Using embryonic morphometrics to diagnose developmental stress and target sustainable management of sea turtle nesting in Malta

1 Introduction

Across the Mediterranean, sea turtle nesting increasingly occurs on beaches subject to intense human use and rapid coastal development (Mazaris et al., 2017; Casale et al., 2018). The western and central Mediterranean basins are currently experiencing an unprecedented, climate-driven geographical expansion of loggerhead sea turtles (Caretta caretta) nesting (Casale and Tucker, 2017; Carreras et al., 2018; Hochscheid et al., 2022). This “secondary colonization” is likely a response to warming sea surface temperatures, pushing nesting activity into higher latitudes (Bentivegna et al., 2010; Mancino et al., 2022; Luna-Ortiz et al., 2024). Small island systems, such as Malta, represent a microcosm of the challenges associated with this expansion. In this study, limited beach width and high recreational demand create a literal “coastal squeeze” (Vousdoukas et al., 2020). In these settings, fragmented management frameworks often compress nesting activity into narrow, suboptimal zones (Zammit, 2020). The limited sandy beaches, comprising only 2.4% of Maltese coastline (Costa et al., 2024), highlights research and conservation needs of this habitat.

While nesting density in the Maltese Islands remains low relative to major rookeries in Greece or Türkiye (Casale et al., 2018), the 2020–2025 period confirms that the islands function as an emergent nesting area (Hochscheid et al., 2022). In marginal systems, individual nest failures require close examination, as each clutch constitutes a substantial fraction of the yearly reproductive yield (Wallace et al., 2011). Traditionally, conservation efforts have concentrated on post-emergence hazards, particularly light pollution (Hamann et al., 2010; Kamrowski et al., 2012; Witherington et al., 2014). However, the principal constraint on sustainability often arises during incubation, when the embryos are highly vulnerable to persistent alterations in the sand's physical, chemical, and thermal characteristics (Ackerman, 1977, 1997; Howard et al., 2014; Staines et al., 2019).

Embryonic development is a delicate metabolic process that is sensitive to subterranean oxygen availability, temperature, and moisture (Bladow and Milton, 2019; Booth et al., 2020; Gravelle and Wyneken, 2022). As embryos reach the final third of their incubation, their metabolic rates and oxygen demands increase exponentially, making them susceptible to “metabolic heating” and hypoxic stress (Ackerman, 1977; Bladow and Milton, 2019; Booth et al., 2020; Gammon et al., 2020). During this stage, embryos are highly vulnerable to the impacts of climate change, as rising sand temperatures can reach lethal limits and significantly decrease overall hatching success (Fuentes et al., 2011; Howard et al., 2014). Climate change is therefore significantly altering loggerhead nesting dynamics in the Mediterranean, driving both a spatial westward and northward expansion into new nesting habitats (Mancino et al., 2022; Ceolotto et al., 2025) and a temporal shift toward earlier breeding seasons (Patel et al., 2016), both of which would benefit from an assessment of embryonic development and hatching success. To resolve these bottlenecks, we argue that embryonic morphometrics can serve as a diagnostic tool, providing a “biological ledger” of the nesting environment (García-Grajales et al., 2021). By dismantling the barriers between embryonic development (biology), coastal geomorphology (physics), and beach management (social science), we can formulate hypotheses about nest failure and apply the “science integration” framework proposed by Vozzo et al. (2025)) to mitigate such failures. This viewpoint uses a causal framework to assess nesting outcomes: Bay Characteristics to Incubation Environment to Embryonic Development to Nest Outcome, proposing that unhatched embryos provide critical data for future conservation strategies. The approach is important for the conservation of this globally vulnerable turtle species (Casale and Tucker, 2017—IUCN Red List), which has received increasing conservation efforts, particularly in the Mediterranean region.

2 Bay characteristics and environmental pressures

Loggerhead turtle nesting studies in the Maltese Islands commenced in 2020 (Koroma and Vella, 2024). Over a 6-year period, nesting was recorded across 23 nests on 6 principal Maltese beaches: Ramla tal-Mixquqa (Golden Bay), Ghajn Tuffieha Bay (Riviera), Gnejna, Ramla l-Hamra (Gozo), Ghadira, and Fajtata [Environment and Resources Authority (ERA), 2025]. These locations vary significantly in their levels of protection and human activity.

The data reveal a “protection gap,” whereby Natura 2000 status does not inherently guarantee high incubation success. While Ramla l-Hamra achieved high hatching success, nests at Ghajn Tuffieha, also a protected site, had to be relocated each time due to beach narrowness and the imminent threat of inundation. This highlights the fact that protection designations alone cannot mitigate the physical limitations posed by rising sea levels and storm surges (Pike, 2013; Katselidis et al., 2014).

The 2020 nesting event at Ghadira Bay offers a compelling observational context regarding the impact of human disturbance. Occurring during the COVID-19 lockdowns, this event coincided with a period of “anthropogenic silence,” characterized by a drastic, albeit temporary, reduction in marine traffic and coastal activity. While our study did not encompass continuous acoustic or photic data logging, this unique period provides anecdotal support for the impacts of sensory barriers. Late-stage aquatic testudine embryos are highly sensitive to substrate-borne vibrational cues, on which they rely to synchronize hatching (Doody et al., 2012; Clabough et al., 2022). Consequently, there is growing concern that anthropogenic vibrations—such as low-frequency vessel noise propagating efficiently through sandy coastal substrates—could mask these natural cues and disrupt embryonic development and hatching synchrony (Peng et al., 2015; Clabough et al., 2022). Furthermore, the general reduction in artificial light at night (ALAN) during the lockdown period aligns with the broader hypothesis that chronic sensory disturbance can deter successful oviposition in female sea turtles (Mazaris et al., 2017), a concern underscored by high-resolution photometric mapping of sky brightness across the Maltese archipelago (Caruana et al., 2020). Observations from such anomalous environmental baselines reinforce the need for strict coastal regulations, such as regional guidelines recommending maintaining beach illumination below 0.1 lux to prevent both adult deterrence and hatchling disorientation (Tuxbury and Salmon, 2005; Witherington et al., 2014; Simantiris et al., 2025). Similarly, on urbanized pocket beaches such as Fajtata, human activities such as trampling and mechanical grooming disproportionately alter the microclimate. We hypothesize that the low hatching success (29.2%) observed at this site may be associated with substrate compaction, which impedes the gas exchange required for late-stage embryonic respiration, effectively suffocating high-metabolism, late-stage embryos (Ackerman, 1980). Therefore, while it is vital to consider changes in natural environmental factors, such as temperature and extreme wave events, it is also important to consider anthropogenic factors, such as coastal urbanization, disturbances, and marine traffic, to understand how different factors may synergistically stress turtle embryonic development and consequent hatching success or failure (Ceolotto et al., 2025).

3 Morphometrics as a diagnostic ledger

Standard hatching success percentages are binary (live vs. dead). In contrast, morphometric analysis of unhatched embryos provides insight into the timing of developmental failure (Miller, 1985; Kaska and Downie, 1999). To establish an evidentiary basis for our conceptual arguments, we analyzed a dataset of 122 unhatched embryo specimens collected from 14 viable nest remains (with specimens that could be measured), out of the 23 naturally incubated nests recorded between 2020 and 2025. These specimens were examined following standard post-emergence necropsy protocols described by Koroma and Vella (2024)). We individually assigned developmental stages to these embryos, referencing standard staging criteria established by Miller (1985)) and applied by Kaska and Downie (1999)) in the Mediterranean. The sampled population's mean straight carapace length (SCL) of 33.45 ± 5.08 mm (Table 1) aligns with stages 28 to 29, indicating that the majority of embryonic mortality across the dataset occurred during the final third of incubation. This is a critical developmental period for loggerhead embryos, as indicated above.

Table 1. Mean and standard deviation of morphometric measures of unhatched embryos (n = 122) in the period 2020–2025.

Mean morphometric measurements (±standard deviation) of unhatched Caretta caretta embryos (2020–2025). Measurements are in millimeters (mm) unless otherwise noted. SCL, straight carapace length; SCW, straight carapace width; HL, head length; HW, head width; BL, body length; FL, forelimb (front flipper) length; HIL, hindlimb (hind flipper) length.

3.1 Developmental arrest and relocation hypotheses

Translating raw morphometrics into developmental milestones reveals a clear bottleneck. The observed mean straight carapace length (SCL) of 33.45 ± 5.08 mm, alongside a mean head length (HL) of 17.99 ± 2.04 mm, corresponds strictly to stages 28–29 of the loggerhead turtle's embryological development index (Miller, 1985; Kaska and Downie, 1999). This finding indicates that development likely did not fail post-oviposition but instead stopped during the final third of incubation—a phase during which late-stage embryos are exceptionally sensitive to hypoxia, substrate compaction, and vibrational stress from relocation. Relocation is a sensitive intervention (Abella et al., 2007; Candan, 2018). Our data compare naturally incubated nests with those relocated due to imminent threats. While natural nests averaged 64% success, relocation introduced variability. In 2022, an artificial incubation event resulted in zero success, with embryos arresting at stages 26–29. This stage corresponds to the “critical period” where oxygen demand peaks (Ahles and Milton, 2016). Even in natural relocation settings, success is contingent on environmental stability. The failure of the October 2024 nest highlights the risks of late-season storm events. However, other failures point to potential handling stress. Relocation should therefore ideally occur within the first 12 h (Abella et al., 2007; Limpus et al., 1979). Any rotation after this period risks detaching the vitelline membrane, leading to the late-stage developmental arrest observed in our dataset (Limpus et al., 1979; Miller, 1985). Therefore, in conservation practice, if a nest must be moved due to an emergency contingency more than 24 h after the female adult turtle has laid it, the eggs must be kept perfectly upright in their original orientation. However, even with extreme care, the vibration and movement associated with relocation at that stage often lead to the late-stage developmental arrest, as recorded in our Maltese dead turtle embryo dataset. While our environmental correlations remain conceptual, the morphometric data strongly support the hypothesis that the physical stress of relocation may contribute to late-stage developmental arrest in this emerging loggerhead turtle rookery.

4 Regional context and pathogens

As indicated earlier, recent years have observed an unprecedented, exponential increase in loggerhead nesting across the Western Mediterranean, particularly in Italy, Spain, and France (Legambiente, 2024; Mancusi et al., 2024; Hochscheid et al., 2022), driven by climate-induced range expansion (Mancino et al., 2022; Luna-Ortiz et al., 2024). However, this expansion brings new risks. For instance, nests expanding into new northern territories such as Veneto, Italy, have exhibited high embryo mortality due to Sea Turtle Egg Fusariosis (STEF) (Pietroluongo et al., 2023). This pathogen (Fusarium solani species complex) thrives in substrates with high organic loads and specific clay/silt content, highlighting the need for integrated sediment management to prevent beach conditions that exacerbate the disease (Sarmiento-Ramírez et al., 2014).

5 Emerging conservation concerns: the geomagnetic mismatch

Looking ahead, the sustainability of these recent turtle nesting grounds faces a concerning challenge: the geomagnetic mismatch. Sea turtles imprint the magnetic signature of their natal beach (Lohmann et al., 2013; Brothers and Lohmann, 2015). However, under a “coastal squeeze,” the physical beach may disappear due to sea-level rise or be replaced by hard infrastructure (sea walls), while the magnetic coordinates remain. Hatchlings internalize both the magnetic inclination and the intensity of the specific beach. As coastlines retreat due to sea-level rise, the specific isolines on which the turtles are imprinted remain at fixed GPS coordinates, assuming minimal shifts in the Earth's magnetic field; however, those coordinates may sit on top of concrete structures in the future. Furthermore, standard coastal armoring utilizes ferromagnetic materials (e.g., steel rebar) that create local magnetic anomalies, distorting the geomagnetic signatures turtles rely on (Irwin and Lohmann, 2003; Putman et al., 2014). While not directly measured in our current dataset, this phenomenon represents an evolutionary trap whereby future turtles may be guided to “empty” paved coordinates. In highly developed small island states such as Malta, recognizing this as an emerging hypothesis is vital for long-term coastal planning.

6 Discussion: advancing sustainability through science integration

The challenges identified above cannot be solved by biology alone. Advancing ocean sustainability requires a paradigm shift toward better science integration, valuing diverse knowledge systems as advocated by Vozzo et al. (2025)). In Malta, “beach management” is often siloed as a tourism issue, while “nesting” is treated as strictly biological. Our morphometric “ledger” bridges this gap by converting biological outcomes (e.g., arrested embryos) into indicators of physical beach quality. This technique allows conservationists to speak the language of planners: sediment quality and acoustic zoning.

Postgraduate research students and volunteers, referred to as the “leaders of tomorrow” and identified by Vozzo et al. (2025)), largely drove the collection of this 6-year dataset. These early-career marine conservation professionals are in a unique position to move between field necropsies and policy advocacy in a smooth and easy way. Formalizing their role in Malta's management strategy ensures that “intergenerational knowledge transfer” (a key Ocean Decade goal) occurs alongside data collection.

To avoid the “protection gap” (Table 2), data can be shared and promoted. Following Vozzo et al.'s (2025)) recommendation for “active and transparent data sharing,” this morphometric dataset acts as an open-access baseline. Integrating these data into unified, cross-border repositories such as the MEDTURTLE database, developed under the LIFE MEDTURTLES framework, allows regional managers to track emergent populations (LIFE MEDTURTLES, 2024). This ensures that local conservation efforts align with the broader RAC/SPA guidelines for Mediterranean sea turtle protection (UNEP/MAP RAC/SPA, 2004).

Table 2. Loggerhead turtle nesting frequency and nest success rates by locations in the Maltese Islands (2020–2025).

¹ For Fajtata Bay (n = 1), the value represents the absolute success rate of the single recorded nest.

²The three nest clutches laid at Ghajn Tuffieha were relocated to Mixquqa Bay due to the risk of inundation.

7 Limitations

It is important to acknowledge the limitations of the current perspective. The relationships proposed between environmental pressures (e.g., coastal noise and substrate compaction) and late-stage developmental arrest are currently discussed at a conceptual level and are supported by broader literature rather than by direct, localized quantitative logging. Our study did not utilize acoustic sensors or soil penetrometers alongside the nests. Future transdisciplinary research should aim to pair morphometric biological data with continuous microclimate and environmental logging to explicitly test these correlations and, when possible, increase sample size. The dead loggerhead turtle embryo specimens collected from the Maltese turtle nests throughout the 6 years of research constituted the total possible sample size (n = 122 from 14 nests). Future research efforts may compare and corroborate the results obtained in this study.

8 Conclusion and management recommendations

Embryonic morphometrics provide a diagnostic tool that goes beyond “nest success vs. failure.” However, applying this tool requires an integrated approach, as advocated by Vozzo et al. (2025)). To transition from opportunistic nesting to a stable rookery, Malta would benefit from the implementation of evidence-based, process-oriented management. We suggest the following specific actions:

• Establish zonal sensory corridors with measurable thresholds: it was proposed that persistent low-light and low-noise corridors be maintained during the approximately 50-day incubation period. Rather than establishing general “quiet zones,” policies should establish precise thresholds, such as restricting artificial light at night (ALAN) below 0.1 lux at the sand surface. The targeted use of low-pressure sodium lamps, amber LED shielding, or strictly enforced curfews on beachfront commercial lighting during the 50-day window may be viable solutions. Low-frequency vessel noise propagates through the substrate and may intensify vibrational stress on late-stage embryos. Therefore, “No-wake” marine acoustic zones should be implemented to mitigate this. Essential natural buffer zones and coastal geomorphology, such as dunes, should also be preserved.
• Implement rigorous, biologically informed relocation protocols: relocation should be regarded as a last resort. In order to prevent late-stage developmental cessation, the Environment and Resources Authority (ERA) must mandate that relocation occur exclusively within 12 h of oviposition, prior to the vitelline membrane's attachment to the inner shell membrane. In order to prevent embryonic shearing, eggs must be transported in containers that are firm and vibration-dampening for a period of up to 24 h. Additionally, they must be maintained in their exact original vertical orientation.
• Institutionalize conservation leadership: early-career marine conservation professionals (ECMCPs) need to be involved in Environment and Resources Authority (ERA) consultations and decision-making committees. This guarantees that management adheres to the criteria of the Regional Activity Centre for Specially Protected Areas (RAC/SPA) and presents innovative, transdisciplinary perspectives on local policy.
• Open-source the “biological ledger”: post-hatching necropsy techniques and Miller's (1985)) staging criteria should be standardized. In addition, it was proposed that annual morphometric evaluations of unhatched eggs to open-access regional databases, such as MEDTURTLE, be disseminated. This approach aligns with the goals of the UN Ocean Decade by enabling cross-border comparisons of subterranean microclimates and “incubation health” in the Mediterranean region.
• Spatial planning to address the geomagnetic discrepancy: it was proposed that Geographic Information Systems (GIS) be implemented to establish historical magnetic inclination and intensity isolines in relation to contemporary urban infrastructure and coastal erosion forecasts. This proactive approach is expected to address the evolutionary challenge of the geomagnetic discrepancy. ECMCPs, coastal planners and authorities, such as the Environment and Resources Authority (ERA), the Planning Authority (PA), and Transport Malta (TM), can work together to pinpoint the precise surface or degraded coordinates where future turtles are physiologically predisposed to arrive by superimposing these records. This would allow for best management practices and development to be effectively coordinated.

By treating the unhatched embryo as a data-rich ledger and integrating this biological data with policy and physical environmental parameters, we can secure a sustainable future for our shared coastlines that conserves protected species, such as loggerhead sea turtles.

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Footnotes

Edited by: Samantha Farquhar, United States Coast Guard Academy, United States

Reviewed by: Luca Ceolotto, University of Padua, Italy