such as under

increased sea water temperatures and variations

in freshwater input linked to climate change.

The experimental findings from this study

forewarn a potential decline of H. baillonii in

Golfo Dulce if marked fluctuations of water

temperature and salinity occurred in the future,

with the addition of the exceptional conditions

of Golfo Dulce as a tropical fjord (Morales-

Ramírez et al., 2015). Halophila baillonii is an

important food source for manatees (Trichechus

manatus L.) in Belize (Ramos et al., 2024; Short

et al., 2006). In Golfo Dulce it has been reported

as a key food source for the green sea turtle

(Chelonia mydas L.) and parrotfish (Bessesen

& Guido, 2012; Samper-Villarreal & Cortés,

2020). Thereby, negative impacts on seagrass

meadows in Golfo Dulce’s ecosystem may also

impact other marine life.

Ethical statement: the authors declare that

they all agree with this publication and made

significant contributions; that there is no con-

flict of interest of any kind; and that we fol-

lowed all pertinent ethical and legal procedures

and requirements. All financial sources are fully

and clearly stated in the acknowledgments sec-

tion. A signed document has been filed in the

journal archives.

ACKNOWLEDGEMENTS

Funding for this project was provided by

the Vicerrectoría de Investigación de la Uni-

versidad de Costa Rica and a Marco Polo grant

from the University of Groningen. Instituto

Meteorológico Nacional de Costa Rica pro-

vided climatic data. Thanks to Latin American

Sea Turtles (LAST), Rebeca Cambronero and

Jairo Moya for help in the field.

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