Giant clam growth in the Gulf of Aqaba is accelerated compared to fossil populations

The health of reef-building corals has declined due to climate change and pollution. However, less is known about whether giant clams, reef-dwelling bivalves with a photosymbiotic partnership similar to that found in reef-building corals, are also threatened by environmental degradation. To compare giant clam health against a prehistoric baseline, we collected fossil and modern Tridacna shells from the Gulf of Aqaba, Northern Red Sea. After calibrating daily/twice-daily growth lines from the outer shell layer, we determined that modern individuals of all three species ( Tridacna maxima , T. squamosa and T. squamosina ) grew faster than Holocene and Pleistocene specimens. Modern specimens also show median shell organic δ 15 N values 4.2‰ lower than fossil specimens, which we propose is most likely due to increased deposition of isotopically light nitrate aerosols in the modern era. Nitrate fertilization accelerates growth in cultured Tridacna , so nitrate aerosol deposition may contribute to faster growth in modern wild populations. Furthermore, colder winter temperatures and past summer monsoons may have depressed fossil giant clam growth. Giant clams can serve as sentinels of reef environmental change, both to determine their individual health and the health of the reefs they inhabit.

Synchronized broadcast spawning by six invertebrates (Echinodermata and Mollusca) in the north-western Red Sea

On the evenings of June 11 and 12, 2019, 5 and 6 days before full moon, broadcast spawning by four echinoderm species and two mollusc species was observed on the Marsa Shagra reef, Egypt (25° 14′ 44.2" N, 34° 47′ 49.0" E). Water temperature was 28 °C and the invertebrates were observed at 2–8 m depth. The sightings included a single basket star Astroboa nuda (Lyman 1874), 2 large Tectus dentatus (Forskal 1775) sea snails, 14 individuals of the Leiaster cf. leachi (Gray 1840) sea star and 1 Mithrodia clavigera (Lamarck 1816) sea star, 3 Pearsonothuria graeffei (Semper 1868) sea cucumbers, and 2 giant clams, Tridacna maxima (Röding 1798). The observations presented here provide relevant information on broadcast spawning of non-coral invertebrate taxa in the Red Sea, where spawning is considerably less well documented than in other tropical geographical regions such as the Indo-Pacific and Caribbean.

Phylogeographic Analysis Suggests a Recent Population Bottleneck in the Rare Red Sea Tridacna squamosina

Giant clams are an important ecological component of coral reefs in the Red Sea, as they enhance the reef’s productivity and provide habitat that can increase diversity. Three species of giant clams, namely Tridacna maxima, T. squamosa, and T. squamosina have been described within the Red Sea. However, due to its scarcity, information about the distribution and ecology of T. squamosina in the Saudi Arabian Red Sea is still lacking. This study used DNA barcoding to confirm the identity of the rare T. squamosina in the Farasan Banks. Six mtCOI fragments (500 bp) of T. squamosina were successfully amplified using the SQUA-primers for the first time. We used our data along with 18 reference sequences (16S) from the online database to assess the genetic diversity and population structure of T. squamosina. Low genetic diversity among the T. squamosina populations inferred from the 16S sequences implies a recent bottleneck for this species, which is supported by their historically higher diversity based on the coalescent-based estimator. Given the small population abundance and limited genetic variation of T. squamosina, it may warrant immediate local protections such as biobanking and fertility preservation programs as well as effective integrated coastal zone management plans.

Drivers of the Abundance of Tridacna spp. Giant Clams in the Red Sea

Giant clams (Subfamily Tridacninae), are important members of Indo-Pacific coral reefs, playing multiple roles in the framework of these communities. Although they are prominent species in Red Sea reefs, data on their distribution and densities in the region are scarce. The present study provides the first large-scale survey of Red Sea Tridacna spp. densities, where we examined a large proportion of the Saudi Arabian Red Sea coast (1,300 km; from 18° to 29°N). Overall, Tridacninae were found at densities of 0.19 ± 0.43 individuals m–2 (±SD). Out of the total 4,002 observed clams, the majority (89%) were Tridacna maxima, with 0.17 ± 0.37 individuals m–2, while only 11% were Tridacna squamosa clams with 0.02 ± 0.07 individuals m–2. We also report on a few (total 6) Tridacna squamosina specimens, found at a single reef. We identified different geographical parameters (i.e., latitude and distance to shore) and local environmental factors (i.e., depth and reef zone) as the main drivers for local Tridacna spp. densities. Our results show that the drivers influencing the densities of Red Sea giant clams are complex due to their co-occurrence and that this complexity might explain the high variation in Tridacninae abundances across the Indo-Pacific, but also within a given reef. We also estimate that giant clam calcification likely contributes to an average of 0.7%, but potentially up to 9%, of the overall mean calcium carbonate budget of Red Sea coral reef communities.

Inventarisasi kima (Tridacnidae) di Pulau Batu Bilis, Desa Kelarik Kecamatan Bunguran Utara, Kabupaten Natuna, Kepulauan Riau, Indonesia

Batu Bilis Island Kelarik Village is surrounded by shallow coral reefs with a fairly bright level of water brightness, making it a habitat for important economic biota in the form of clam shells (Tridacnidae). The purpose of this study is to identify the types of clam shells and determine the value of shellfish. Closing data is carried out on 9 (nine) points. Using the Benthos Belt Transect method, with an observation area of 140 m². The results were found 2 (two) types of clams, namely Hole Clams (Tridacna crocea) and Small Clams (Tridacna maxima). The highest density of Hole Clams is at point 9 (nine) with a value of 1.142.9 individual/ha, then the lowest density at point 7 (seven) 142,86 individual/ha. The highest density of Small Clams is at point 7 (seven) with a density value of 928,57 individual/ha and the lowest density at point 8 (eight) with a value of 71,43 individual/ha. The quality of physical-chemical parameters in Batu Bilis waters is still in the normal range for the life of clams shells such as temperature 29,4⁰C, salinity 29,5‰, current speed 0,08 m/s and brightness 100%.

Are giant clams (Tridacna maxima) distractible? A multi-modal study

To properly assess risk, an animal must focus its attention on relevant external stimuli; however, attention can be reallocated when distracting stimuli are present. This reallocation of attention may interfere with an individual’s ability to effectively assess risk and may impede its response. Multiple stimuli presented together can have additive effects as distractors, and these include stimuli in different modalities. Although changes in noise and water flow are detectable by some bivalves, this has not been studied in the context of risk assessment or distraction. We experimentally exposed giant clams (Tridacna maxima) to changes in water particle movement through underwater sound (motorboat noise) and increased water flow to determine whether these stimuli, individually or together, modified risk assessment or caused distraction. We found that clams responded to sound, flow, and their combination by increasing frequency of mantle retractions (a potential anti-predator response) when exposed to a stimulus. Sound alone did not change risk assessment in either the latency to close or to reemerge following closure. However, when exposed to both stimuli simultaneously, clams increased their latency to close. We suggest that clams perceive sound and flow in an additive way, and are thus distracted. Interestingly, and uniquely, clams discriminate these multimodal stimuli through a single sensory modality. For sessile clams, anthropogenic noise is detectable, yet unavoidable, suggesting that they be especially vulnerable to marine noise pollution.