Revista de Biología Tropical, ISSN: 2215-2075, Vol. 74 (S1): e20267683, abril 2026 (Publicado May. 06, 2026)

Emerging filamentous red algae blooms:

A rising threat to coral reef restoration

Carlos Toledo-Hernández1*; https://orcid.org/0000-0003-3958-6283

Claudia Patricia Ruiz-Diaz1; https://orcid.org/0000-0002-8183-5186

Alex E. Mercado-Molina1,2; https://orcid.org/0000-0002-7306-9426

1. Mar Et Terra Ambiens Solutio, San Juan, Puerto Rico; cgth0918@gmail.com (*correspondence), ruizclaudia@metaspr.

org, alex.mercado1@upr.edu

2. Universidad de Puerto Rico-Recinto de Bayamón, Puerto Rico

Received 27-VIII-2025. Corrected 14-I-2026. Accepted 05-III-2026.

ABSTRACT

Introduction: The in-situ cultivation of acroporid corals in Culebra, Puerto Rico, has been impacted by blooms

of filamentous red algae in recent years. Since 2022, the onset of this bloom has occurred from March to the end

of April, with a consistent recurrence pattern. The filamentous algae become entangled with corals both in nurs-

ery structures and after outplanting to the reef, often leading to partial or complete tissue smothering.

Objective: Document, for the first time, the recurrence and ecological impact of a filamentous red algal bloom

observed since 2022 in the coral nurseries and restoration sites of Culebra, Puerto Rico.

Results: The mortality rate resulting from each bloom event, including corals on farms and transplants, ranged

from 5 % to 20 %. The specific algal species or assemblage remains unknown; however, visual inspection is con-

sistent with species of the genus Cottoniella, characterized by fine, red filamentous thalli, profuse branching, and

the formation of dense, entangling mats.

Conclusions: The environmental factors that promote these blooms and their origin remain to be elucidated.

Furthermore, the origin and the geographical distribution of these blooms have not been determined; at pres-

ent, it is yet unclear whether they are a local or regional phenomenon. It is vital to note that these events, when

considered in conjunction with other environmental disturbances, have the potential to compromise the success

of coral reef restoration and conservation efforts.

Key words: Red Algal Bloom, coral farming, Acropora cervicornis, Culebra, Puerto Rico.

RESUMEN

Floraciones emergentes de algas rojas filamentosas: una amenaza creciente

para la restauración de los arrecifes de coral

Introducción: El cultivo in situ de corales acropóridos en Culebra, Puerto Rico, se ha visto afectado por floracio-

nes de algas rojas filamentosas en los últimos años. Desde 2022, el inicio de esta floración ha ocurrido de marzo

a finales de abril, con un patrón recurrente constante. Las algas filamentosas se enredan con los corales tanto en

las estructuras de vivero como después de trasplantarlos al arrecife, lo que a menudo provoca una asfixia parcial

o total del tejido.

Objetivo: Documentar, por primera vez, la recurrencia e impacto de afloramientos de algas rojas observados

desde el 2022, en fincas de corales y áreas de restauración de Culebra, Puerto Rico.

https://doi.org/10.15517/9j22kx29

SUPPLEMENT

2Revista de Biología Tropical, ISSN: 2215-2075 Vol. 74 (S1): e20267683, abril 2026 (Publicado May. 06, 2026)

INTRODUCTION

Macro-algae are essential components of

coral reefs, providing shelter and food to a wide

array of organisms and playing a key role in the

nutrient cycle. However, disease outbreaks—

such as the mass mortality events of Diadema

antillarum in the early 1980s and again in 2022

(Rodríguez-Barreras et al., 2023)—along with

the overfishing of key reef fishes and inverte-

brates, have contributed to a shift from coral-

dominated to algae-dominated ecosystems.

In response to current coral reef conditions,

the conservation community has increasingly

adopted active restoration strategies, such as

human-assisted coral propagation. Coral farm-

ing has become a leading strategy for propagat-

ing and restoring acroporid populations in the

Caribbean, largely due to the relatively high

survival and growth rates of coral fragments

during the nursery stage (Ruiz-Diaz et al.,

2022). However, these operations face threats

similar to those encountered by corals in the

wild, including rising water temperatures, dis-

eases, and hurricanes. In recent years, a new

and persistent threat has emerged for coral

farm managers: (Fig. 1).

These algal blooms have recurred annu-

ally from 2022 to 2025, persisting for only a

few weeks between March and April. During

bloom events, the algae form dense, pink-to-

red, filamentous masses that drift within the

water column and readily become entangled

on the branches of Acropora cervicornis and

A. palmata. This entanglement is associated

with coral mortality rates ranging from 5 % to

20 % per event, which have been documented

in both nursery-reared corals and corals out-

planted onto restored reefs (Fig. 1B–E).

Although we were unable to conduct

microscopic or molecular analyses to conclu-

sively identify the species or species complex,

visual inspection of the algal morphology sug-

gests affiliation with the genus Cottoniella.

The observed thalli exhibit a distinct pink-to-

red coloration and consists of slender, delicate

filaments with extensive, pseudodichotomous

branching that produces a dense, bushy appear-

ance. This combination of morphological traits,

along with the algae’s propensity to form drift-

ing, entangling mats, is consistent with descrip-

tions of Cottoniella spp.

Additional support for a tentative assign-

ment to the genus Cottoniella comes from its

known biogeographic distribution and eco-

logical traits. The genus was first described

from the Antilles in the early 20th century and

exhibits a broad geographic range, extending

from the southern coast of Brazil to the Florida

Keys, as well as the Mediterranean Sea and the

Canary Islands (Gavio, 2021; Gil-Rodríguez et

al., 1985). To date, seven species and one vari-

ety of Cottoniella have been recognized, four of

which (C. arcuata, C. filamentosa, C. fusiformis,

and C. sanguinea) have been reported from

tropical and subtropical regions of the western

Atlantic (Freshwater & Idol, 2013). Species

within this genus are commonly observed as

epiphytes on macroalgae such as Sargassum

(Fig. 1A; Suárez et al., 2015) and have also

Resultados: La tasa de mortalidad resultante de cada floración, incluyendo corales en granjas y trasplantes, osciló

entre el 5 % y el 20 %. Se desconoce la especie o conjunto específico de algas; sin embargo, la inspección visual es

consistente con especies del género Cottoniella, caracterizadas por finos talos filamentosos rojos, profusa ramifi-

cación y la formación de densas esteras enredadas.

Conclusiones: Los factores ambientales que promueven estas floraciones y su origen aún no se han dilucidado.

Además, no se ha determinado el origen ni la distribución geográfica de estas floraciones; actualmente, no está

claro si se trata de un fenómeno local o regional. Es vital señalar que estos eventos, cuando se consideran en

conjunto con otras perturbaciones ambientales, tienen el potencial de comprometer el éxito de los esfuerzos de

restauración y conservación de los arrecifes de coral.

Palabras clave: Afloramiento de algas rojas, fincas de corales, Acropora cervicornis, Culebra, Puerto Rico.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 74 (S1): e20267683, abril 2026 (Publicado May. 06, 2026)

been documented in deeper reef environments,

occurring at depths of up to 92 m (Gavio, 2021;

Suárez et al., 2015; Taylor, 1972).

Currently, two fundamental taxonomic

and ecological questions remain unanswered

regarding these algae blooms: (1) The species

composition (whether single or multiple spe-

cies) has not been confirmed. This challenge

stems from the unpredictable nature of the

outbreaks, which complicates the availability

of materials, such as chemical solutions, that

preserve the physical integrity of the collected

specimens to perform the necessary micro-

scopic and genetic analyses. (2) The source(s)

of these blooms and the environmental factors

driving them remain unknown. Understanding

whether these events are localized or occurring

across the wider Caribbean is crucial for the

success of coral restoration efforts (Fig. 1F).

Given their relatively high prevalence in

the last couple of years and their detrimental

effects on acroporids, addressing these ques-

tions are essential to mitigate the impact of

these floating algae blooms on coral reha-

bilitation initiatives. To bridge the knowledge

gap, establishing a regional network or similar

initiative to report, monitor, and document

floating algae blooms will be vital. This note

represents an initial step toward documenting

these events and highlighting an emerging issue

that, if persistent, could significantly impede

reef restoration initiatives and add yet another

stressor to the already vulnerable coral reef

ecosystem.

Ethical statement: The authors declare

that they all agree with this publication and

made significant contributions; that there is

no conflict of interest of any kind; and that we

Fig. 1. Images of algae belonging to the genus Cottoniella spp.. A. A floating Cottoniella spp. entangled in a branch of

Sargassum spp. (image taken in 2022). B. Image from the 2022 event of Acropora cervicornis nourished in tree farms

entangled with Cottoniella spp.. C. Image from 2023 of Cottoniella spp. entangled in an individual A. plorifera. D. Image from

2025 showing Cottoniella spp. entangled in an A. cervicornis set on a floating coral farm. E. Image from 2023 of Cottoniella

spp. entangled in an outplanted colony of A. cervicornis. F. Image taken in 2024 showing Cottoniella spp. entangled in an

outplanted A. cervicornis. Notice that some sections of the colony are dead.

4Revista de Biología Tropical, ISSN: 2215-2075 Vol. 74 (S1): e20267683, abril 2026 (Publicado May. 06, 2026)

followed all pertinent ethical and legal proce-

dures and requirements. All financial sources

are fully and clearly stated in the acknowledg-

ments section. A signed document has been

filed in the journal archives.

ACKNOWLEDGMENTS

We thank Pedro Gómez, Juan L. Sánchez,

and Juan David Murcia for their assistance with

fieldwork.

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