Читать книгу Marine Mussels - Elizabeth Gosling - Страница 35

There are two types of excretory organs in bivalves, the pericardial glands and the paired nephridia or kidneys. In Mytilus, the reddish‐brown elongate kidneys lie ventral to the pericardial cavity surrounding the heart and dorsal to the gill axis, and in fact extend the complete length of the gill axis from the labial palps to the posterior adductor muscle (Figure 2.6). One arm of each kidney is glandular and opens into the pericardium, and the other end is a thin‐walled bladder that opens through a nephridiopore and empties into the exhalant chamber of the mantle cavity. See Pirie & George (1979) for a more detailed description of the excretory system in M. edulis.

The brown‐coloured pericardial glands, sometimes referred to as Keber’s organs, develop from the epithelial lining of the pericardium and come to lie over the auricular walls of the heart. Waste accumulates in certain cells of the pericardial glands and is periodically discharged into the pericardial cavity, and from there is eliminated via the kidneys. Other cells of the pericardial glands are involved in filtering the haemolymph, the first stage of urine formation. The filtrate then flows to the glandular part of the kidney, where the processes of secretion and reabsorption of ions occur. The end result is urine that has a high concentration of ammonia and smaller amounts of amino acids and creatine. Most aquatic invertebrates excrete ammonia as the end product of protein metabolism. Ammonia is highly toxic but its small molecular size and high solubility in water ensure that it very rapidly diffuses away from the animal.

While the kidneys and pericardial glands are the major excretory organs, excretory products are probably also lost across the general body surface and particularly across the gills (see Chapter 7 for details on excretion and osmoregulation). The kidney also plays a very important role in the storage and elimination of radionuclides and heavy metals such as silver, cobalt, mercury, manganese, lead and zinc (Metian et al. 2011 and references therein; Pouil et al. 2015). In scallops, Metian et al. (2009) have shown that several of these metals are sequestered in renal concretions, mostly of calcium carbonate, before being eliminated in the urine.