Evolutionary Derivation of the American Lobster Cardiovascular System: An Hypothesis Based on Morphological and Physiological Evidence

Abstract

The cardiovascular system of the American lobster includes a large muscular heart that pumps blood into seven arteries, each of which ramifies extensively. Portions of the system may be viewed as relatively primitive, while others are highly derived. We have confirmed earlier findings that the sternal artery is not a single vessel, but a paired structure. The sternal artery and its partner closely resemble the medial branches of the segmental lateral vessels from the dorsal abdominal artery in anterior segments of the abdomen, and they may be homologous. We report that the walls of the dorsal abdominal artery contain blocks of striated muscle cells and that the artery can be induced to contract in response to electrical stimulation or perfusion with proctolin. These observations provide the basis for an attempt to trace the evolution of the heart and arteries from that of primitive malacostracans to its state of development in lobsters. Additional key words: Crustacea, Homarus americanus, evolution, heart, dorsal abdominal artery, sternal artery We present here anatomical and physiological evidence supporting the hypothesis that the cardiovascular system in adult decapod crustaceans, as illustrated for Homarus americanus (e.g., McLaughlin 1983), is derived, by migration and regional specialization, from the evolutionarily primitive plan of a dorsal, longitudinal, tubular, muscular heart with a pair of ostia in each segment running from the head to the telson (Siewing 1963; Hessler et al. 1982). It has been postulated, based on the Cephalocarida, that crustacean ancestors displayed strong serial homology in body plan (Sanders 1955), possibly arising from annelid ancestral antecedants such as an extended nauplius-like larva (Hessler et al. 1982). The Cephalocarida and Branchiopoda (Anostraca, fairy shrimps) are closest to this prototype. This primitive plan is still the general plan in stomatopods (Siewing 1963; McLaughlin 1980). In the ancestral plan, the heart tube lies immediately dorsal to the gut and extends the entire length of the body. The heart tube supplies hemolymph to a short non-muscular anterior median artery to the brain, to some form of serially homologous segmental lateral arterial supply in each body segment, and to an unpaired posterior artery to the telson. The segmental a Author with whom to correspond. lateral arteries are short and do not branch extensively. Paired ostia admit returning hemolymph to the heart tube in each segment. In adults of H. americanus, the single-chambered heart is suspended in the pericardial sinus in the dorsal thorax by several pairs of alary ligaments (Lochhead 1950, pp. 428-431; McLaughlin 1980, pp. 138-141). These elastic ligaments are stretched during systole, and they expand the heart during diastole. During diastole, the heart passively fills with hemolymph from the pericardial sinus through three pairs of muscular,