The Predation of Pinna nobilis (Mollusca) Juveniles by the Spiny Sea Star Marthasterias glacialis (Echinodermata) in the Sea of Marmara

Abstract

The critically endangered bivalve species, the pen shell Pinna nobilis, has been suffering from an epidemic disease in the Mediterranean Sea, and its populations have totally disappeared in many areas across the basin. The Sea of Marmara’s populations of the pen shell, which were previously assumed to have remained unaffected by the epidemic disease because of the special hydrographical condition of the sea (brackish water), have been devastated after the catastrophic mucilage event suddenly outburst in November 2020 in the region. The settlement of healthy juvenile specimens of the species was frequently observed in the area after the mucilage event. However, an unexpected predator of juvenile specimens of the pen shell, the spiny sea star Marthasterias glacialis, was encountered in the area in January 2023. Two cases were observed and the spiny sea star has a capability of crashing one valve of the sea-pen through which it consumes its flesh. This predation might have the potential to affect populations of P. nobilis, which has been struggling to survive in the region after its mass mortality due to the mucilage event. Such data are of utmost importance to imply effective conservation planning for the sustainability of the remaining populations of the pen shell in the region.

1. Introduction

The fan mussel or pen shell Pinna nobilis Linnaeus, 1758, endemic to the Mediterranean Sea, has been suffering from an epidemic disease mainly caused by a sporozoon parasite, Haplosporodium pinnae, Catanase et al., 2018 associated with various bacteria [1,2,3,4,5,6]. The pathogens are host-specific as they do not infect the congeneric species P. rudis Linnaeus, 1758 in the Mediterranean Sea. However, detailed investigations approved that microbial infections are crucial in some mortality events, in the absence of H. pinnae. Moribund individuals of the species collected at several localities indicated the presence of bacteria such as Mycobacterium simiae Karassova et al., 1965 complex and Vibrio mediterranei Pujalte and Garay, 1986 [7]. The disease first emerged in Autumn 2016 [1] on the Spanish coast—at least, it was first reported from there—and then spread across the Mediterranean Sea a few years later [6]. The mortality of P. nobilis is very high, the death rate has reached up to 90–100% in the whole Mediterranean basin [1,8], as well as 99% in the Çanakkale Strait [9], a gate to the Sea of Marmara. The epidemic disease is seriously threatening the survival of P. nobilis in the region and, therefore, IUCN has recently regarded it as a critically endangered species [10].

The current system in the Mediterranean Sea [2], an intermediate host (may be a planktonic species) [3], or the movement of vessels among harbors (through ballast water) [11] were thought to be probable reasons for the rapid spread of the disease across the Mediterranean Sea. The outbreak of the disease might also be closely related to environmental variables; for instance, parasites/bacteria are frequently effective in temperatures above 13.5 °C and in a salinity value between 36.5 and 39.7 PSU [3]. Within this perspective, the low salinity character (between 18 and 24 PSU) of the surface waters of the Sea of Marmara (0–25 m depth) due to the influx from the Black Sea of brackish water was considered to constitute an ecological barrier for the spread of the pathogens to the region, and many healthy populations (>95% were alive) of the pen shell were reported at different localities in the region in September 2020 [12]. This unique feature had made the Sea of Marmara a refuge area (disease-free area) for the species. However, after a catastrophic, widespread mucilage phenomenon that was first seen in November 2020 and affected the whole basin including the Çanakkale and İstanbul Straits, many habitats and species were adversely affected by mucilage accumulations and oxygen depletion due to bacterial biodegradation, causing the mass mortality of several species including P. nobilis; 88% of its population were reported to be dead in the southern Marmara Island region [13]. A more optimistic scenario was represented in the same period (years 2020–2021) from the Gemlik Peninsula [14,15], in which a relatively high rate of healthy individuals (>80%) was observed.

In addition to the anthropogenic and climatic factors adversely affecting the distribution and population density of the pen shell P. nobilis in the Mediterranean Sea, predation by carnivorous species in different animal groups (vertebrate and invertebrate species) causes the natural mortality of the species. The common octopus, Octopus vulgaris Cuvier, 1797, was reported to be the main predator of P. nobilis adults. The common octopus tightly holds the pen shell with its muscular and strong arms, slowly pulls the pen shell valves apart, and eventually preys upon its flesh [16,17]. In an experimental study, a fan mussel of 45–46 cm shell length was fed by an octopus specimen of 250–500 g in weight, and the time it took to open the valves by the octopus was observed to be 137 min [16]. The juvenile phase of the pen shell is more susceptible to predation, and the gastropod Hexaplex trunculus (Linnaeus, 1758) [18] and sparid fish Sparus aurata [19] led to the mortality of the pen shell juveniles. However, other species of Sparidae and fish from different families, crabs, and molluscs might also predate on juveniles of P. nobilis [20]. Loggerhead sea turtle, Caretta caretta (Linnaeus, 1758), was also observed to feed upon pen shell adults along the coasts of Cyprus, Lebanon, and Türkiye [10].

The present study reports a previously undocumented case of the predation of the pen shell (P. nobilis) juveniles by the large spiny sea star Marthasterias glacialis (Linnaeus, 1758) in the Sea of Marmara. The study also gives an abundance of M. glacialis and live P. nobilis juveniles at monitoring stations and depth intervals. Some null hypotheses as to whether species abundances vary depending on locality and depth were tested.

2. Materials and Methods

2.1. Study Area

The Sea of Marmara is located between the Mediterranean and the Black Seas and its surface area is approximately 11,350 km². It is connected to the Aegean Sea by the Çanakkale Strait (Dardanelles) in the south, and to the Black Sea by the Istanbul Strait (Bosphorus) in the north. There are three deep basins separated by sills in the sea, namely Tekirdag Basin, Central Basin, and Çinarcik Basin. The deepest point of the sea is 1390 m, the widest point in the north-south direction is 80 km, and the longest axis in the east-west direction is 280 km. As it is located between two seas of different hydrographical features, the Sea of Marmara consists of two stratified water bodies; the upper layer contains the Black Sea-originated water that is less saline (from north to south 18–25 PSU) and the lower layer contains the Mediterranean-originated water that is highly saline (almost homogenous across the basin 36–38 PSU) [21]. The study area, the southern Marmara Islands, consists of 23 islands of different surface areas. Marmara Island, Paşalimanı Island, Avşa (Türkeli) Island, Ekinlik Island, and Koyun Island are relatively big islands in the area.

2.2. Samplings

Within the scope of the MARIAS project (Addressing Invasive Alien Species Threats at Key Marine Biodiversity Areas Project), several scientific cruises were performed in 2020–2023 to find out the distribution of invasive alien species [especially Rapana venosa (Valenciennes, 1846) and Asterias rubens Linnaeus, 1758] at 10 stations in the south Marmara Islands and Kapıdağ Peninsula in the Sea of Marmara (Figure 1).

At the monitoring stations, two 100 m ropes, perpendicular to the coastline, were placed on the seafloor, starting from a 0.5 m depth. An area of 4 m wide (2 m left and 2 m right from the rope) along the rope was observed by two scientific divers; the total surveyed area of each transect was 400 m². The abundances of alien species as well as some common species (such as the pen shell P. nobilis and the native spiny sea star M. glacialis) were noted along the transects.

The present paper is only focused on the data gathered during the January 2023 cruise of the MARIAS project to the south Marmara Islands. In order to assess the M. glacialis’s abundance at stations and depths, each station is divided into two depth intervals: 0–10 m and 10–20 m depths. In order to find out the importance of the geographical locations on the distribution of M. glacialis and alive juveniles of P. nobilis, the stations in the area were clustered into four localities: locality I comprised the stations (stations 1–3) near the mainland Kapıdağ Peninsula; locality II possessed the stations (stations 4–6) located around the western islands, which are far from the mainland Kapıdağ Peninsula; locality III included the stations located in the southern (station 8) and western (station 7) parts of the south Marmara Islands; and locality IV had the stations (stations 9 and 10) located in the northern part of the Marmara and Asmalı Islands where the depth interval of 10–20 m had dense beds of the black mussel (Mytilus galloprovincialis Lamarck, 1819).

The spiny star individuals occurring along the two transect lines at each depth of the stations were counted and noted underwater on a waterproof stone notebook by the scientific divers. Some stations (stations 1, 4, 5, and 8) were shallow and the transects only extended to a depth between 0 and 10 m (3 m depth at station 1).

To quantify the distribution and depth preferences of the juvenile individuals of the pen shell P. nobilis in the area, as in the study on M. glacialis populations, depths of 0–10 m and 10–20 m at each station were investigated separately. The lengths of the unburied shell of the juvenile P. nobilis individuals were measured by a ruler in situ. Along the transects at each depth interval, the number of live P. nobilis individuals were counted.

During the cruise to the study area in January 2023, large specimens of the spiny sea star M. glacialis were observed grasping juveniles of the pen shell P. nobilis at two locations (stations 1 and 2). This uncommon, unreported case was recorded in situ with an underwater camera (GoPro Hero 9, Teknosa, İzmir, Türkiye). The underwater photographs of M. glacialis at stations were taken by the Nikon D7000 camera (Promar, İstanbul, Türkiye) with a Sea & Sea housing and two strobes (Sea & Sea YS-D1).

2.3. Statistical Analysis

Using the locality (four levels) as a random factor and the depth intervals (two levels) as a fixed factor, a two-way distance-based permutation multivariate analysis of variance (PERMANOVA) [22] was run to test if the abundances of the spiny star M. glacialis and the fan mussel P. nobilis differ significantly in the depth intervals and localities. The euclidean distance matrix was constructed for the analysis. Differences were considered significant at p < 0.05. The analysis was performed by using the software PRIMER v7.

3. Results

3.1. Distribution and Abundance of Marthasterias glacialis in the Area

The spiny sea star M. glacialis was observed both in the soft and hard substrata of all stations. It occurred in all dived depths (0–25 m depth) and habitats, but dominantly inhabited mussel beds at 10–20 m depths. No specific density of this species was estimated in the depth strata of the stations, but it attained maximally 2–3 ind. m⁻² at 15 m depths at station 9. The juveniles and adults of the species were encountered at stations, with the larger arms being around 25–30 cm. It was frequently observed feeding on black mussels (Mytilus galloprovincialis) in the area (Figure 2).

The abundances of M. glacialis in the area ranged from 1 (station 1, 0–3 m depth) to 95 (station 9, 10–20 m depth) (

Table 1. Abundances of the spiny sea star Marthasterias glacialis and juvenile pen shell Pinna nobilis at localities, stations, and depth intervals. Abundances are the sum of the individuals of the species counted along the two transects in each depth interval.

Locality	Station	Depth Interval	Habitat	M. glacialis	P. nobilis
I	1	0–3 m	Sand	1	5
I	2	0–10 m	Rock	2	4
		10–20 m	Rock/sand	4	-
I	3	0–10 m	Rock	2	-
		10–20 m	Sand	5	-
II	4	0–10 m	Rocks/sand	2	1
II	5	0–10 m	Sand	1	1
II	6	0–10 m	Rocks/sand	1	2
		10–17 m	Sand	3	-
III	7	0–10 m	Rock	6	-
		10–20 m	Rock/sand	9	-
III	8	0–10 m	Rock/sand	7	3
IV	9	0–10 m	Rock	10	-
		10–20 m	Mussel bed	95	-
IV	10	0–10 m	Rock	12	-
		10–20 m	Mussel bed	80	-

). A relatively high number of individuals of the species were counted at stations along the coasts of the Marmara and Asmalı Islands. The abundance of the species differed significantly among the localities (PERMANOVA test, p < 0.05), but not the depth intervals (PERMANOVA test, p > 0.05) (

Table 2. Main tests of PERMANOVA based on the abundance data of Martasterias glacialis in the region. Locality was used as a random factor, and the depth interval as a fixed factor.

Source	Degree of Freedom	Sum of Square	Mean of Square	Pseudo-F	P (perm)
Locality	3	6109	2036	140.6	0.01
Depth Interval	1	2021	2020	0.99	0.515
Depth Interval × Locality	2	4078	2039	140.8	0.01
Residue	9	130	14.5
Total	15	12,300

). The combination of the locality and depth interval also significantly affected the distribution of the spiny star in the Sea of Marmara.

3.2. Distribution and Abundance of Juvenile Pinna nobilis in the Area

In the January 2023 cruise to the south Marmara Islands, several healthy juvenile individuals were observed in the area. Along the transects, the abundances of live juvenile individuals of P. nobilis changed between 1 (stations 4 and 5) and 5 (station 1). At four stations (stations 3, 7, 9, and 10), no live P. nobilis individuals were observed. Stations 1 and 2 were known to have dense beds of P. nobilis before the catastrophic mucilage event happened at the end of 2020. The abundances of live juvenile P. nobilis did not change significantly among the localities and depth intervals (PERMANOVA test, p > 0.05) (

Table 3. Main tests of PERMANOVA based on the live juvenile abundance data of Pinna nobilis in the region. Locality was used as a random factor, and the depth interval as a fixed factor.

Source	Degree of Freedom	Sum of Square	Mean of Square	Pseudo-F	P (perm)
Locality	3	4.96	1.65	0.69	0.59
Depth Interval	1	7.29	7.29	4.34	0.15
Depth Interval × Locality	3	4.96	1.65	0.69	0.59
Residue	8	19.17	2.40
Total	15

).

3.3. Predation of Pinna nobilis by Marthasterias glacialis

At stations 1 and 2, two specimens of M. glacialis with the larger arm of 25 cm were observed on the shells of live juvenile P. nobilis individuals. The depth of observation was 3 m and the habitat was a sandy bottom with Cymodocea nodosa (Ucria) Asch. meadows. The lengths of the unburied shells of P. nobilis were around 10–12 cm, and the width of the shells was 8–9 cm. While the individuals of M. glacialis were gently removed from the shells of the P. nobilis juveniles, it was observed that one shell of the juveniles was broken by a sea star, through which whose guts were engulfing the juvenile’s flesh. One arm of the sea star was holding the opposite shell and three arms were holding the broken shell (Figure 3). The flesh of the juvenile P. nobilis individuals was observed to be partly consumed by the spiny sea stars.

4. Discussion

The distribution and healthy status of the pen shell P. nobilis were previously studied in the south Sea of Marmara [12,14,15,23]. The fate of the pen shell in the region can be divided into two stages: before and after the catastrophic mucilage event suddenly outburst in November 2020. Before the mucilage event, a number of healthy individuals of the pen shell were encountered on the soft substrata of the shallow water of the south Marmara Islands where the present study was carried out. Although the mass mortality of the species was reported at the south gate of the Sea of Marmara, the Çanakkale Strait and the north Aegean Sea, because of the deadly epidemic disease caused by the sporozoon Haplosporidium pinnae and other pathogens [24,25], the Sea of Marmara population of the species appeared to be free of this disease as the upper layer of the sea contains brackish water (0–25 m), which might have acted as a physical barrier for the spread of the epidemic [12]. In the September 2020 cruise to the area, healthy populations of the species (>90% of all individuals) were determined at stations where no sign of the epidemic was observed. The maximum density of the species was estimated as 40 ind. 10 m⁻², with an average density ranging from 0.6 ind. 10 m⁻² to 24 ind. 10 m⁻² at stations. After the mucilage event, the mass mortality of the sessile and sedentary benthic species including algae, sponges, cnidarians, molluscs, and crustaceans were observed, and the previously observed healthy P. nobilis beds have been largely devastated, with the dead shells standing as tombstones to signify the mass mortality of the species [13,14,15]. Although changing with the locality in the region, almost 80% of the southern Marmara Islands’ populations of the species were dead [13,14]. However, healthy juvenile specimens were frequently observed in the area, showing a sign of the species’ recovery. Live juvenile individuals of Pinna nobilis were observed at depths ranging from 0.8 to 8 m, and its maximum density was estimated as 0.5 ind. 100 m⁻² at station 1 [13]. In the last cruises to the area in January and March 2023, several healthy juvenile specimens were encountered in the shallow-water soft substrata of some stations. In the January 2023 cruise, a total of 16 live juvenile individuals of the pen shell were found along the transects at stations, the majority of which were settled at stations 1 and 2 where dense P. nobilis beds were previously reported [11]. It seems that juvenile individuals of P. nobilis prefer a sandy bottom with the seagrass Cymodocea nodosa in the shallow waters. However, its abundance appeared not to be significantly changed in depths and localities. However, more data are required to assess its depth and locality preferences.

In the January 2023 cruise, we come across an unexpected enemy of the juvenile individuals of the sea pen, the spiny sea star Marthasterias glacialis. The spiny sea star is known to be a major predator on a variety of marine animals, including crustaceans, bivalves, and sea-urchins, at depths 0–180 m [26,27,28,29]. It is the biggest sea star in the Mediterranean Sea and its diameter can reach as much as 84 cm [26]. It abundantly occurs in some localities in the Atlantic Ocean and Mediterranean Sea. Its density is estimated to be 1.7 ind. m⁻² in the east Atlantic [28] and 1 ind. 250 m⁻² in the western Mediterranean Sea [29]. In the Sea of Marmara, it occurs in the whole basin, including the Black Sea entrance of the İstanbul Strait [26]. No study specific to the distribution and population characteristics of this species in the Sea of Marmara has been carried out to date, except for [30], who studied its relationship with the invasive alien sea star Asterias rubens and estimated its maximum density as 0.54 ind. m⁻² in the northern Sea of Marmara. Çinar et al. [31] also encountered this species on the soft substrata collected by bottom-trawl haulings between 40 and 90 m depths around the southern Marmara Islands, and reported its maximum density as 17 ind. 0.032 km⁻² and the maximum biomass (wet weight) as 166 ind. 0.032 km⁻². In the present study, it occurs at all stations and its density was found to reach up to 2–3 ind. m⁻² at a 10–15 m depth in a mussel bed at station 9. The statistical analysis indicated that this species significantly differs at localities and depths, showing its patchy distributional pattern in association with the presence of mussel beds at 10 m downwards. However, a purpose-oriented long-term observation at different depth strata and habitats is required to determine its real spatio-temporal population distribution in the region.

During the cruise to the Sea of Marmara in January 2023, two cases of juvenile pen shell individuals being eaten by the spiny sea star were observed. The lengths of the unburied shells of P. nobilis were around 10–12 cm, with a width of 8–9 cm, a size that remains within the “refuge size (8 cm width)” for the species proposed by [20]. Juveniles of the pen shell are known to be more susceptible to predation, mainly by fish, other gastropods, and crabs [20]. Adults of this species can be consumed by large predators such as the common octopus O. vulgaris [32], which uses its muscular arms to open shells of P. nobilis. However, to this day, the sea star M. glacialis has not been reported to prey upon P. nobilis juveniles. The predation was not rare, as it was observed at two stations out of seven (ca. 30%). Considering the wide distribution of M. glacialis in the Sea of Marmara and its abundant occurrences in some habitats and depths, this predation might have the potential to considerably affect populations of P. nobilis, which seems to be at the initial phase of recolonization in the region after its mass mortality due to the mucilage event in the region at the end of 2020. It is unknown at this stage whether the voracious predator M. glacialis normally feeds on P. nobilis juveniles, or has incorporated it in its diet after the decline in the mussel populations in some areas due to the mucilage event and the predation by two invasive alien species, namely Rapana venosa and Asterias rubens.

5. Conclusions

It has been well documented that the Mediterranean heritage, the pen shell Pinna nobilis, faces extinction because of its mass mortality across the Mediterranean Sea, which is mainly caused by the protozoan H. pinnae and concurrent polymicrobial infections. In addition, several human-implicated factors such as pollution, anchoring, harvesting, habitat degradation, and global warming have caused the decline of P. nobilis populations in the region. The adult and juvenile individuals of P. nobilis are known to be subjected to predation by several native species, such as octopus and fish. Indeed, the mortality of this species by the predation of native species has not been considered to significantly affect its survival in the region. However, taking the mass mortality of this species in both the Mediterranean Sea and the Sea of Marmara and its sensitive status (critically endangered species) into account, the spiny starfish predation might significantly reduce the survival chances of P. nobilis in the regions. In this respective, we highly recommend developing a cage system to keep juvenile P. nobilis individuals away from this predation, at least until the healthy P. nobilis population reaches a certain level in the region. Future studies to be performed on the bio-ecological features of this species will shed more light on the conservation strategies to be developed to sustain the population of this species in the region. In this respective, a long-term monitoring study needs to be urgently implemented in order to assess the healthy status and population characteristics of P. nobilis and to reveal the effects of the predation of M. glacialis on the P. nobilis populations in the area.