Regeneration time and morphology of the inhalant siphon of Donax denticulatus Linnaeus, 1758 after amputation

Journal of Shellfisheries Research, August, 2004 by Patricia Miloslavich, Pablo E. Penchaszadeh, Ana Karinna Carbonini, Diego Schapira

ABSTRACT To study the process of regeneration and its effect on the morphology of the inhalant siphon of a tellineacean bivalve, Donax denticulatus, individuals were artificially amputated and placed in aquaria under laboratory conditions, in the absence of predators. Alter amputation, two individuals were sacrificed twice a day (9 AM and 5 PM) during a 9- day period, and their siphons were examined in the microscope. Within the first 24 hours after amputation, the process was slow, and only the rudiments of the primary tentacles were visible. However, between the second and the third day a fast transformation of the siphon was observed, with regeneration of secondary and tertiary tentacles. The siphon was fully regenerated on the fifth day after amputation. One month later, the siphon showed the same aspect than at the fifth day. Despite the delay observed in the regeneration of the siphon within the first 24 hours after amputation, probably due to physiologic constrains associated with the lack of well developed tentacles, regeneration showed to be a fast process. Development of primary and secondary tentacles must therefore play an important role in providing basic functionality to the siphon, as well as to the whole organism.

KEY WORDS: siphon, regeneration, Donna denticulatus, tellineacean bivalve, fish cropping

INTRODUCTION

To maintain their basic physiologic functions (i.e., feeding, defecation, and gas exchange), infaunal organisms must establish contact with the sediment-water interface. Soft bodied species of ten achieve this interaction by exposing parts of their bodies outside the sediment in the form of specialize structures such as ramified tentacles (e.g., polychaetes) or siphons (e.g., bivalves) that maximize the area of contact with the water column, thus optimizing the interaction with environment (McLachlan et al. 1995). However, the exposure of such structures can enhance the risk for predation, making infaunal organisms more vulnerable to tissue browsing by benthic feeders such as, crab and shrimp (Kamermans & Huitema 1994), pelagic juvenile flatfish (Trevallion et al. 1970), and birds (Zwarts 1986). Siphon cropping has been reported for several bivalve species (Trevaillon 1971, Hodgson 1982, Peterson & Quammen 1982, Zwarts 1986). After amputation, siphons can regenerate providing a renewable source of secondary production that has been shown to maintain higher trophic levels (Penchaszadeh 1983, Pekkarinen 1984, Riera 1995, Luzzatto & Penchaszadeh 2001). However regeneration occurs at the cost of growth and reproduction, thus affecting the fitness of these organisms (Geller 1990, Peterson & Quammen 1982, Kamermans & Huitema 1994, Irlandi & Mehlich 1996).

The bivalve Donax denticulatus Linnaeus, 1758, locally known as "chipi-chipi" occurs intertidally on sandy beaches throughout the Caribbean all year long (Wade 1969). In Venezuela, it is commonly found in the intertidal zone of fine and medium grain dissipative sandy beaches in the central west coast, together with the bivalves Donax striatus and Tivela mactroides (De Mahieu & Gamba 1980, Penchaszadeh 1983, Penchaszadeh et al 2000). D. denticulatus is an important species in the food chain as a primary consumer of phytoplankton and detritus and is also eaten by a wide range of predators including the fishes Menticirrhus littoralis, Conodon nobilis, Trachinotus carolinus. Trachinotus goodei and Umbrina coroides, and the ghost crab Ocypode verreauxi (Penchaszadeh 1983, Riera 1995).

The siphons of this species are both flexible and extensible as are those of Donax hanleyanus Philippi 1847 (Luzzatto & Penchaszadeh, 2001) and other tellinacean bivalves. They are comprised of 6 primaries, 6 secondaries, 12 tertiaries, and 24 quaternaries tentacles (Wade 1969). When buried, the siphons of D. denticulatus extend through the sediment into the water column and look like a white crown from above. This highly ramified structure prevent large particules (>250 [micro]m) from entering into the stomach (Wade 1969).

In this study, we followed a detailed examination of artificially amputated siphon regeneration of D. denticulatus, to establish time of regeneration and the effects of amputation on the morphology of siphons.

MATERIALS AND METHODS

Individuals of D. denticulatus were collected at the intertidal zone of Tucacas beach located in the central west coast of Venezuela (10[degrees]45'55"N, 68[degrees]19'24"W). Organisms with a shell length of 1.5-2.0 cm were collected to ensure that all were already sexually developed. They were transported to the laboratory in an icebox with wet sand. In the laboratory, 2 groups consisting of 50 animals each (control and experimental groups) were placed separately in aquaria with aerated seawater at 35 [per thousand], 23 [degrees]C and a layer of sand deep enough for burrowing. They were fed with Advanced Invertebrate Formula (Marine Enterprises, Inc.) once a day. The animals were maintained for acclimation under these conditions for 5 days.

After this period, the experimental organisms were placed in Petri dishes with seawater. Once the siphons were extended, an anesthetic solution of magnesium chloride 7.5% in seawater was slowly added, and then the petri dish was placed in the refrigerator at 4 [degrees]C for 20 min. After this treatment, the siphons are completely elongated and do not retract in the shell, so we proceeded to cut the siphon tip with dissecting scissors. The lips were fixed in a glucamine-acetate buffer containing 6% formalin (Miloslavich & Penchaszadeh, 1997) and kept in the refrigerator. The animals were left to recover in their aquaria and then sacrificed in groups of 2 individuals twice a day (9 AM and 5 PM) during a 10-day period and then at day 30, cutting and fixing the tips by the described procedure. Observations of the fixed tips were done with a stereoscopic microscope. Also, we observed the siphons of the recovering animals to corroborate activity and functionality.