Observations on the population characteristics of Apionsoma ( Apionsoma ) misakianum ( Sipuncula : Phascolosomatidae ) , a new species for the Mediterranean fauna

The phylum Sipuncula comprises a total of 31 species in the western Mediterreanean, 25 species from the eastern Mediterranean and 33 species in the whole Mediterranean basin (PancucciPapadopoulou et al., 1999; Açik et al., 2005). The first record of Sipuncula from the Turkish coasts was given by Ostroumoff (1896), who found Nephasoma (Nephasoma) diaphanes diaphanes (Gerould, 1913) (as Petalastoma minutum Keferstein, 1863) at 45-77 m depth in the Sea of Marmara. Later, Aspidosiphon (Aspidosiphon) muelleri Diesing, 1851, Phascolion (Phascolion) strombus strombus (Montagu, 1804), and Nephasoma (Nephasoma) diaphanes diaphanes SCIENTIA MARINA 71(3) September 2007, 571-577, Barcelona (Spain) ISSN: 0214-8358

The genus Apionsoma Sluiter, 1902 is represented by two species in the Mediterranean Sea: Apionsoma (Apionsoma) murinae bilobatae (Cutler, 1969) and Apionsoma (Apionsoma) trichocephalus Sluiter, 1902.This genus is mainly characterised by having four retractor muscles, a long introvert, rings of hooks (absent in Apionsoma (A.) trichocephalus) that have accessory spinelets at the bases, and two biloped nephridia.Outside the Mediterranean, Apionsoma (A.) murinae bilobatae was previously reported from the North Atlantic and Indian Oceans, and Apionsoma (A.) trichocephalus from the western and eastern Atlantic, western Pacific and Indian Oceans (Cutler, 1994).
The examination of benthic materials collected from the Aegean Sea between 0 and 195 m depth yielded a new Apionsoma species new to the Mediterranean fauna, Apionsoma (A.) misakianum Ikeda, 1904.This species was originally described from the Sea of Japan and was subsequently found from the Indian, Pacific and western Atlantic Oceans (Cutler, 1994).It is easily distinguished from the other species of Apionsoma by its small rounded papillae on the posterior part of the trunk, mostly equal bilobed nephridia and small hooks with basal spinelets on the introvert.
The present paper aims to re-describe this species based on the Aegean Sea material and to add a contribution to its distributional and ecological features.

MATERIAL AND METHODS
Specimens of Apionsoma were collected at 25 stations in the Aegean Sea between July and September 2000 (Fig. 1).Samples were taken by an anchor dredge at depths ranging from 0 to 195 m.On board the ship, benthic materials were first sieved with a 0.5 mm mesh and the retained fauna was put into separate jars containing 10% seawaterformalin solution.In the laboratory, the samples were sorted according to taxonomic groups under a stereomicroscope and preserved in 70% ethanol.Afterwards, sipunculans were identified and counted under stereo-and compound microscopes.
A number of biometrical features (i.e.length and width of trunk and introvert, length of papillae, hooks, etc.) of the smallest and largest individuals of the species were measured using an ocular micrometer.The wet weight of 50 specimens was estimated using a balance of 0.0001 g sensitivity.
To characterise population structures of Apionsoma (A.) misakianum in the area, regression and Pearson product-moment correlation analyses were performed to assess the relationship among the biometrical measurements taken.
Photographs of specimens of Apionsoma (A.) misakianum were taken using a digital camera (Olympus, Camedia C-7070) attached to the compound microscopes. The
The majority of specimens of this species were found on sandy-muddy sediment (92.3%), followed by muddy-sand (7%) and sand (0.7%).Specimens of this species were generally associated with serpulid tubes and mollusc shells in this study.They inhabited the empty tubes of serpulid tube worms such as species of Vermiliopsis and Serpula, the empty shell of Antalis panorma (=Dentalium panormum) (Chenu, 1843) and layers of empty bivalve shells.Some specimens were also found in pores of calcareous rocks in the study area.
The other sipunculan species co-occurring with Apionsoma (A.) misakianum at the sampled stations were Aspidosiphon (Aspidosiphon) muelleri and Onchnesoma steenstrupii steenstrupii.The dominance of A. (A.) misakianum over these species was observed at 10 stations out of 25, comprising more than 50% of specimens.The lowest dominance of A. (A.) misakianum was found at Stations 8 (0.63%), 253 (0.72%), 280 (1.21%) and 105 (1.72%), where O. steenstrupii steenstrupii was the most dominant species.Biometric analysis.The trunk length of the specimens of Apionsoma (A.) misakianum ranged from 1.7 to 7.8 mm and the highest length frequency value (34%) was estimated between 2 and 3 mm.The wet weight of the specimens of this species ranged from 0.1 to 7.2 mg and the highest weight frequency value (48%) was estimated to be between 0.1 and 1 mg.
The size-weight relationship (trunk length and individual wet weight) was estimated on 50 complete specimens of Apionsoma (A.) misakianum and was analysed using regression analysis (potential model) (Fig. 4).The correlation coefficient between the parameters was positive and significant (r=0.85,p<0.05) (Fig. 4).

DISCUSSION
The morphological features of the specimens of Apionsoma (A.) misakianum found during this study were similar to those found in the previous studies.However, the maximum trunk length (7.8 mm) of Apionsoma (A.) misakianum from the Aegean Sea is much smaller than those reported from the Indian (49 mm) (Cutler, 1994) and Pacific Oceans (25 mm) (Stephen and Edmonds, 1972), but somewhat similar to those from southern Brazil (5 mm) (Cutler and Cutler, 1980).In addition, the number of intestinal coils (6-10 coils) given by Stephen and Edmonds (1972) for this species was lower than that of the present specimens (11-23 coils).The number of hook rings (28-40) on the introvert of the Aegean Sea specimens of Apionsoma (A.) misakianum was lower than that (60 rings) reported by Stephen and Edmonds (1972).The above-mentioned biometrical parameters seem to be size-dependent and such differences can be expected between distant populations of a given species.
This species was previously reported from the Indian, Pacific and western Atlantic Oceans (Cutler, 1994).The report of the species in the present study extends its distributional range to the Mediterranean Sea.Although Murina (1964) reported Golfingia (Mitosiphon) hespera (Chamberlain, 1919) from the Mediterranean Sea in deep waters (173-1234 m), which was later synonymised with Apionsoma (A.) misakianum (see Cutler, 1973), Cutler (1973) examined Murina's specimens and proved that they in fact belonged to the species Apionsoma (Apionsoma) murinae Cutler, 1969.As A. (A.) misakianum is widely distributed in the area (not confined in a recipient area such as harbours or near canals) and, most probably, was previously confused with the other Apionsoma species in the Mediterranean Sea, it is unclear at this stage whether this species is alien to the Mediterranean Sea.The previous reports of Apionsoma species should be re-examined to determine the real distributional boundary of A. (A.) misakianum within the Mediterranean basins.
Concerning the reproductive characteristics found, Rice (1981) reported similar egg sizes on specimens collected near Florida, where the egg sizes of this species attained up to 77 µm (smaller axis) x 108 µm (longer axis).
Apionsoma (A.) misakianum was found in shallow waters (1-77 m) of the Indian Ocean (Cutler and Cutler, 1996;Saiz Salinas, 1993); from intertidal to 160 m depths in Japan (Cutler et al., 1984) and at 166 m from the outer Continental Shelf of Southern Brazil (Cutler and Cutler, 1980).In the present study, this species was found at depths ranging from 41 to 195 m.
Apionsoma (A.) misakianum was previously reported from coral sands and algae in shallow waters of the tropical and subtropical zones (Pagola-Carte and Saiz Salinas, 2000), and from sediments made up of diatom skeletons in roots of the green algae Vaucheria sp. and red algae which were growing on the rocks (Cutler et al., 1984).This species was frequently found within cavities of empty tubes of serpulid polychaetes and mollusc shells in the Aegean Sea.It was also found in pores of calcareous rocks and between layers of dead bivalve shells.
The present study sheds more light on the ecology, reproduction and population characteristics of Apionsoma (A.) misakianum.Future studies to be undertaken on the sipunculid fauna of the Mediterranean Sea will improve knowledge of its distributional and ecological features in the region.
FIG. 1. -Map of the study area with the location of sampling sites.